# Ground Effects on the rotor

#### kolibri282

##### Active Member
These numbers are a bit odd. Disk loadings are within 1% of each other but rrpm differs by about 27%? Currently I have no idea how to explain this.

#### JAL

##### Member
These numbers are a bit odd. Disk loadings are within 1% of each other but rrpm differs by about 27%? Currently I have no idea how to explain this.
Maybe its the bearings, one has more resistance than the other??

#### Jazzenjohn

##### Gold Supporter
Offhand, I can think of 2 ways to explain it Kolibri, the first is more likely. Rotor blade pitch has a great deal to do with RRPM. lower the pitch 1 degree and the RRPM matches up fairly well, and second, published rotor diameters aren't often very exact. It would take a different way of measurement (such as measuring blades with hubbar vs blades alone without hubbar for instance) to make the required difference.

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#### All_In

##### Gold Supporter
These numbers are a bit odd. Disk loadings are within 1% of each other but rrpm differs by about 27%? Currently I have no idea how to explain this.
I wish we could swap blades to test a speculation of mine.
When I fly an old Cherokee 140 with the big fat Hersey Bar wings they have so much more ground effect that we had to buy a Cessna 150 to train students how to land because it's more like a sheet of plywood and anyone can land it with a swish and not a bag. It' floats so much more than a modern Warrior's Wings yet they are the same height above the ground.
I've always assumed that it was because the fat less-aerodynamic wings have more vortisices to be straightened out with so much more of it's wing now able to provide lift but only in ground effect.

I would suspect that the difference is this case is one set of blades are just more aerodynamic???? = One set as more vortices to straighten out than the other.

One thing is true for sure the more ground effect you have the less aerodynamic your wings are! At least that is my observations and understanding of ground effect. But they all have vortisices and some cannot be straighten out into lamer flow without almost hitting the ground I suspect.

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#### All_In

##### Gold Supporter
BTW:
Have any of you noticed you float more when your blades are dirty?

That would be an observation worth knowing.

#### All_In

##### Gold Supporter
PS:
Based on Vance's observations I would use/buy the blades that have no or little float on any ride and not the ones that float... as the rest of the spec's are about the same.

But we would need to swap them to really know it's not the ground effect of a flying body as it's much closer to the ground... or the body at all.

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#### All_In

##### Gold Supporter
PS:
If our only disbeliever had actually experienced, as I have, ground effect in about every make and model of high and low FW aircraft and examined the wings of old and new at the same height above the ground.. he would understand that all wings have ground effect! Even spinning ones.

The less aerodynamic they are the more ground effect they have!

PS:
I'm so glad we had two threads that were not gyroplane specific. I do miss this site!

#### Vance

##### Gyroplane CFI
I feel like my point may have been lost.

I feel like my point may have been lost.

In my opinion both the gyroplanes I fly have measurable rotor ground effect.

I don’t think it is important and I will not try to explain why.

#### All_In

##### Gold Supporter
In my opinion both the gyroplanes I fly have measurable rotor ground effect.

I don’t think it is important and I will not try to explain why.
Oops...In that case I miss read it and retract what I wrote about select one set of blades over the other!!!

I assume the others must have miss read it too as we were looking for a difference as to why.

I'm really confused at a higher level!!

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#### NoWingsAttached

##### Unobtainium Member
These numbers are a bit odd. Disk loadings are within 1% of each other but rrpm differs by about 27%? Currently I have no idea how to explain this.
This is getting off topic, but...

Disk loading is just one variable. There are many other variables to take into account, this is totally apples to oranges in Vance's case.

Here's a mind-bender example, my personal experience: Two 28' RFD rotors, same tip weights, profile, chord, length/diameter, disk loading, and installed on the SAME gyro - at different times, obviously. Guess what? One spins and flies at 280 RRPM, the other at 320.

Why? Angle of attack, it is the only variable not accounted for, above. One had more twist at the tips than the other.

In Vance's case, you are talking different EVERYTHING, and trying to account for such different RRPMs is chasing your shadow until you have all the variables accounted for: blade pitch + disk pitch (together = AOA), weights and disk loading, blade mass and tip weights, chord and lifting surface area, profile (lift and drag coefficients), RRPM x diameter = tip speed, etc.

And don't forget drag/lift are a function of speed SQUARED, so differences in tip speed drastically affect lift.

Look at how RRPM factors in to produce similar tip velocity in three common size rotors:

RRPM ______ 414 ____ 340____ 317.5
Diameter ___ 23 _____ 28 ____ 30
Tip, MPH ___ 340 ____ 340 ____ 340

I've always been fond of the way the equation for tip speed works out for 28" rotors. RRPM = tip speed MPH.

+++++++++++++++++++++++++++++++++++++++++++

I admit to having read just a few posts on this thread...it is about ground effects, the old discussion of whether or not gyroplanes are affected by it, right?

It's useful to think in terms of Frisbee aerodynamics, not propeller dynamics. There can be no doubt as you experiment with tossing a frisbee close to the pavement that ground effects do affect the vehicle.

Why get all caught up with helicopter downwash vs. autogiros? Does a FW produce helicopter-type downwash? No, yet it is accepted by all that ground effects do come into play when landing a FW, right? Why should it be any different for a rotating wing?

Further, in the end, both helos and gyros have powered rotor systems - believe it or nuts. Not in terms of a direct-drive link to a motor, but in terms of the fact that it takes power input to get a lift component output which flies the vehicle. What are you doing when you are standing on the ground, hand spinning up your Bensen, other than powering the blades? Are you going to tell me that as those blades are turning, even at 5 RRPMs, that they produce no downwash, no vertical lift, and no forward lift? Of course not.

In the case of a helo, it is obvious where the power is coming from. In the case of a gyro, it is more subtle, less so, but is there, nonetheless. The vehicle is powered forward by either gravity in a glide, or a prop when the throttle is up, and instead of a drive shaft coupling power to the rotor the apparent wind is the coupling between gravity/engine power and rotor.

Either way, an autogiro rotor is powered in the end.

Lift is perpendicular to its surface of origin. EVERY wing has a certain component of forward lift. This is very easy to demonstrate using a foam wing. Drop a section of foam wing, perfectly parallel to earth, and it will move FORWARD some good distance before it drops its leading edge and gathers speed as it continues its descent to the ground due to gravity, and perhaps noses back up, stalls, etc - depending on its shape and balance.

Always.

It will ALWAYS move forward, flat and level, before it nose-dives at all.

That demonstrates how an autogiro rotor works, very well.

It is thought that the outer 1/3 dia. of the rotor "powers" the lifting area of the rotor - the center 1/3 dia. of the disk. I wonder if this is true. I heard of a story of a famous gyro builder/designer/pilot/test pilot that goes like this (This is only what I heard, so take it for what it's worth and dont' shoot the messenger).

An untested, never-before-tried set of rotor blades was being flown for the first time. Wide-cord, rather flexible spar and chord-wise twisting rotors were installed on a gyrocopter. Pilot flew the thing, and at some point the blades twisted chord-wise to the point that the tips no longer had the 3-degrees pitch they needed to continue to create vertical lift. With the rear-ward component of drag reduced, the forward component of lift was no longer in balance with that drag, and the rotors sped up...which caused more untwisting, which created more forward speed, and the closed-loop system caused the gyro to produce too much forward lift and not enough vertical lift until the vehicle plummeted to the ground, killing the test pilot.

If this account of events is in fact true, then we need to rethink our notion of how the autogiro rotor "engine" actually works. For if, as the tips UNWIND, they then create LOTS of forward lift, and lots of forward blade speed, while the middle section of the rotors remains somewhat close to the same AOA, then that gyro should have experienced INCREASED vertical lift, not loss of vertical lift. The blades were a progressive spring of which the OUTER portion untwisted a LOT more than the area closer to the root, after all.

It is therefore a mistake to think in terms of the typical FAA Rotorcraft Handbook notion of the outer 1/3 areas of a disk powering a gyrocopter's inner second third of the disk's so-called "lifting area".

The outer third of the rotor is just as critical for vertical lift as it is for forward lift - the engine's power connection to the autogiro's rotor blades. And it is far more critical for vertical lift than the second third of the disk diameter. The accident described above proves it beyond a doubt.

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#### kolibri282

##### Active Member
We are talking steady state values here and the program I am using predicts rotor speed to within an error of less than 5% for the fairly large number of test cases for which I have measured values. I have calculated rrpm for an MTO type gyro 740 lb 27' rotor at about 100 mph.
For pitch angles of 2° and 4° and mean drag coefficients of 0.0085 and 0.0145 the results are:

thaN.......dN...........rrpm
4°.......0.0145.........298
2°.......0.0145.........338
2°.......0.0085.........371

The 25% difference for the extreme cases is indeed close to Vance's numbers, but for two fairly modern gyros with - I assume - stock blades by fairly experienced manufacturers I can hardly believe that the difference in drag coefficient would be anywhere near the 70% needed in my example to get the rrpm difference we are looking for. Could you please give us the pitch settings for the two aircraft, Vance?

PS: Chuck posted the quote below to another thread, really love it:

“I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”

Lord Kelvin

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#### Jean Claude

##### Junior Member
The 25% difference for the extreme cases is indeed close to Vance's numbers, but for two fairly modern gyros with - I assume - stock blades by fairly experienced manufacturers I can hardly believe that the difference in drag coefficient would be anywhere near the 70% needed in my example to get the rrpm difference we are looking for.?
After check, I am surprised as you, Juergen.
For rotor dimensions and loads gived by Vance my simulations gives, in flight:
Blade pitch = 4.4° (rotor 30 'x 8.5" for 1100 lbs and 315 rpm at see level )
Blade pitch = 2° (rotor 28 'x 8" for 950 lbs and 400 rpm at see level )

Not forget that blade pitch setting at rest is different to flight pitch because the dynamic twisting, despite Cm=0

In my opinion, on this bases, one degree higher for Cavalon, will gives better efficiency. one degree lower for Predator, will gives better safety.

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#### kolibri282

##### Active Member
It seemed such a good idea to prescribe rrpm and backwards calculate pitch, Jean-Caude, that I couldn't resist and ran such a calculation...;-) The mean rotor drag coefficient I used was 0.01 for both cases. The results are:
Blade pitch = 4.63° (rotor 30 'x 8.5" for 1100 lbs and 315 rpm at see level )
Blade pitch = 1.93° (rotor 28 'x 8" for 950 lbs and 400 rpm at see level )
These results are so close that I think we've got it right to within the limitations of the theories we are using. I also thinkg that our approaches are so different that we can exclude the possibility that we both make the same mistakes.
I do hope that Vance will provide values for the pitch settings, that'll be very interesting indeed!

##### Newbie
I haven't read all 21 pages and my apologies in advance if someone has said this already, but as we all know, air is fluid and behaves a little like other fluids, such as water. If anyone drives a boat, especially a jet boat or air boat, that can run in deep or shallow water.... or even water skiing/wake boarding where you carve in over shallow water, once the water becomes quite shallow, the boat/ski/board rides higher in the water and goes faster much easier.... Evidently, it is because the pressure that is normally absorbed and dissipated in deeper water, hits and bounces off the bottom of the river bottom and as a result reflects back and produces more pressure under the hull... not unlike aquaplaning car tires, or even the guy surfing on a motorbike just recently....

So, air, thinking of it as a fluid type thing, any time a wing of any description is producing lift in close proximity to the ground, whether it is a helicopter rotor or a tilt wing, the driven props/rotors will force the air down when in ground effect, the air has nowhere to go, so it will create a cushion.... fixed wings also create lower pressure above the wing, so that equates to more pressure under the wing.... when that pressureised air' under the wing cant be absorbed because of the ground, it will also do a similar thing to an aquaplaning tire.... the air, like the water under a tire will carry extra load.... I cant help thinking also of skimming stones on water, the stone will skim easier, further and more 'bounces' when skimmed on shallow water than deep water... and we have all seen the 'Bonniville fails' when cars at high speed get too much air under them and it has nowhere to go either, it lifts vehicles off the ground.... usually with predictable and disasterous results.

Like someone said, a heli gets its lift thru powered blades (wings) being driven, but a FW or even a car at Bonniville get their lift energy from their speed..... but lift is lift, however it is achieved..... it is my opinion that just as a FW gets noticeable ground effect, a heli gets noticeable ground effect, so does a gyro.... I would say, so does a kite on a string...

In my head, anything aerodynamically 'flying' which is somehow creating lift thru wings, rotors or even flat surfaces like the bottom of a car at speed, will experience some form of ground effect....

I'm not saying I am right, my head is more hollow than Birdys :lol: this is just simply how it appears to me :yo:

#### C. Beaty

##### Gold Supporter
Strange numbers indeed for Cavalon Vs. Predator. Both use essentially the same airfoil, the NACA 8H12 but the Predator’s Sport Copter fabricated blades no doubt have better surface quality than MTO’s extruded blades..

A 28% increase of tip speed doubles profile drag power.

Perhaps MTO’s fixation on training pilots to do things by the numbers led to the lower incidence settings with a reduced chance of blade flap during rotor start.

Another possibility is earlier retreating blade stall of the extruded blades required higher tip speed in order to get acceptable top speed.

Now, if we could get Vance, in the interest of science, to fly his SC blades on an MTO…..

##### Newbie
Does any one on here happen to have an MTO with SC rotors? Or, has anyone ever tried it? :noidea:

#### eddie

##### RAF, turbo subaru 230hp
GREG; the FAA handbook has the outer 1/3 section generating the lift and the inner

section as the driver section.

Regards,

#### Vance

##### Gyroplane CFI
A clarification.

A clarification.

I do hope that Vance will provide values for the pitch settings, that'll be very interesting indeed!
I am in Buckeye, Arizona now and it will be several weeks before I can measure the pitch on The Predator.

Please remind me if I forget. I have a lot of things going on and a poor memory.

I will start a new thread as this one is about rotor ground effect.

The Cavalon has the standard pitch settings and I don't expect to have the opportunity to measure the pitch so someone else will need to provide that information.

I was writing about ground effect so I was less precise than I should have been.

The White Cavalon I am flying (509PH) at 6,500 feet is showing between 400 and 410 rotor rpm. Her take off weight was close to 950 pounds out of Provo, Utah.

I have not flown 509PH below a density altitude of 4,000 feet and I have not noticed the rotor tachometer below 400 rotor rpm.

The Red Cavalon I had for a year (509QB) was always above 380 RPM at sea level.

The tachometer is small and hard to read so even those numbers are not precise.

The Predator at sea level typically flies level at 315 rotor rpm but I have seen close to 400 rpm at 12,500 feet. At a density altitude of 6,500 feet she flies around 360 rotor rpm.

#### Vance

##### Gyroplane CFI
No opportunity.

No opportunity.

Now, if we could get Vance, in the interest of science, to fly his SC blades on an MTO…..
I will not have an opportunity to do that Chuck.

I don't know if Sport Copter has done that yet.

#### LGoodhind

##### Member
>Does any one on here happen to have an MTO with SC rotors? Or, has anyone ever tried it?

No amount of empirical evidence would influence the handful of people who refuse to acknowledge that for all practical purposes the term CG and CM are the same.

Doug gathered similar empirical evidence about the effectiveness of a horizontal stabilizer when I had literally no gray hair. None. His method had the advantage of not requiring an aircraft and you could run dozens of trials to take care of outlier data.

Now I do have a gray hair or two. I am not aware of anybody who thought that a gyroplane didn't need a horizontal who was persuaded by science and empirical experiments to recognize their previous position as flawed. Obviously I could be wrong- feel free to share your "I saw the light" story but there are apparently still CFIs who tell their students its bunk.

The people I saw who really changed their mind did it in conversations like this one by staying engaged. The rest just recognized that it wasn't PC to argue the point, didn't want to deal with blowback, and walked away head held high leaving everyone else to stumble around without their enlightened input. The experiments are important to determine the constants and dial in the factors that matter but they won't stop flat earthers from arguing with you.

Experiments confirm what you believe but people have the capacity to completely immunize themselves from factual information that causes a paradox for them- it is a self-preservation mechanism since we define ourselves by what we believe and when that changes you are changed.

I have a perfectly serviceable RFD rotor head- if you want to mount it on a boom that you can raise and lower and run down the road a couple dozen times you're welcome to borrow it. You can also drop lead and wooden balls from the airport tower and watch them hit the ground at *pretty much* the same time. Physics is obliging when it comes to predictable deterministic fun.

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