Food for Thought

[Dean_Dolph]

Okay, wife has been in Hong Kong for two weeks visiting her daughter/son-in-law who are there on a two year assignment and I'm setting here bored out of my mind! Sooo......

Take a look at this site and see if there is a gyro application. http://www.fly-seagull.com/ Slick composite pod, retractable landing gear and etc. The spec sheet shows a Cordura seat on an aluminum frame which reminded me of the seats in a new non-flying gyro at Mentone this year. This gyro, by the way, is shown in Tim O'Connor's Mentone '05 write up on page 9 of the September issue of Rotorcraft.

I've attached about half the pictures I took of it. The seats in it are RANS recumbent bicycle seats. What?!! You didn't know that RANS started out as a bike manufacturer?!!:D

There are some other neat/unique features on this machine. If it had been flying it would probably have picked up some awards because of the features and the workmanship.

Okay, guys, see if you can have some fun with what is on the web site and the pictures.

[robertstodaro]

I saw this machine in the mag, and I was disapointed that no comment was made on it's configuration.

Could somebody please fill me in on the advantages of a rear mounted tractor engine.

I been in Aviation too long and I'm stuck in the "thought Box"

I see nothing but dirty air going over the tail and into the aft rotordisc.

It also looks like too much weight on the tail wheel.

A more conventional pusher rans hybred idea is posted at

http://www.rotaryforum.com/forum/showthread.php?t=4013

The new forum would not let me print the pic here

[GyroRon]

Could somebody please fill me in on the advantages of a rear mounted tractor engine.



No advantage, but that engine appears to be aircooled and using propwash over the fins to keep it cool. Hirths aren't like a mac.... they need either belt driven fans to blow air on the fins, or strong amount of propwash, or they overheat. Looks like this guy designed the gyro around this engine.

[robertstodaro]

Is there a support for the engine thru the prop shaft to the frame aft of the rear seat?

[mike marckel]

ok , ill try to answer some of your questions on my michine .tractor pusher? advantages,prop clearance to rotor blades w/short mast. Better visibilty compared to a normal tractor, Prop in very clean air compared to other two place designs (no radiators oil coolers etc.in front of prop), engine noise behind prop, Exhust spring etc.etc. falling off engine wont go thru prop.Free air cooling on hirth f-30 saves horsepower and is much quieter. Yes the engine is supported by a tube frame member under the engine,thru prop,then triangulated to frame. As for tail wheel weight,remember the pilot is in front of the mains. How much is too much????? The overall design was to build a quiet ,smooth ,lightweight ,easy to preflight and transport, C.L.T.gyro with true S.T.O.L performance.

[Olbod]

G,day.
I asked before somewheres but still havent been answered.
With this configuration, having the prop closer to the mast and nearer
to being under the rotorhead, would it make the Gyro, possibly more stable ?
What I am getting at is, theoretically, would PIO - PPL be less of a problem
if the drive (prop) was under the rotorhead and the loaded machine was balanced somewhat by the location of the tail and other bits.
I have read everything written and discussed over the years about stability
and such but have not seen this question raised before ?
Cheers.

[Dean_Dolph]

......With this configuration, having the prop closer to the mast and nearer to being under the rotorhead, would it make the Gyro, possibly more stable ?
What I am getting at is, theoretically, would PIO - PPL be less of a problem
if the drive (prop) was under the rotorhead and the loaded machine was balanced somewhat by the location of the tail and other bits.
I have read everything written and discussed over the years about stability
and such but have not seen this question raised before ?
Cheers.Robert, I will try to answer your question and then let our mentors, C. Beaty et al grade it!

First, any and all balancing on any aircraft in any direction is done around the center of mass (CoM).

PIO and PPO are longitudinal stability issues exhibited by pitch excursions from level flight. One, PIO, is what the term implies, pilot induced oscillation. It is the result of the pilot over controlling the pitch and getting out of phase with the machines natural stability. I say machine because this can happen in fixed wing aircraft also. A stable aircraft will stop the pitch excursions if the pilot will let it. But in an aircraft that is unstable it will only get worse. You will see our mentors use the term divergent when explaining this instability. PPO exacerbates the problem of PIO. Regardless of cause, pitch excursions represent rotation around the center of mass (CoM).

So, I think you can answer your question if you visualize the center of mass as a ball. Pick your size! Let’s use a basketball.

To simulate a pusher gyro lets say that basketball has rods sticking out the top, and the bottom, that represent the mast. In reality, it isn't likely that the CoM is located right on the mast but for our purposes lets say it is. And also that the rods/mast are perfectly vertical. Lets also say the basketball has a nose cone the is perpendicular (90 degrees) to the mast.

Now visualize a prop thrust line that is perpendicular to the rods (mast). Let’s say that you have the capability of moving the prop thrust line up/down while still maintaining it perpendicular to the rods (mast).

With a movable prop thrust line, you can now place it vertically any where you want. So, place it where the force from the prop thrust is dead center on the basketball (CoM).. Do you think that it will cause the nosecone on the basketball to rotate up or down, as it would during a pitch excursion, or would it exhibit no pitch excursion at all?

Next move the prop thrust line (keeping it perpendicular to the rods) up where the prop thrust is putting a force on the rod sticking out of the top of the basketball (the CoM). Now what do you think the nose will do?

What happens if you move the prop thrust line down (keeping it perpendicular to the rods) and put force on the rod sticking out of the bottom of the basketball.

I think you can see that it is the relationship between the prop thrust line and the CoM that affects PIO and PPO. And that the prop distance from the mast and its relationship to the rotor head have no bearing on PIO and PPO.

[PW_Plack]

The question of location of the prop along the longitudinal CoM is a little different matter. If the engine were behind the prop, and the engine weighed enough to move the CoM slightly behind the prop, the machine would become more stable. It would become, technically, a tractor, even though the pilot is in front of the prop.

[chuter]

I think I remember reading something from Chuck B. on the old forum that said it doesn't matter if it's a tractor or pusher; it's thrust with a vector that runs through the airframe at some point. The airframe doesn't know where the engine is.

If a tractor engine was attached with a ujoint to the airframe, then the frame would tend to stay behind the engine.

What say the experts?

[robertstodaro]

removed message

[Dean_Dolph]

The question of location of the prop along the longitudinal CoM is a little different matter. If the engine were behind the prop, and the engine weighed enough to move the CoM slightly behind the prop, the machine would become more stable........Paul, could you explain why a machine would become more stable? And by stable what do you mean?

I know what you mean by longitudinal CoM but as D. Riley pointed out, in another thread, there is only one CoM in an object and it is described in three dimensions so it is misleading to say longitudinal CoM. I would think that saying longitudinal balance point would be more accurate.

All the forces that act on an aircraft act on and around the CoM and not the longitudinal balance point. The balance point just identifies the place (in a postitive gravity environment) that is directly beneath the CoM. In a zero G environment, balance points are not going to discussed.

In my reply to Robert, I was addressing the part in his post that mentioned PIO & PPO.

[Olbod]

Thanks Guys.
Dean I'm sorry that I mislead you, I should not have mentioned PIO-PPO.
I was actually referring to the prop along the longitudinal line but not exactly
a centre of mass location either. Lets assume that the Gyro is balanced
following the normal hang tests and so on but we have the prop in front
of the engine and Directly Below the Rotorhead! Not back some distance
behind that vertical line below the rotorhead.
Will that effect the tendancy to slightly nose up or down with throttle
changes etc. Seems to me that with the prop back some from below that
vertical line, there must be an angle of thrust working against the Rotor ?
In a FW aircraft, it is exactly that, A FW ! but in a Gyro, the rotor is not
fixed to the airframe but swivels about.
Does this angle of thrust, as I call it, have any bearing on the things
stability in any small way or doesnt it matter where the prop is located
along that logitudinal line if the aircraft is balanced ?
Cheers.

[Dean_Dolph]

Robert, I'm not the one to explain the hang test since I've never performed one. But the purpose is to assure that the cyclic has the proper range of motion with the intent, as I understand it, to have the cyclic centered with the gyro in flight configuration. The test is performed minus the blades since they not needed for this test. The gyro is hung from the teeter bolt but it is not a balance test per se. I need to point out that it doesn't matter if an aircraft is fixed wing or a rotorcraft, all the same rules apply when it comes to forces and the CoM.

So, with out intending too, the stability I think you are talking about is pitch stability and pitch instability takes form as PIO/PPO. And a rotating wing that 'swivels' as opposed to a fixed wing is not a factor in pitch stability.

The placement of the engine and prop along the longitudinal line has no affect on gyro's pitch stability or a tendency for the nose to rise or drop with throttle changes. It is the prop thrust relation to the CoM (not the rotor) that causes this, as I explained by using a basketball with rods.

You have to remember that the mass of the gyro includes the rotor blades. Consequently any force that is applied that is not directly on the Center of Mass (CoM) will cause it to rotate the machine, rotor blades included. And yes, of course, the blades are providing some thrust/drag resistance to rotation. But not enough to keep the machine from performing a power push over (PPO) in a zero ‘G’ situation which can happen during PIO or turbulence.

The rotor thrust vector is primarily in an upwards direction but does have a rearward component. The real problem develops when the rotor thrust is lost when a zero ‘G’ situation is encountered. At that point the rotor thrust is gone and any prop thrust that is not applied exactly on the CoM will cause the gyro to rotate. The further the prop thrust line is away from the CoM, the more force is applied via a longer moment (lever) and the larger the problem. And the more difficult it is to compensate with a horizontal stabilizer.

You can simulate this by using a ball to represent the CoM and a pusher stick to represent prop thrust. As long as you push exactly (admittedly very difficult to do!) on the center of the ball it will just slide. But get it even slightly off center and it rotates.

I hope I don't confuse the issue by saying this but rotation around the CoM can be in any direction; pitch, yaw, roll and otherwise. Using the ball and stick will demonstrate this very well.

The prop thrust line most discussed here is the high one as represented in the RAF machines and in the Fetter's era Air Commands. These machine are defined as high thrust line (HTL) gyros. The prop thrust on these machines is always pushing the nose down.

Knowledgeable RAF owners have installed horizontal stabilizers in an attempt to provide enough down force to the imaginary rod sticking out the back of the CoM to counter the prop thrust pushing forward on the imaginary rod sticking out the top of the CoM.

The imaginary rod out the top of a RAF CoM is around 10 -12 inches long. While on a Dominator, the imaginary rod sticks out the bottom of the CoM and is some where around 2 - 3 inches long. If you say that a Dominators prop thrust is always pushing the nose up you are right. The Dominator is considered a low thrust line machine (LTL). It is doubtful if there are any true center line thrust (CLT) machines. Gyros such as the GyroBee have a prop thrust line CoM relationship that is less than 3 inches high and is considered a near center line thrust (NCLT) machine.

Always remember that all forces on an aircraft are applied to the CoM or around it. I probably should have used a golf ball or a marble in my original explanation as it might make it easier to see that forces off dead center of the CoM will cause rotation. Okay, guys don’t jump on me for being redundant in that last sentence! Little prop line offsets apparently are not difficult to counter. But as they get to be more than three inches then it gets tougher.

I believe it was C. Beaty who posted pictures/diagrams of some of the early tractor gyros that have the engine tilted so as to provide a prop thrust line that is obviously not parallel with the longitudinal axis. Apparently the CoM on these machines does not lie on the longitudinal axis. The original Bensen plans, as I understand it, had a installation spec for engine to angle the prop thrust thru the CoM.. So, the prop thrust can either pull/push on/or around the CoM. Those that think tractor gyro's can't have a prop thrust line/CoM issues, need to think this over again. If people say that it is easier to get a tractor's prop thust line to pass thru the CoM and any offsets are easier compensate for, then yes, for the most part they are correct.

There is a place or two that I’m not convinced I’ve been clear and may have made it seem more complex that it is. If that is the case then hopefully one of our experts will provide another slant for you.

Hang in there and sooner or later one of us will offer an explanation that makes sense. In the mean time I get to see if I know what I’m talking about. If I don’t then our mentors are not bashful about letting me know! In the process we both learn.

[barnstorm2]

ok , ill try to answer some of your questions on my michine .tractor pusher? advantages,prop clearance to rotor blades w/short mast. Better visibilty compared to a normal tractor, Prop in very clean air compared to other two place designs (no radiators oil coolers etc.in front of prop), engine noise behind prop, Exhust spring etc.etc. falling off engine wont go thru prop.Free air cooling on hirth f-30 saves horsepower and is much quieter. Yes the engine is supported by a tube frame member under the engine,thru prop,then triangulated to frame. As for tail wheel weight,remember the pilot is in front of the mains. How much is too much????? The overall design was to build a quiet ,smooth ,lightweight ,easy to preflight and transport, C.L.T.gyro with true S.T.O.L performance.


Thank you Mike. Your machine is a work of art. Can you tell us about your pre-rotator?

[Doug Riley]

Many people guess that a tractor prop arrangement with the prop in the nose is more stable because (they think) the machine will "trail" nicely behind the "pull" of the prop. This notion probably arises from experiences trying to back up trailers, or perhaps even trying to back up your little red wagon when you were a lad. We vaguely surmise that a pusher aircraft is always trying to jackknife in the air.

Fortunately, these experiences don't translate to aircraft. The problem of jackknifing when the thrust is at the rear of the vehicle only occurs when there's a joint in the vehicle ahead of the thrust source. A vehicle without a trailer doesn't jackknife. A tandem semi-truck, with TWO swivel joints, is unbackuppable, as far as I know. Railroad trains, with many swivel joints, can only back up under control because they're restrained from jackknifing by their tracks.

A pusher aircraft doesn't jackknife. The engine is mounted rigidly to the frame. There's no swivel. If the frame yaws or pitches, the engine and prop move with it and continue pushing in the same direction (relative to the frame). If the engine were mounted on a swivel, there WOULD be trouble with a pusher setup.

There's no stability advantage to putting the prop at any particular place along the fore-aft axis, at least from the point of view of simple push-pull physics.

There may be other advantages, as the designer points out. OTOH, a frame that has the big masses close together will generally be lighter and simpler for a given strength than one that has the masses spread out. That's one reason why Bensen-style gyros are just about the simplest aircraft around. It's why side-by-side aircraft have simpler frames than tandems.

Trying to build a structurally sound bridge around the prop to connect the passenger area's mass with the engine's mass is a challenge. We have enough trouble trying to build such a bridge to connect just the tail surfaces and tailwheel to the rest of the gyro.

[Cobra Doc]

A double trailer can be backed, carefully, but it really messes with your head.
Go around a corner in rear-wheel drive and punch it hard, then do the same in a fron-wheel drive. The rear-wheel is lot more fun! Any time the center of thrust/pressure is behind the center of mass the rear end will try to come around if able. If you can put the prop at the center of mass, that leads to some interesting dynamic questions. I can visualize some wild precession behavior.

[mike marckel]

[QUOTE=Doug Riley]

There's no stability advantage to putting the prop at any particular place along the fore-aft axis, at least from the point of view of simple push-pull physics.

Hi Doug, would a gyro with a prop mounted say 10 feet behind com act the same in a yawed or pitched state (including air drag frame etc)as one mounted in a normal manner?? I dont know but would like your imput.

[Olbod]

Thanks for your imput Guy's.
Its a very interesting discussion.
I believe Cody hit the nail on the head tho when he said " if you could put the prop at the centre of mass" !
That to me is the interesting question.
All facets of the Gyro are facinating be it theory or practice and certainly
easier and cheaper than trying to build a Shuttle, eh.
Cheers.

[mike marckel]

Quote=Doug Riley.... Trying to build a structurally sound bridge around the prop to connect the passenger area's mass with the engine's mass is a challenge. We have enough trouble trying to build such a bridge to connect just the tail surfaces and tailwheel to the rest of the gyro.[/QUOTE] Doug,
Just incase you were refering to my tandem,the structural bridge goes thru the prop, Makes the bridge easy and strong. As for weight,dry weight 415lbs w/o blades

[REDHORSE556CES]

In looking at the pictures, surely the pilot would want to warn the backseat passenger NOT TO PUT THEIR ARMS OUT AND LET THEM DRIFT ANYWHERE BEHIND THE SEAT!

That prop seems to be verrrrrry close to the G.I.B.

[mike marckel]

Hi Charlie , check out the photos in the raf 1000thread of my tandem,have added a cage around prop should make the guy in back a bit safer??

[chuter]

Regarding Cody's example of the rear drive car vs. the front drive car; I think there are other forces in play besides just the thrust being behind or in front of the COM.

In the case of rear wheel drive, the front wheels act like a pivot due to drag on the pavement, sort of like having the tail feathers on the front of a gyro, it wants to swap ends. Plus while in a corner you have centrifugal force acting on the COM. Punching it spins the rear wheels causing them to loose traction. Less drag on the rear than the front, so the rear swings out.

Front wheel drive, the drag is at the rear, helping to keep it straight.

I think if you had a front drive car and a rear drive car, both with all 4 wheels locked in the straight ahead position, and no other forces acting on the car, they would both go straight ahead.

Just a shade tree engineers thoughts...........

[Cobra Doc]

Mike, that would would be true if a steady-state situation really existed. There will always be outside forces. I really want to know what would happen with the groscopic forces at center of mass under perpendicular gyroscopic forces above center of mass. I supose I can go down to the toy store and buy a couple of gyroscopes. Just a simple rudder turn sounds like it would be an interesting maneuver. ( I always score high in "spatial orientation". Probably explains why I really enjoyed ATC.)

[chuter]

Ahh, I see what you're getting at now.

I have no clue about that; interesting question.

[Doug Riley]

Gyroscopic reactions resulting from tilting the gyroscope's axis are a function of the angular velocity of the axis-- jargon that means that the reaction is strong or weak according to how fast you try to tilt the gyroscope. These reactions are pure couples -- that is, pure torques with no net straight-line force component.

Whether the 'scope is at the center of mass or way out on a boom fore/aft, the angular velocity during a given rotation of the aircraft body is the same. IOW, if your gyroplane is yawing at 90 degrees per second, the gyroscopic reaction at the center of the prop hub will be the same no matter where on the machine the prop is located. This reaction will be felt at the aircraft's C.M. as the same torque, no matter where the prop happens to be.

This is weird and not intuitive, but true. Check any engineering textbook.

What may throw our thinking off is that, if the prop is way out on a boom, there's some mass out there, too. This affects the aircraft's moment of inertia (flywheel effect) and may slow down its reactions to torques.

[Cobra Doc]

When the gryscope (prop) has no arm moment in relation to the COM, what happens when there is no dampening effect? I've seen one other airplane that may have the prop almost on the COM. Without looking it up I think it was called the Optica. Fuselage looked like a Bell 47, ringed prop, twin tail boom. I may check with Budd Davisson and see if he ever flew one. Just one more neat little concept to play with. I don't see any fault with it, just curious about the effects. I do see a need for a triple boom to bridge the gap effectively. Probably two lower rails and one upper.

[mike marckel]

This is great, I didnt consider gyroscopic effects .Iwould think with the longer boom,the gyroscopic effect would be less but would have a longer arm ????(with the same airframe movement rate and degrees) ...I was thinking if the com and the cg were not in the same exact place in all flight configurations.The angle this creates carried out over a longer distance would cause a greater thrust offset compared to a short coupled machine??Sorry Iam thinking out loud...

[Cobra Doc]

Mike, we are all just kind of thinking out loud. You will have to let us know how it flies. Maybe I should have been a mechanical engineer. I have this odd way of seeing forces at work. I'm not always right, but I'm generally pretty close. I doubt that there is anything unsafe about it, but there could be just a little "weirdness" hiding somewhere, probably in abrupt yaw or pitch movements. Then again every contraption ever invented has a weird spot somwhere in it's performance envelope. Don't ask me to explain angular mototion 'cause I can't. I do know the best way to turn a motorcycle to the left is to apply pressure (I didn't say turn) to the right. If you apply just the right amount of left pressure comming out of the left turn, the bike will wheelie out of the turn quite nicely. That involves an abrupt change in angular momentum. If we were talking about a unicycle instead of a motorcyle, would the unicyle pop up off the ground? OK, I have a weird spot, also. That's a given.

[robertstodaro]

I didn't want to mention it before, hoping somone else would.

The tail feathers look too small to be effective at low or no power settings.

[Ron Marlett]

I didn't want to mention it before, hoping somone else would.

The tail feathers look too small to be effective at low or no power settings.


I was thinking the same thing. Mike do you have any pictures that show the whole airframe from the side(3 Oclock or 9 Oclock position)?

I hope you have good hearing protection for the GIB... their head will be awfully close to the prop tip:)

Have you thought about seperated/turbulent airflow into the prop with that large squarebacked fueltank butted right up against the prop? I recall a very nice looking little composite bi-winged pusher seaplane that had problems with this. I can't recall the name but I think it had the word Hawk in its name (seahawk?). The fuselage was pretty wide and they did a very rapid fuselage taper in the back over the engine. This caused a real nasty air seperation several inches before the prop. This turbulent air caused some pretty nasty prop vibrations. If I recall correctly they even had props structurally fail because of it but I think those were wood props. Their fix for it was to install vortex generators up and down the left and right side of the fuselage some distance in front of the seperation. Some of these were clear plexiglass and glued on the rear portion of the rear windows. I think they also switched to a composite prop.

Your workmanship looks first rate.

Ron

[mike marckel]

Re.tailfeathers,etc. ...have you guys ever seen a parsons 2 place fly with a little benson style tail??If the rudders prove to be to small,Ill change them.As for air flow into prop,look at almost any water cooled gyro,they seem to always mount the radiator directly in front of the prop??If you look carefully you can see the tanks are tappered..in on the sides.If compared to (normal)gyro air flow into the prop is cleaner,(no radiator ,oil cooler,exhaust ,or engine )ahead of it ... Remember this is a prototype....If the gyro was designed to fly at high speeds more consideration would be given to seperated or turbulent airflow in this and other areas.. .Prop noise???time will tell,..this is 72" and turns slower than most..The biggest noise issue might be the prop tips passing the support tubes of the cage????Again ,time will tell,...so far seems very quiet!....thanks for your comments,

[robertstodaro]

Thanks Mike.

It's a neat machine, I've always liked the Rans nose and windshield.

Hope you forgive our armchair quaterback comments, we're still little gun shy right now.

[robertstodaro]

It's been very quite, You got that bad boy in the air yet?

[mike marckel]

Not yet Robert,still too cold outside,today the high will be about 30.I dont have heat in the airport hanger,Have most of my homework done!!!so when its time to fly or test,no distractions! hope it warms up soon...

Mike Marckel

[Dean_Dolph]

Mike, the answer to the following question may be buried in this thread somewhere but I'm too lazy to look! Do you have any idea what the seat from RANS weighs?

[mike marckel]

Dean, I dont know,but if I had to say,maybe 3 lbs.Its alum.apx.3/4"tube with nylon webbing type material streatched on it.When you pick one up for the first time its hard to believe it so lite !!!!Next time iam at the hanger Ill try to weigh one. PS ( very comfortable)
Mike