A simple HS science project

[John Stahl]

First I need to put in a disclaimer I’m a wantabee pilot.
I put this example together to help me understand how a horizontal stabilizer works.
It is really simple and I recommend you do it too.
The Fan representing the prop is facing toward the ground. I placed the fan about eight feet off the ground. I did this to prevent ground effect from messing up the test. The HS hangs about six feet off the ground.

[John Stahl]

These photos show the HS during a simulated level flight.
The first photo shows a fully immersed HS.
Notice how the prop vortex affects the HS.
In the second photo I found it interesting how the center flags would rap around each other.

[John Stahl]

These photos show the HS during a simulated level flight.
The photos shows a partially immersed HS.
I was surprised how much the prop vortex affected this stab.

[John Stahl]

Now to simulate PIO
In this photo the two fans that are hanging at an angle are simulating clean air and its effect on the HS.
The prop fan is set on high the clean air fans are running on there low speed.
I’m using two fans for the clean air for I was told that air entering the prop at an angle will still move in its original cores after going through the prop.
The first test I did was to see if this was true.
I turned the prop fan on and the upper clean air fan on. I left the lower clean air fan off.
I then looked closely for any difference. I even turned the clean air fan on and off looking for any change.
I saw no change
Then I turned the clean air fan up to medium then high.
I saw no change
If there was any effect it was not visibly noticeable

[John Stahl]

To simulate PIO I wanted to keep as many things constant as possible.
To achieve this goal I moved only the prop fan to simulate the different tail configurations.
In this photo the HS is fully immersed. It wasn’t necessary to simulate both a nose down and a nose up scenario because the results would have been the same.

[John Stahl]

In these photos the HS is partially immersed in the props wash
The first photo is a nose up scenario.
The second is a nose down scenario.
In the nose down scenario, I was surprised at how little affect the HS had.
I was also impressed at how much effect the HS had in the nose up scenario

[John Stahl]

Conclusions
I feel that the total immersed HS is the most stable because the HS has effect in both the nose up and nose down scenario.

Prop wash affects the efficiency of the HS more than I had imagined. It appeared to reduce the efficiency of the HS as much as 50 %.

So you understand what I’m going to describe. Most partially immersed HS look like an upside down T. most fully immersed HS look like a ( + ).
I think that two HS in an ( I ) configuration would be the most efficient HS. With this efficiency we will find the most stability.

Well what do you think?

[banaari]

Where to begin... first off, major respect for going to considerable time and trouble to try and test this stuff for yourself, but, point-by-point:

a) You've got nowhere near enough "clean air" source there to model free-stream air entering & then exiting a propeller. In real life the entire airmass is moving at the speed of the aircraft, not just a relatively tiny amount in, and fighting against, an otherwise static volume.

b) Your "HS" isn't. It's a set of indicators showing what the unmodified flow is aft of the prop, in the absence of an HS.

c) It doesn't show what happens when you introduce a solid aerofoil-shaped body into that flow modifying it & deflecting it. Nor does it show what the resultant force on that body might be - which is what's needed.

I can appreciate how visualising the airflow in that region might be seen as a useful investigative technique - but I don't think the conditions are realistic and there's a LOT more going on with a real HS. Again though - good to see somebody grappling with this.

If you're really keen - suggestion might be to build a to-scale wind tunnel (do some digging around aerodynamics theory and Reynold's number to find out HOW to scale it properly) and do some testing with a model HS - with some sort of force-measuring apparatus embedded in it. Better yet - get on with building a full-size airworthy stabiliser - and a gyro to carry it about on! :D

cheers,
John

[Dean_Dolph]

In these photos the HS is partially immersed in the props wash
The first photo is a nose up scenario.
The second is a nose down scenario.
In the nose down scenario, I was surprised at how little affect the HS had.
I was also impressed at how much effect the HS had in the nose up scenarioJohn, I'm puzzled. But then this is a common state for me!

If I'm looking at the photos correctly then it appears, by the prop fan placement in the two photos, that you moved the simulated 'prop' flow instead of the simulated free air flowing around the gyro. If this is fact then I'm not sure what you have demonstrated. Too early to think that deeply!

If it were me, I would leave the prop fan fixed in place and move the free air fan on opposite sides of the horz stab to simulate the nose up or down. And I wouldn't expect to see different results from placing it on opposite sides of the prop fan.

In the real world I suspect there is a different air flow pattern in a nose up or down situations. It is difficult to simulate that.

[Dean_Dolph]

Now to simulate PIOFirst of all John, I want to say I appreciate the effort you have gone thru to visualize what is taking place and provide the photos of your work. But there are a few things that need to clarified.

First PIO stands for pilot induced oscillation. I don't see your experiment as demonstrating that. I think what you are trying to do is demonstrate PPO, which is Power Push Over. These are two different animals. PPO can be exacerbated by PIO but PPO can also happen independent of pilot control input. But in fact the air flow to the horz stab is the same during PIO or PPO in the nose down attitude.

I want to throw in a disclaimer at this point and say it too early (before 10:00 a.m.!) for me to make any comments but I may not have another chance today.

In this photo the two fans that are hanging at an angle are simulating clean air and its effect on the HS. If you want to simulate clean air then the clean air should be flowing in the same direction as the prop flow and not affecting the horz stab. This assumes that the horz stab has no incidence and is just going along for the ride.

The prop fan is set on high the clean air fans are running on there low speed.
I’m using two fans for the clean air for I was told that air entering the prop at an angle will still move in its original cores after going through the prop. I believe you are trying to perform the experiment that Doug Riley described and suggested you do in another thread.

You need to go back and perform the experiment exactly as Doug described it with out he second ‘clean air’ fan. Its use clouds the results. I think I know what you were trying to achieve but for the purposes of showing the affect of ‘clean air’ angled in flow to the prop you need to follow Doug’s outline.

I also think that, for the demo Doug was describing, the ‘clean air’ fan is should be directed more into the prop instead of air flow split between the prop and the horz stab which it appears you have done. I believe if you moved the ‘clean air’ fan to other angles of prop input that you would more clearly see what Doug is saying.

What you are doing in this experiment with the ‘clean air’ fan (kind of a misnomer!), is simulating the flow of air passing by/around a gyro. Direct it more into the prop fan intake and at different angles, or maybe at different distances, and I believe you will see different and maybe clearer affects on the prop fan outflow.

[John Stahl]

Dean
After doing the experiment that Doug gave me. Seeing how the string was affected by moving another fan around the intake of the fan. I thought maybe I could take that experiment to the next step. I thought this simulation would show that the angle clean air enters the prop would have a large impact on the HS. But in this simulation I couldn’t SEE it.
Why?
Most likely the simulation set up was flawed.
I’m clueless as how to improve it.

There may be physics at work I’m unaware of within the prop vortex

The rest of the simulation was designed to assist me in deciding which tail configuration I will put on my Gyro.

[Frosimo]

Some interesting stuff about HS:
http://www.geocities.com/donshoebridge/h-stab.html

[C. Beaty]

The following NACA report clearly illustrates the energizing effect of centering the horizontal stabilizer in the propeller slipstream:


http://naca.larc.nasa.gov/reports/1940/naca-report-690/naca-report-690.pdf

The downwash angle of the wing muddies the water in some instances but in general, the horizontal stabilizer located in the propeller slipstream was more effective.

Propeller driven T tail airplanes sacrifice horizontal stabilizer effectiveness in the interest of improved spin characteristics. Conventionally located stabilizers blanket the vertical tail surfaces at the high angles of attack encountered in a spin.

[Doug Riley]

John:

Your setup, of course, is a different experiment than the one I described. The pro designers often use small lengths of yarn to help visualize local airflow over wings. OTOH, long streamers of fabric with no tension on them flap and swirl around (sailors call it "luffing") so much that their motion tends to obscure the air's direction. They will help you in a general way to see the swirl effects and some of the angle-of attack effects, though.

We need to tease out the concept of HS "efficiency." In the case of a prop slipstream that deflects in response to the inflow direction, the change in direction of the outflow is less than the change in direction of the inflow. The exact ratio of the two will depend on their relative speeds.... naturally, it's not always 1:2. The important point is that the aerodynamic force on the HS varies as the SQUARE of the airspeed, but directly (linearly) as the angle of attack up to stall. Therefore what is lost in smaller changes in angle of attack is more than made up for in the higher airspeed of the prop blast.

A Dominator-style fully immersed gyro HS is not likely to see any air that has not first travelled through the prop disk (the prop disk is 5-6 feet in diameter). Thus, in your setup, a fan aimed at the HS from position DOWNstream of the "prop" fan (or way off to one side) doesn't accurately model this type of HS. The fan simulating the free stream must exhaust its flow into the intake of the fan simulating the prop. IOW, the fans should be lined up in "series." The changes in inflow direction in the real world are on the order of 10-20 degrees, except perhaps in a vertical descent. Most airfoils (including HS's) stall at angles of attack of 12-14 degrees.

On of the biggest problems with our close-coupled pusher aircraft is turbulent inflow to the prop. It's a case of "turbulence in, turbulence out." If the inflow is turbulent, the prop's blades will be receiving air locally at all manner of angles of attack, from zero on up beyond stall. The outflow will be even more turbulent than the inflow. This makes a racket and reduces the prop's efficiency, as well as that of the HS. At gyro speeds, however, the HS still produces more stabilizing force when immersed than when not immersed, as long as the engine is producing appreciable thrust. This fact is instantly obvious when you do an idling approach at your regular cruise speed in a craft with an immersed HS.

[John Stahl]

C. Beaty
I still don’t buy it.
A fixed wing HS is also a control surface that requires it to be in the slip stream to work properly. The last I looked on a gyro there is no control surface on the HS.

A horse looks like a mule but you can’t use s horse saddle on a mule or vice versa. You can’t use a horse shoe on a mule. You can’t train a mule like a horse.

Like a horse shoe and a mule shoe differ. I suspect that the HS on a fixed wing and a gyro will differ also.

The lack of a real testing on the function of a HS is disturbing to me. I find it intriguing that gyro builder’s take so much time and care in building there craft. Are guessing and taking others word as gospel, when it comes to a HS.
So far the only thing I believe is the farther from the center of mass a HS is the more efficient it is.
In the final analyses the distance from the CM will probably be more of a factor in stabilizing flight, than where the HS is placed.

[Doug Riley]

If a FW's elevator had to be in the slipstream to work, then FW gliders and canard airplanes would not be controllable. In fact, elevators will work either immersed or not. They are typically more powerful (or sensitive, if you prefer) when immersed.

There's no clear qualitative distinction possible between a movable (control) surface and a fixed (stabilizer) surface. Both generate a range of amounts (and directions) of lift in the face of changing airspeeds and angles of attack. An elevator simply allows you rapidly to alter the camber of the HS, which in turn biases the various amounts of lift that the HS will create at various angles of attack. Hold the elevator in one position and it functions exactly the same as a fixed fin: it creates lift (usually downward) that is directly proportional to its angle of attack. This angle of attack will vary constantly in turbulence.

"Fixed" HS's (on gyros and otherwise) can in fact be adjusted either in flight or on the ground. The mechanism may be as fancy as a fly-by-wire digitally controlled jackscrew or as crude as stuffing washers under some mounting straps. Are these HS's then "control" surfaces or mere stabilizers? Where do you draw the line? Is there any reason to draw it?

All HS's do the same basic job, no matter to what degree, how fast or by what means they are adjustable. In order to work, they all have the same need, which is a flow of air of adequate speed that varies with the aircraft fuselage's angle of attack.

[Udi]

John - Although I also like to sometimes experiment with physics to prove some things to myself, we don't have to re-invent the wheel when it comes to the 100 year-old science of aerodynamics. Basic aerodynamic phenomena are well known and documented, and they apply to any flying object from birds to planes to rotorcraft. Obviously, one has to be mindful of the specific application of the science to their own specific application.

I know nothing of mules and horses, but I am sure there are even some commonalities between a mule shoe and a horse shoe. I assume that a horse shoe maker can probably make a shoe for the mule if he understands the differences between the two animals.

One known aerodynamic phenomena, that is true to every flying surface, is that the lift of a flying surface is proportional to the square of the velocity of the air. For example. A given stab on a given gyro is generating a lift of 20 lbs when flying at an angle of attack (AOA) of 4 degrees in 30 mph airspeed (measured at the stab). The same stab, at the same AOA, would generate a lift of 80 lbs when flying at an airspeed of 60 mph. The airspeed has doubled, so the lift is 2 squared = 4 times the original lift. If you want, you can verify this law of physics in your experimental setup by taking a lifting surface (a flat plate will do) and measuring the lift (you can use a simple spring weigh scale) as a function of airspeed (use a pitot tube to measure airspeed).

Now, another known law of aerodynamics: We know that the angle of the air going thru a propeller will change directly proportional to the acceleration of air speed. For example - if the airspeed going into the prop is 20 mph and the airspeed going out of the prop (measured a few feet away to allow for complete acceleration) is 40 mph, the angle of the air going out of the prop will be half of the angle going into the prop. You can also verify this law with your setup.

Applying this knowledge to the aerodynamics of a gyroplane stabilizer - Let's say a gyro is flying at 40 mph and the prop is doubling the airspeed in the prop wash to 80 mph. I am using these numbers because they are easy to handle mathematically. Now let's compare 2 stabs - one is located inside the center of the prop wash and the other is located outside the prop wash (they never are completely outside the prop wash, but this is the assumption for this example).

The gyro is hitting an up-draft that is causing the relative wind to hit the gyro at a 2-degree angle from below. Let's compare the two stabs. The stab that is flying outside the prop wash will see air at 40 mph, hitting it at an angle of 2 degrees. This will generate a certain lift (or a nose-down pitching moment).

The stab that is flying inside the prop wash will see air at 80 mph, hitting it from only 1 degree below (the air was accelerated by 2, so the angle is half). The lift is linear with the AOA, but it is a square function of the airspeed. So the lift that the prop wash immersed stab would generate is (1 divided by 2 for the angle, multiplied by 4 for the speed = 2) twice the lift that the stab that is located outside the prop wash would generate.

John - this above calculation is no speculation, it is a simple application of the laws of physics to our specific application. I have made a few assumptions - and of course the results of my calculation are as accurate as the assumptions that I have made. We can argue all day about the assumptions we make, but we cannot argue about the laws of physics.

Udi

[Frosimo]

Maybe a test like this will give a realistic result...?:
http://home.no.net/frosimo1/testjig.jpg

[mceagle]

The gyro is hitting an up-draft that is causing the relative wind to hit the gyro at a 2-degree angle from below. Let's compare the two stabs. The stab that is flying outside the prop wash will see air at 40 mph, hitting it at an angle of 2 degrees. This will generate a certain lift (or a nose-down pitching moment).
The stab that is flying inside the prop wash will see air at 80 mph, hitting it from only 1 degree below (the air was accelerated by 2, so the angle is half). The lift is linear with the AOA, but it is a square function of the airspeed. So the lift that the prop wash immersed stab would generate is (1 divided by 2 for the angle, multiplied by 4 for the speed = 2) twice the lift that the stab that is located outside the prop wash would generate. Udi, it would be interesting to compare popular stabilizer effects, including distance from C of G. Take for example a popular H/S fully immersed with a 4 ft moment arm, and compare it to a popular under fin (not immersed) H/S with a 6 ft moment arm. Throw in a more practical cruise airflow variation of 90 mph immersed and 70 mph not immersed, and what are the results?
It is obvious that the difference in efficiency wouldn't be so dramatic, which would allow builders to make choices on the other pros and cons of immersed Vs non immersed stabilizers and rudders.

[Udi]

My example was intended to illustrate how placing the stab inside the prop wash can add power to a given stab, Tim. As you have mentioned, there are other factors affecting stab power, and the distance of the stab from the CG is one of them.

Anyone can use the basic rules to compare different stab configurations. The basic rules are:

1. Stab power is directly proportional to the stab distance from the CG (force x arm).
2. Lift is directly proportional (only within the linear range) to the stab AOA.
3. Lift is proportional to the square of true airspeed.

Here is one that is often neglected:

4. Stab damping power is proportional to the SQUARE of stab distance from the CG.

I don't know if anyone has made a serious attemp to compare the airspeed accross the stab at different locations. Some people claim that a keel-mounted stab is geting a fair amount of prop wash. Nobody can make a serious comparison without having solid data.

I think nobody can argue with the statement that the best stab configuration is a stab mounted as far away from the CG as practical, and located inside the prop wash. A tractor configuration, such as the Little Wing comes to mind.

Udi

[John Stahl]

For those that do not know equines
A mules shoe is half the width and 1.5 times the length of a horse shoe. Mules are more sure footed than horses.
The back of a Mule has a slope coming of the back bone that is 6 degree’s steeper than a back of a horse. Saddles don’t role as easily on a mule
A mule is twice as smart. Three times more honoree than a horse is. That is why I don’t own one.
Almost sounds like I’m comparing a fixed wing aircraft with a gyro.

Udi I don’t know what you do for a living but I hope it is teaching.

C. beaty Chuck if I had realized who had sent the info I wouldn’t have walked over your information with muddy combat boots. Sorry, I’m just frustrated with the lack of apples to apples data on HS on gyros.

I just want to build a craft that is a joy to fly, and as safe a design as possible.
John

[donshoebridge]

I think nobody can argue with the statement that the best stab configuration is a stab mounted as far away from the CG as practical, and located inside the prop wash. A tractor configuration, such as the Little Wing comes to mind.

Udi,

This post isn't specifically directed at you. But more to the subject and peoples attitude over the past year or 2. It's just that your post happen to push the right button.

As far away from the CG as possible, granted. However, as for in or out of the prop wash, I my not be able to argue the point very well, but I can surely disagree. And I disagree for one main reason - because it's dirty air. And this is the exact reason why me, and John, and a bunch of other people want to see REAL TEST DATA, a real apples-to-apples comparison. Not just a "it feels better" flight report.

Everyone here can throw formulas, and vector charts, and lip service at everything that his said about a HS in or out of the prop wash, but until someone does REAL TESTING, nobody is going to know for absolutely sure what the hell is going on behind the prop. I suggested tufting the tail feathers a few months ago, but was blown off because "air flow didn't matter". The hell it don't! This would be a great first step in seeing what is happening. Load cells between the tail feathers and the airframe would be another light weight testing apparatus that could shed more light on a dimly lit subject.

I'm tired of seeing homemade bitmap images that are intended to make the creator out to be some kind of a gyro wizard, no-it-all, expert. Where's the real data that supports peoples claims? You can search all over the web, dig through countless aerodynamic books, and run computer simulations till your blue in the face. But until someone has application (gyro) specific, tail feather test data, all the talk in the world means nothing.

[John Stahl]

HALALUYA
I am not the only one that would like to have apples to apples data! You don’t know how good that makes me feel, I thought I was dense or something.

I think it would be good to have a basic HS Bible.
Using Uni’s four basic HS rules, I should say commandments to start with.

Then others can be added like
5. The closer the HS is to the CG the larger the HS surface area needs to be.
And
6. The farther from the CG the HS is the less important the placement in the slipstream becomes.

I feel that Ausie Paul’s new design is on to something.
In five years we will no longer be debating where to put the HS but How far back from the CG is back.

It sure would be nice to have reliable data helping us determine the size and placement of our stabs.

[darrellwittke]

Although I sure like lists. The point of disagreement is that horizontal stabilizers affect flight qualities and people vary in their tastes of flight qualities.

Some examples: C. Wall liked his gyro with the partially immersed stabilizer best (as stated on the lepton reports on Norm's forum). He stated it had a feeling of being on rails but able to maneuver more quickly. He had flown a full span tail with immersed stab previously.

Art Evans bensen with horizontal stabilizer (worth reviving I say) had a ride report that it made it fly like a cessna 210 as reported in Rotorcraft, I believe.

Some would say that Art Evans gyro was unsafe and did not fly right. Others would say C. Walls gyro was unsafe and did not fly right. (I use old examples so as not to cause a stir on current designs)

The point being that, as long as the gyro is safe, people can choose the stabilizer that fits their preferred flying "feel." Greg G. has done a wonderful thing to have guidelines formed to have manufacturers meet at least minimum safety guidelines. Pilots are then free to argue about the best horizontal stab/placement positions for perpetuity, but only (hopefully) after safe minimums are met. My .02 cents, thanks for listening.

Oops, just reread your post, John. I have to agree with you that I would also like apples to apples comparisons of H'stabs.

[John Stahl]

Darrel
Do you fly?
Do you have a gyro?
It is good to hear there are more of gyro folks near me.
Some times I get the feeling of total isolation.
John

[Udi]

First let me say that I respect other people's opinions. I don’t want to get into a pissing contest with anyone and my only objective is to better my understanding of gyro aerodynamics and, if I can, help others.

With regard to "apples to apples", there's plenty of that. I have read countless reports from people who've modified their gyros in one way or another and reported the changes they perceived. Obviously, most of us don't own an on-board data collection system and most reports come from “the seat of our pants”. Still, you can't discount the reports. I collect these reports as they come and give them the credibility I feel they deserve. With time, I form an opinion based on the accumulated "data". My subjective impression is that most people feel improvement in the response of their gyros' stab once they have moved it into the prop wash.

Don - your argument against placing the stab in the prop wash seem to be that a stab behind the prop is getting "dirty air". I don't know what that means. Do you mean the flow from the prop is not clean because the bulky airframe/pilot ahead of the prop is generating turbulence? Or do you mean the air is dirty because the prop is spiraling the air and making it all turbulent (or both)? I am sure there is some of both, but the reality is that it aint hurting anything.

I have flown gyros both with a keel-mounted stab and a stab in the center of the prop. I didn't “feel” anything that would indicate that a stab behind the prop is getting anything but smooth air. The Sparrow Hawk is flying like a Rolls Royse, pretty much like a Magni with its huge tail "outside" the prop wash.

Furthermore - when you look at any single place gyro from the front or the back you can see that the shadow that is created by the pilot and the airframe covers a small portion of the prop. Most props are 60" or better, but most pilots are no more than 20" wide. These props see a lot of clean air.

My only grudge with the tall tails is that, in my opinion, they are too close the CG, and they miss out on some important pitch damping. Unlike static stability, dynamic stability is NOT enhanced by placing the stab in the prop wash. To improve dynamic stability (the result of damping), one must move the stab back, make the stab larger, or both. You get the most bang for your buck by moving the stab back because damping is proportional to the square of the distance from the CG, while being directly proportional to stab size and air speed (didn't mean to throw formulas around, Don, but this is how my brain works).

In any case, Don and John, until we have more solid data to base our decisions, we will all use our best judgments to build our own birds. This is what experimental aviation is all about. Both of you are critical thinkers and whatever you decide will work out fine. I love flying gyros and I don't really care much if their stubs are on the keel or behind the prop - as long as they have an effective stab.

Udi

[darrellwittke]

For your well written post and especially for sharing your knowledge with us. "Damping is proportional to square of the distance from the CG, while being directly proportional to stabilizer size and airspeed" didn't sink in until I read it the second time. You and the other engineers, Chuck B, Raghu, Doug R., really are wonderful to share your knowledge here with us. I've said it before and I will say it again. Thank you.

Re John Stahl, yes I have a gyro and yes I do fly but haven't since last year. I am rebuilding (took off the left hand jugs and replaced the base gasket on my mac 72 last night) and you are more than welcome to come by and talk gyro's. I thought you sent me an e-mail perhaps a month ago inquiring about the same questions? I know the feeling of total isolation you are talking about, there are gyro folks up north here though, if you look around hard enough. Best regards.

[C. Beaty]

The thing which got me thinking about the energizing effect of the propeller slipstream happened more years ago than I like to admit.

My early flying experience with a Mac powered Bensen nearly always resulted in a landing with a dead engine. My first landing with the engine still running was a bit scary.

The Bensen had excellent weathercock stability in yaw, being essentially dead-beat as long as the engine was running. The airframe tracked perfectly with feet off the rudder pedals through “S” and any other maneuver.

Once the engine died, keeping the airframe aligned in yaw became quite a chore. The airframe absolutely refused to track the relative wind through turns without aggressive rudder input.

The vertical tail surface of a Bensen, as well as most other gyros, is almost entirely immersed in the propeller slipstream.

[donshoebridge]

With regard to "apples to apples", there's plenty of that. I have read countless reports from people who've modified their gyros in one way or another and reported the changes they perceived.

This is exactly what I'm talking about. I don't care about what people are "saying". I want to see a graph where I can point to a spot and say "here's where my gyro performs, and over here is an RAF, and here is the Dominator". But more to what John is thinking, a chart that shows aspect ratio, area, percent immersion, sweep angle, and distance from the prop. Then there are no porsonalized comparisons which leave huge holes for interpratation.

Don - your argument against placing the stab in the prop wash seem to be that a stab behind the prop is getting "dirty air". I don't know what that means. Do you mean the flow from the prop is not clean because the bulky airframe/pilot ahead of the prop is generating turbulence? Or do you mean the air is dirty because the prop is spiraling the air and making it all turbulent (or both)? I am sure there is some of both, but the reality is that it aint hurting anything.

Both, because it's not predictable prior to flying. The idea from an engineering point of view is to know as many of the variables before flight testing, so there's no suprises. Remember the saying about old and bold pilots?

In any case, Don and John, until we have more solid data to base our decisions, we will all use our best judgments to build our own birds. This is what experimental aviation is all about. Both of you are critical thinkers and whatever you decide will work out fine. I love flying gyros and I don't really care much if their stubs are on the keel or behind the prop - as long as they have an effective stab.

But even fixed wing experimental aircraft designers have a good idea of how their aircraft should perform prior to first flight. For gyros, almost all faith has been put into the rotor blades. We know every little detail about the rotor system - how it functions, how to size it, what the limitations are, etc. Now I haven't seen charts and graphs for rotor systems, but it's pretty obvious that it has been refined to a point that everyones rotor heads are the same basic configuration and the can be said for blade selection. But for tail feathers, there's basically nothing to draw from - only personal accounts.

[John Stahl]

It appears to me that the consensus is.
More data on HS is needed.
Then how do we go about getting the data?
Who do we go to?
What is the best approach to use? To improve our chances of getting the research data we want?

[C. Beaty]

The thrust produced by a given propeller at a given airspeed is:

Thrust = air density x propeller area x freestream velocity x (propeller slipstream velocity – freestream velocity) Units are: feet, seconds and slugs. The mass of air at sea level is 0.0023 slugs/cubic foot.

Flying at 50 mph with a 4’ diameter propeller with a slipstream velocity of 100 mph supplies a thrust of 156 lb.

Flying at 50 mph with a 5’ propeller diameter with a slipstream velocity of 100 mph supplies a thrust of 244 lb.

If velocities are constant, thrust is directly proportional to propeller area.

Air is always turbulent. In spite of that, wings work and so do stabilizer surfaces in the propeller slipstream.

If Don’s “theories” were correct, a B-17 couldn’t possibly have flown. But Boeing, not knowing about Don’s theory of “extreme turbulence,” went ahead and designed the B-17 with perhaps 75% of the wing buried in the propeller slipstream.

The fact is, wings and flaps centered in the propeller slipstream enabled the B-17 to lift off with a heavier load than would have been the case had it been a pusher.

********************

The British CAA funded a study of gyroplane stability that was conducted by the University of Glasgow.

Part of the testing required wind tunnel testing of a ¼ scale model of a Magni, subcontracted to the University of Prague in the Czech Republic.

An electric motor mounted in the model driving a propeller simulated the full-scale propeller slipstream. Pitch and yaw force was measured at various angles of the model with respect to the freestream.

Normally, the horizontal stabilizer of a Magni is clear of the propeller slipstream except at large nosedown angles.

One of the most striking things I recall from reading the U. of G. report is how much greater was the restoring force on the model at high nosedown angles as the horizontal stabilizer entered the propeller slipstream.

I have a copy of that report around here somewhere and will extract the data on pitch force if I can find it.

[rehler]

John, you asked: "Then how do we go about getting the data?"

I suggest:

As has been done in the past, some one or group can go to several gyro flyins (Mentone, Bensen Days, etc.) one year and measure all of the gyros, noting the gyro brand, the HS dimensions, the distance from HS to CG (this can be found - 9 degree line down from rotor head teeter bolt and near of below the thrust line depending on the gyro configuration - close enough for comparative information), distance from HS to prop thrust line (height), distance of HS from prop (horizontal), size of prop, dimensions of rudder and location, rotor diameter, etc.

This is a good start, but a pilot also needs to fly as many of the different gyros as possible and make notes on the how they fly.

Within a year the results could be found and a comparative analysis could be made and published in PRA Rotorcraft magazine. It would be rough information but helpful.

Also, all gyro kit sellers could be asked to provide the information on each of their kits.

THe challange is finding someone or group who will be willing to do the work!

[Dean_Dolph]

Don, to satisfy your desires, as an engineer/designer, to have credible data points available would require a well designed project. One that requires resources that I doubt are available to anyone outside the few professional organizations such as CarterCopter and Groen Bros. Could a group of people pull this off? Certainly, but we are talking credible data and I quite frankly wouldn’t trust data points that were generated in less than controlled/documented conditions. Even then………

I worked for an R&D organization pretty much my whole career and I have seen how data can be manipulated to serve a purpose and to reach the conclusions that the leader wanted. I worked with one engineer that would take the data points we collected/graphed and throw out any that didn’t fall on the his preconceived proof line. I say proof line instead of the results line because this guy was committed to selling his proposals. Yeah, there are always outliers that should be tossed but the percentages that were being tossed was discouraging for those of us who were doing the data collecting. What was really amazing was the fact that this guy was very seldom wrong and had a history of successful projects.

What I’m saying is that with the proper primary info, which in our case is simply the laws of physics, and empirical info, that more often than not our intuition will lead us in the right direction without going thru the data collection effort. Sometimes we are better off not stressing ourselves and just accept what has proven to work without getting into the details.

I know that won’t satisfy you and a bunch of us that have this inner need to understand. So, in order for you to get you desires satisfied then it will be up to you to organize/lead such an effort. Oh yeah, you are also going to have to be a salesman to get buy in from those who you would need to help.

[Dean_Dolph]

John, unless your intent is to design a machine from scratch then there is no reason to not use the empirical info available. This info doesn't include a bunch of data points but does demonstrate what works and the successful designs don't violate the laws of physics. I wouldn't say that a consensus exists to collect this type of info and with good reason, its not needed by the back yard builder.

Unless one want to be different just for the sake of being different then there is no reason not to adopt what has already proven to work. Yes, it is always nice to understand as much as possible the 'whys when making these adoptions. But in the case of the gyro tail we have all the proof we need of the performance so we can accept the existing designs on the machines that have proven stability. This is why you see the tall tail being used. I have issues with the all flying cruciform tall tail but that is just me since it is an accepted design that has proven itself.

[donshoebridge]

If Don’s “theories” were correct, a B-17 couldn’t possibly have flown. But Boeing, not knowing about Don’s theory of “extreme turbulence,” went ahead and designed the B-17 with perhaps 75% of the wing buried in the propeller slipstream.

The fact is, wings and flaps centered in the propeller slipstream enabled the B-17 to lift off with a heavier load than would have been the case had it been a pusher.

There it is. Typical! If you can't produce test results, the next best thing is to discredit those that challenge you. This isn't even a case of taking anything out of context either. You just flat made that crap up! What a bunch of HOGWASH! When and where did I EVER say that the propeller, by itself, causes the prop wash to be "turbulent"? I never said that crap! What I said was, and Doug also stated it, that if the in-coming airflow is turbulent, than, chances are, the prop wash will also be turbulent - garbage in, garbage out!

John,

I think we are barking up the wrong tree on this topic. Let's just leave these "people" to themselves to do whatever they want. I seriously doubt that any of these "people" will lift a finger to generate any technical documentation to substantiate what they’ve already been preaching for years. You know... Preconceived notions.

[donshoebridge]

Don, to satisfy your desires, as an engineer/designer, to have credible data points available would require a well designed project. One that requires resources that I doubt are available to anyone outside the few professional organizations such as CarterCopter and Groen Bros. Could a group of people pull this off? Certainly, but we are talking credible data and I quite frankly wouldn’t trust data points that were generated in less than controlled/documented conditions. Even then………

I worked for an R&D organization pretty much my whole career and I have seen how data can be manipulated to serve a purpose and to reach the conclusions that the leader wanted. I worked with one engineer that would take the data points we collected/graphed and throw out any that didn’t fall on the his preconceived proof line. I say proof line instead of the results line because this guy was committed to selling his proposals. Yeah, there are always outliers that should be tossed but the percentages that were being tossed was discouraging for those of us who were doing the data collecting. What was really amazing was the fact that this guy was very seldom wrong and had a history of successful projects.

What I’m saying is that with the proper primary info, which in our case is simply the laws of physics, and empirical info, that more often than not our intuition will lead us in the right direction without going thru the data collection effort. Sometimes we are better off not stressing ourselves and just accept what has proven to work without getting into the details.

I know that won’t satisfy you and a bunch of us that have this inner need to understand. So, in order for you to get you desires satisfied then it will be up to you to organize/lead such an effort. Oh yeah, you are also going to have to be a salesman to get buy in from those who you would need to help.

Dean,

What I'm saying is that there is a HUGE gap between NASA DAQ systems and box fan testing. Anything more complex than a box fan is going to be light years better than nothing at all. Load cell film from Omega isn't terrible expensive and can be applied directly to the keel tube of a gyro. Not only would this allow the measurment of X and Y loads, but could also provide tortional loads as well. If additional cells in the tail feathers are required, fine, load'm up. The cells on the tail boom can be used for calibration so that the dat variation between tails can be converted to a base line. The only thing required is the data logger. A simple PIC porcessor will do nicely or MPU from Microchip. The parts are not the costly. Finding someone that is willing to do is another matter as obvious by the tone of this thread.

[C. Beaty]

No matter what objects are in front of a propeller, the angle of the propeller slipstream reflects the freestream entry angle.

The path length around the leeward side of an obstructing object in yawed flow is shorter than the path length around the windward side so that the entry angle into the propeller is about the same as it would be without the obstruction.

Vortices from blunt objects do generate turbulence but that’s not very relevant. The slipstream of an isolated propeller is a series of vortex sheets. Airfoils in general respond to mean velocities because stall has a hysteresis (lag of response); an abrupt increase of angle of attack beyond the static stall angle does not produce dynamic stall.

Also, Don, a draftsman with a fancy title is still a draftsman in my book.

For those who care, the following graph is from the NACA site. It shows the effect on maximum wing lift for tractor Vs. pusher propellers.

This was an investigation into the benefits of engine installation buried in the wing for a large 4-engine bomber. Tests were made with propellers located near both trailing and leading edges, driven by extension shafts.

The vertical scale is maximum lift and the horizontal scale is normalized power to the propellers. In both cases, lift coefficient increases from the power off value of 1.1 to a maximum value of about 2.4 at high power setting.

The pusher propellers located at the wing trailing edge increased the power on lift because they drew air over the wing but not as much as the leading edge propellers.

http://naca.larc.nasa.gov/reports/1938/naca-wr-l-456/

[Harry_S.]

CB...you will always amaze me. :D


Cheers :)

[donshoebridge]

Gee, Chuck. I really under estimated you. You're a bigger a-hole than I first figured.

[darrellwittke]

I'm sorry Don but I remember answers like yours coming from RAF at the beginning of Norm's forum, what 10 years or better now? Please refute what Mr. Beaty says with math or some form of logic, but please, no name calling. What Chuck has to say is very valuable to I and many others on this forum, especially when he backs it up with drawings, graphs, mathematical formula's, etc. All things which take time and effort to compile, research and post. RAF could not and/or would not refute the math and/or argument for horizontal stabilizers that Chuck B. has preached for many years. They then resorted to name calling and mud-slinging.

Just speaking for myself, I cannot speak for other forum readers, I prefer you refute arguments with facts or logical statements and refrain from name calling. Thanks for listening. darrellwittke

PS to John Stahl, I reviewed my e-mail and found it was from a forum member named montflyer. He is also in Missoula and perhaps you two could get together? Maybe both of you could come over to Helena here and look over my gyro? Please reply via e-mail or private message, thanks in advance.

[GyroRon]

Reading this whole thread I feel like I am reading a bunch of nothing. There is enough evidence of what one type of tail will do over another, proof is in the pudding as they say. I mean what does, or will, this discussion result in? Some type of new tail that is better than what is out there already???

It is tail feathers folks! Look at airplanes, some are pushers, some tractors. Some have all flying tails, some have fixed tailplanes with moveable surfaces. Some have Airfoil shaped surfaces, some are just round tubes covered in fabric.

If you take what is already out there and look at each types pros and cons, you can make the choice on what is best for you. The math has shown that the stab and rudder is more effective in the prop wash than not in the propwash. The math also shows that having the vertical surface tall enough that the propwash above and below the thrustline has advantages in countering P factor without pilot input.

There is enough information out there as it is to draw a conclusion on what would be the most effective stab and rudder combo already, and there is plenty of reports from pilots on what each type does or doesn't do.

As for me I believe the best combo is what Carl S. uses on his current version of his gyrocycle. It has all the benifits of the tall tail and the stab being in the propwash, and yet with the fixed wertical and moveable rudder, he eliminates the problems of the all flying tail such as what is on a Dominator.

[John Stahl]

Sorry guys I didn’t mean to start a shouting match.
I should have realized that this was a contentious subject.

Chuck
Looking at my experiment I was surprised at how much the prop vortex was still evident with the partial HS stab.
Did this indicate that the HS was still in the wash?
How immersed does a stab need to be?
If I fell in a river and went under one inch, I would come out of the river just as wet as if I had jumped of a cliff and gone under water ten feet.

Is it possible that we are losing our tempers over two stabs that are both immersed in the wash?

[C. Beaty]

I looked for the paper on wind tunnel testing of the Magni gyro (actually a VPM-M16) but couldn’t find it.

I did however locate the final report by the University of Glasgow, Department of Aerospace Engineering by Dr. S. S. Houston: “Report #9710, Identification of Gyroplane Lateral/Directional Stability and Control Characteristics From Flight Test.”

This reference was made in respect to yaw stability:

“Given the relatively low flight speed, the strong primary damping in yaw Nr is probably due to the energising effect of the propeller in close proximity to the fin and endplates. This parameter has been shown previously, Ref. 24, to have a dominant influence on dutch roll damping, and is primarily responsible for this mode being so well damped. The weathercock stability derivative Nv is of interest because it is stabilising and significant in magnitude. The concern with light gyroplanes of this configuration is the amount of side area ahead of the centre of mass, which will tend to be de-stabilising. Again, the energising effect of the propeller on the tail surfaces probably helps in this regard.”

In my mind, it’s not much of a stretch to extend the energizing effect of propeller slipstream on vertical surfaces to horizontal surfaces.

[C. Beaty]

John, the energizing effect is there even though tail surfaces aren’t centered.

But there are additional benefits from centering tail surfaces in the propeller slipstream.

A vertical fin/rudder that extends from keel to about prop center is subjected to a corkscrewing slipstream that tries to move it sideways. On takeoff roll as you increase power, you also have to feed in rudder to keep the machine straight. Throttle/rudder coordination is one of the things slowing student progress. A full span vertical tail like a Dominator does not exhibit throttle/yaw coupling and is much easier to handle during the early learning phase.

Immediately following liftoff, the machine tends to roll in the direction opposite to propeller rotation. This must be compensated by stick movement away from the direction of roll.

A cruciform tail removes most of the propeller slipstream rotation and in so doing, applies a torque to the airframe that nearly balances propeller torque. A machine so equipped doesn’t have a tendency (or at least one much reduced) to roll following liftoff.

[darrellwittke]

Another way to counter torque roll is to make the horizontal stabilizer a full span at least propellor length wide, utilizing two smaller elevators at either end. This allows greater rotor clearance from the vertical stabilizer while allowing the whole assembly to be placed farther back from the CG. Examples of this may be Ken Rehlers gyro, Ernie's LFINO I think, and the Cartercopter.

"Stab damping power is proportional to square of stab distance from the CG" as stated by Udi, perhaps this is the "ideal" design if you are designing for best safety in a pusher design? Are my assumptions correct, Chuck B? anyone? Thanks in advance.

Oh, the drawbacks to the design, extra weight for the two booms, or if you are doing a single tail boom design, then the extra weight to beef up the horizontal stabilizer/vertical stabilizer to handle the load?

(It's funny, now that you folks have educated me somewhat, I keep realizing how smart Juan de la Cierva and the early gyro pioneers were since the tractor gyroplane design solves everything so practically and neatly. Only problem is I sat in a Eich JE-2 once and didn't like the view! I really like the openness/unimpeded view of my flying chair/Bensen! Oh well, pilot preferences which put us right back where we started...perpetuity :) )

The Glasgow study makes reference "the energising effect of the propellor being in close proximity to the fin and rudder." Does this make much of a difference? I noticed in the hydrogen peroxide helicopter video that the inherent steam revealed that the rotor down wash seems to tighten up (ie. get narrower) and not spread out as I assumed it would do. This being the case for propellors also, then placing the full span horizontal stabilizer farther from the center of gravity should still be as effective in reducing torque roll, correct? Sorry about all the questions, always searching for the ideal. Thanks for any answers. darrellwittke

[C. Beaty]

The problem with twin tail booms, Darrell, is the structural offset involved in feeding the loads back into a central mast. Doglegs aren’t structurally efficient.

Another possibility where belt redrive is employed is to use a large diameter tail boom running through the prop center.

Neglecting the panoramic view, a tractor is the most sensible solution; the large diameter tail cone has high structural efficiency and, for a given mast height, permits the largest propeller diameter possible.

But efficiency isn’t everything as many have alluded to previously. The stock Bensen, with the ability to hand start the rotor from a seated position and a mast height that permits it being rolled through standard door openings has a lot of appeal and with a 90 hp Mac, who needs efficiency?

All a Bensen really needs is something a little better than the ox cart suspension and a Ron Heron style “T” tail.

[darrellwittke]

Thanks for the reply Chuck, sums it up quite accurately, at least how I view it. Hope my Bensen turns out well, I am opening up the exhaust as you suggested and putting it on a diet. Can't stand those Mac's unmuffled though....lol.

Oops...forgot to ask. Does the propellor stream lose much of its energising effect with 3-4-5 feet of distance?

[C. Beaty]

Contrary to what most people expect Darrell, propeller slipstreams contract and speed of downstream of the prop. At least for several diameters before spreading out again. You can easily observe this effect by taping streamers to the sides of a box fan.

[Udi]

Another way to counter torque roll is to make the horizontal stabilizer a full span at least propellor length wide, utilizing two smaller elevators at either end. This allows greater rotor clearance from the vertical stabilizer while allowing the whole assembly to be placed farther back from the CG. Examples of this may be Ken Rehlers gyro, Ernie's LFINO I think, and the Cartercopter.
This would be my preferred design, Darrell, short of using counter-rotating props. Add to your list of gyros that do that Ron's LW. Even with the huge distance from his prop to the stabs, minor differential elevators counter the prop torque. I don't think you need super beefy tail boom to move the tail feathers another 2-3 ft back - and an over the porp top support will work out well too.

Contrary to what most people expect Darrell, propeller slipstreams contract and speed of downstream of the prop. At least for several diameters before spreading out again. You can easily observe this effect by taping streamers to the sides of a box fan.
Yep - a direct result of the Bernoulli principle.

Udi

[John Stahl]

I got the answer to all the stability problems.
Let’s use the same balancing system that a Segway uses.
Well we will need two, one for the X axes and one for the Y axes.
The Segway makes corrections every ten milliseconds.
That should keep our gyros stable.

Here is a link that got me thinking.
When I think it is a dangerous thing.

http://www.tlb.org/scooter.html

[darrellwittke]

John, we have to get together! Further back in this thread I told you how I had taken two jugs of my Mac and replaced the base gaskets. What I didn't say is that I spent 6 hours just prior to that welding up a truss frame to use with my gyro loading ramps (Can be seen on old Drive it home! thread in for sale section- along with pics of my gyro) to make a bridge. (that's why my gyro is taking so long, I end up doing other projects cause my girls don't have much interest in the gyro!) The reason I wanted a bridge is because I found a great camping spot exactly 20 miles behind my house. A nice creek, trees, great view of mountain, and railroad tracks. But it is next to the road. But across the stream is a nice little peninsula and, if I had a bridge, I would have my own little camping spot. Well, now I have a bridge.

(Brings up some engineering questions for Chuck B. and Udi though, if I could. I have a span of 22 ft of walkway, (the ramps.) I welded a pyramidal truss (with the top third cut off) that fits underneath the centerpoint of the span. The truss has a 12 inch drop (or length) to it and is splayed out 6 inches from the edges of the walkway, or ramps, above. I strung 1/4 galvanized cable through the bases of the pyramid truss which are in parralell with the walkway. I tightened the whole thing up with galvanized turnbuckles, which are the weakest link in the whole cable system, rated at 1080 pounds safe working limit. So I have two of these underneath, and when tightened up they raise the walkway up about 4 inches. I (245lbs) can walk across with appx. 3 to 3 1/2 inch drop. The question I have is: What is the maximum load this bridge is likely to carry safely? Any answers appreciated, no replies necesary, as always)

So the reason for this campspot is because it is right next to an unused railroad (not yet abandoned, but unusable do to a washout near Great Falls, Mt) with approximately 25-30 miles running through the very beautiful missouri river canyon, most often right next to the river, with 5 tunnels and all kinds of wildlife along it. The Montana legislature had a bill proposed to buy it and turn it into a recreational trail (which has worked wonderfully in Idaho, the hiawatha trail) but the bill failed due to opposition of the landowners next to the railroad right of way. Well, that pissed me off and I said "Well, if we all can't use it at least I'm going to!" So I modified my mountain bike to ride on the railroad tracks. It took 60 dollars and some aluminum tubes I found in the scrapyard, 4 pair of rollerblades from the thrift store, some 18 guage metal and some creative welding. Took me three tries and two months but I finally got it working passably good. So this campsite with my bridge makes a perfect base camp for riding the rail bike. (I've got pictures I could probaly have put on a disk and post if you're interested.) So I have an interest in riding the rails (and it is really worth it, very beautiful!)

I called Kaman corp. at that time and talked to a guy about whether a Segway could be modified to ride the railroad tracks (doesn't make sense to me to tear up those rails to make a trail, why not use them?) He wasn't very encouraging since the Segway couldn't stand up by itself on the 2 1/2 inch wide rail and had to have the drive differential of the two wheels to make turns. (Also said there wasn't enough force to keep it balanced if using rounded profile tires on a 2 1/2 inch railway in a motorcycle or bicycle way of steering/riding. Can't believe that, works on a bicycle, how much force is there anyways? Chuck? Udi?

In looking at the german unicycle, with its wide wheel, my gut feeling is it could be made to ride on a 2 1/2 inch rail. Not for everybody, as I hoped, but in the realm of possibility for an athletic rider, I'll bet. Way over my head to make though, my skills stop at crude metal working. But still, if I'm thinking this then somebody else is thinking it, and in 20 years or less I bet somebody has a working contraption. (May be me, I sent an e-mail inquiring if they sell them, just have to get off this forum and keep studying for my second job RN license to make the money to make these silly dreams happen!)

So it appears you and I have kindred souls, John, or at least similarly warped minds. (Hmmm....dunno if that's a good thing.) I gave montflyer an e-mail and he said he has your number and would contact you. Several gyro's (sportcopters) down by Darby, maybe we could do a field trip in January?

Sorry for the long off-topic post, you hit on one of my eclectic interests. No harm intended.

On the more relevant topic of gyro's, Udi, you state propellor slipstreams contract as "a direct effect of the Bernoulli principle." The Bernoulli principle stating that "as the speed of a moving fluid increases the pressure within the fluid decreases" explains the narrowing of the slipstream. The question is if the friction of the outside layers of the accelerated slipstream against the unaccelerated air causes an appreciable narrowing of the slipstream also. (On my basic level, I am wondering if the propellor slipstream is more like a fire hose and nozzle rather than a sheetrock texture gun, for instance. (or an aerosol paint can?) Thanks for listening to my gibberish, darrellwittke

[Udi]

Lots of questions, Darrell...
...What is the maximum load this bridge is likely to carry safely?...
Sorry, civil engineering is my weakest side. I bet Ken Rehler can pull an answer right off his sleeve.

...He wasn't very encouraging since the Segway couldn't stand up by itself on the 2 1/2 inch wide rail and had to have the drive differential of the two wheels to make turns. (Also said there wasn't enough force to keep it balanced if using rounded profile tires on a 2 1/2 inch railway in a motorcycle or bicycle way of steering/riding. Can't believe that, works on a bicycle, how much force is there anyways?...
Obviously the guy you talked with wasn't thinking outside “his box”. The Segway principle of operation can be applied beautifully to do exactly what you want. Basically, you want to balance a standing person on a "ball". When I say on a ball I mean stabilize the person both on the x and y axes. The Segway is balancing only on the x axis right now. To build your contraption you would need to place the rail wheel over a sideways rail that can move the bottom of your "Darrellway" right and left in order to balance you on the y axis. Then you have two balancing circuits - one is controlling the rail wheel for lateral balancing and the other is controlling motors that move the base left and right to keep the person upright (like a bicycle). Easy! Heck – I think this is a great idea for a “super Segway”, a Segway that rides on one wheel instead of two. It’s a unicycle Segway – good for more sporty riding than the geriatric Segway currently in use.

Who wants to partner with me (i.e. pay the cost) to develop this one?

...On the more relevant topic of gyro's, Udi, you state propellor slipstreams contract as "a direct effect of the Bernoulli principle." The Bernoulli principle stating that "as the speed of a moving fluid increases the pressure within the fluid decreases" explains the narrowing of the slipstream. The question is if the friction of the outside layers of the accelerated slipstream against the unaccelerated air causes an appreciable narrowing of the slipstream also. (On my basic level, I am wondering if the propellor slipstream is more like a fire hose and nozzle rather than a sheetrock texture gun, for instance. (or an aerosol paint can?) Thanks for listening to my gibberish, darrellwittke
I don't think I am following your logic there Darrell. Obviously, the friction is accelerating some air on the outside of the moving column of air, but I don't see how it would help narrowing it down. I think it is pure conservation of energy (i.e. Bernoulli).

Udi

[C. Beaty]

The pressure immediately behind the propeller disc, Darrell, is slightly higher than that of the surrounding air.

As the air expands, its pressure drops and its velocity increases so in keeping with Bernouli, the slipstream must contract.

Another phenomenon, known as vena contracta, may also play a role. Air is drawn into a propeller not just from straight ahead but from the sides as well. This air has momentum and tends to keep going in its original direction.

The nozzle of an aerosol can is pinhole size; most likely, if you could view it under a microscope, you would see contraction until 2 or 3 nozzle diameters had been passed. Also, the spray head on your bug bomb has passages that draw in outside air and direct it crosswise to the liquid jet.

[John Stahl]

udi
you mean something like this?
http://www.tlb.org/eunicycle.html

[Udi]

Another phenomenon, known as vena contracta, may also play a role. Air is drawn into a propeller not just from straight ahead but from the sides as well. This air has momentum and tends to keep going in its original direction.
Good point, Chuck. The vena contracta effect, as you explaind, is causing further narrowing of the air stream, and is making the air accelerate to an even faster speed.

Udi

[Udi]

udi
you mean something like this?
http://www.tlb.org/eunicycle.html
Yes, only this unicycle has only one axis contorl - laterally. I bet it is hard to stand still without moving. Unicycles, or moving on a rail, don't have a good sideways balancing mechanism (you can't change your direction of movement to achieve stability, like when riding a bike).

Udi

[John Stahl]

Chuck
Let me make sure I understand what’s up.
Right after air goes through the prop there is a high pressure bubble then Bernoulli principle causes the air to contract for 2 to 3 prop diameters. Then vena contracta starts to spread the prop wash out.

How far does the high pressure bubble extend beyond the prop?

In my simple experiment the string attached to the fans grating showed defiant incoming air influences. Because the string was in the high pressure bubble right behind the fan
Would I be wrong to hypothesize that the HS in my experiment was in the air affected by the Bernoulli principle. That is why the directional influences of the incoming air were not evident.
Had I moved the HS farther away from the fan into the vena contracta then the incoming air influences would have become evident again?

If I understand this correctly then wouldn’t the HS work best in the high pressure bubble behind the prop and in the vena contracta?

[Mayfield]

Chuck/Udi

I'm having difficulty getting my arms around quantifying the vena contracta effect. In honesty I had not considered it in relation to a propeller before.

I know that a typical propeller, or fan, produces an outflow pattern that look like a lumpy hourglass.

The only time I have considered the effect is when designing air inlets or outlet jets.

Even then I know it exists, but that's about all. I know that often, if I am trying to design an air inlet, I will size for what should get maybe 500 CFM but I'll end up with 300.

R/S

Jim

[C. Beaty]

I’m not sure vena contracta plays a significant role in propeller slipstream contraction; perhaps a good bit when the prop is stationary but not much in forward flight.

Most of the slipstream contraction results from pressure equalization.

The attached drawing from Gessow & Myers shows how the slipstream contracts and speeds up in the process of pressure equalization.

The pressure in front of the propeller disc is slightly negative and the pressure behind is slightly positive with respect to that of the surrounding air. For the pressure to reach equilibrium, it must speed up and in the process contracts in accordance with Bernouli’s Law.

Experiments with box fans tend to be confusing. The grilles and housing produce effects not encountered with an isolated propeller.

I expect to produce accurate modeling, the fan ought to be removed from the housing and clamped to a post or saw horse. The fan hub occupies a significant portion of the total area so a spinner should be attached to the outlet side to eliminate the central core of stagnant air.

[darrellwittke]

Thanks, Chuck. "The nozzle of an aerosol can if viewed microscopically, you would likely see a contraction of the stream until 2 or 3 diameters."

That keeps my practical experience system of beliefs in harmony with the laws of physics (not that the laws of physics cares! :) )

What I hear you saying about the phenomena of vena contracta is that the air drawn into the propellor has velocity. Velocity being defined as speed WITH direction; the air impacts the slipstream at an angle that helps narrow the slipstream. (ie. momentum)

What I don't hear anybody saying, but have drawn a conclusion that it is correct, is that the entrained propellor slipstream air looks more like my fire hose and smooth bore nozzle water stream than I would have intuitively guessed.

John, (thanks for the e-mail, I'll call soon) regarding "then vena contracta causes the prop stream to spread out." The vena contracta evidently causes a very very small narrowing of slipstream. The spreading out of the slipstream is due to the reduced velocity of the airstream as it gets more distant from the propellor. This reduces the Bernoulli principle, thereby causing the spreading out of the propellor stream, if I deduce correctly.

Creative dreaming again, and tying together the off topic subject of Segways and gyro's, I am wondering what possible result would occur if a HyperSonic Speaker was placed in front of a propellor. HyperSonic Sound was chosen by Popular Mechanics as best new invention of 2002, over the much publicized Segway. HyperSonic Sound directs sound much as a laser directs light (up to 200 ft or more.)

In trying to gain efficiency through slights of physics rather than aerodynamics, would there be any increase of propellor efficiency as it chops and pushes energised (ie initial inertia overcome) air molecules downstream?

(My hope is perhaps it would act as soap does in water, cutting cohesiveness and thereby reducing friction loss, allowing farther projection of water stream)

Thanks again, everybody, for the answers, challenging thinking and healthier entertainment than watching TV!

(PS. I understand what you are describing Udi, wish I had the money to invest :) Interestingly, there is a gentleman in Washington state who conducts tours on rail bikes via commercial leases of raillines. He can be found via internet search for railbikes.)

[darrellwittke]

In trying to acheive more efficiency through slights of physics rather than the more often tried improved aerodynamics section, it seems sound waves can play a role.

Al H. sent me this link, www.newscientist.com/channel/mech-tech/aviation/dn7867. I noticed the sound sheets can increase lift by up to 22%! Must act like vortex generators, although I don't think vortex generators can increase lift in an airfoil by 22% typically, can they?

Maybe Ernie B. will try some on the inboard driving region of his rotors, or maybe the folks down under?

Anyhow, I'm glad there was something substantiating my intuition. Best regards. darrellwittke