Gyrocopter Aerodynamics - Page 9 - RC Groups
 This thread is privately moderated by mnowell129, who may elect to delete unwanted replies.
Jan 23, 2007, 10:03 AM
I'm not as bad as they say.

# Twist and shout

A quick note on blade twist.
Assume a model with the rotor turning 1200 rpm, this is 125 radians/second (Ω). The blade speed at any radius, R, is = Ω*R. So at .25 feet the speed is 31, at 1.5 feet it's 187.
Assume the model is going 20 mph, or 30 feet per second
and the rotor is tilted 15°.
We now look at the local airflow on the blade at the root and tip. A convenient trick is that we can draw the airflow as vectors (arrows) and show what's going on. Those of you who are trig minded can derive the formulas but I didn't show them to clutter up the diagrams.
The result is that angle A is the real airfoil angle of attack at that particular blade radius on the advancing side. For the tip, the tip speed is high compared to the model speed and angle A is 1.5°.
At the root however the model speed is large compared to the rotor speed and the angle A is 7.5°. So the tip airfoil "sees" 1.5° and the root airfoil "sees" 7.5°.
To help this situation you twist the blade. Assuming the tip is quite happy at 1.5° (which is really agressive considering that the clark Y when set at geometric 0 is actually set at +3° making the real tip angle of attack about 4.5°. This is why model gyrocopter blades are set at minus angles, even at -1° with respect to the mast, this airfoil will be running at +3.5° real angle of attack), you set the root at the same angle relative to the local airflow, or -6°. Now the root and tip are running at the same angle of attack and the root is actually doing something useful.
Note that this is washin in the typical airplane sense, the tip is set to 6° higher angle than the root, or more appropriately the root is set 6° lower than the tip.
Note also that this is completely the opposite of propeller or fixed pitch helicopter blades and why fixed pitch heli blades with twist are absolutely no good for gyrocopter blades.
One final note the 6° twist figure here is based on a model speed of 20 mph and a tiltback angle of 15°. The twist will only be correct at one model speed. At higher speeds you need more twist, at lower speeds you need less (obviously with no forward velocity the twist need would be zero). So there is no perfect twist. I've used 8°-10° with good results, i.e. very low airspeeds and easier spinups. The net result is that a little twist can make a big improvement. How much? You'll just have to experiment.
regards to all

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Last edited by mnowell129; Jan 23, 2007 at 10:08 AM.
Jan 23, 2007, 10:35 AM
Registered User
Mickey,

tractor vs. pusher: of course I heartily disagree with you, at least when we're talking about dc head gyros.

You start off by saying that a pusher will always have to have the motor/prop relatively lower in the airframe. On a dc head pusher the prop is practically under the rotor hub and needs little more than 1/2" of clearing. On a tractor the distance from a prop placed at the same height to the rotor is obviously bigger, but you need this distance, because when your landings aren't real smooth, the blades will flap down by a considerable amount and may hit your prop. Don't ask me how I know. That means, that the vertical position of the motor relative to the rotor is about the same for a tractor or a pusher, which also means that the centerline of thrust has about the same distance to the rotor in both cases.

Then you say that the motor/prop dictates ground clearance. This is only true for tractors, the ground clearance for pushers is dictated by the length of the grass you want to take off from. Let's say that the distance from the tailboom of a pusher to ground is about the same as the distance of the tractor's prop from ground. You're right, the pusher will be taller than the tractor, but only by the thickness of its tail boom plus a bit.

Now let's consider the cg situation. The cg of a convetional tractor with the motor in the fuse is practically always above the thrustline of the motor and you need down thrust to overcome the drag of the rotor. When you're building a pusher, you have the whole distance from the tail boom to the motor to place your rc equipment and batteries and you can easily adjust the position of the cg relative to the thrustline of the motor without any downthrust. Using upthrust or downthrust with pushers isn't very effective anyway, the motor is too near the cg to create the necessary torque. It's far better to shift the cg or the motor position up or down.

Your explanations of the static and dynamic effects of thrust changes are of course right, as always, but I think you overestimate their importance - especially if your gyro has a little horizontal stab at the rear end. All my pushers have the thrustline near but above the cg to counter the rotor drag. But all of them will still climb when I apply power. It's - like the tractors downthrust - just a matter of adjustment.

Another advantage of the pusher besides the flexibility of the adjustment is that your prop and motor are in a safe position when something goes wrong again. It's only real disadvantage, as far as I can see, is the fact that the rudder is very near the prop. The wash of the prop will swing the gyro around if you don't take any measures to counteract this. Right now I'm using about 10° of sidethrust to get my pusher to fly straight, which is quite a lot. A better solution may be a lateral offset of the motor, but that's still on my 'to test' list for next spring.

My first gyro that really flew - after half a year of trying and about 60 chrashes - was a tractor design by Karel Pustka (first picture, see here for my version: https://www.rcgroups.com/forums/atta...mentid=541582). For an inexperienced gyro pilot like me, this gyro had several drawbacks: the bottom plate got ripped off on most of my landings and I had constant tail boom strikes, which in the end disintegrated the 6 mm cf tube (I'm still using the Aerobalsa blades, though). That's why I redesigned it, which, with my limited experience, meant that I took down the tail boom and strengthened the construction, but kept the cg and all the angles. The result was a Minimum puller (picture 2). Now, if you print out the puller and the Minimum pusher (picture 3) and hold the printouts above each other, you'll see that there's really no change, apart from the motor position. If I'd continued with the development of the puller, the puller motor would most likely have ended up there where the puller has it now. So what's the real difference bewteen a tractor and a pushher? Nothing much.

Jochen

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Jan 23, 2007, 11:58 AM
I'm not as bad as they say.
Quote:
 Originally Posted by JochenK You start off by saying that a pusher will always have to have the motor/prop relatively lower in the airframe. On a dc head pusher the prop is practically under the rotor hub and needs little more than 1/2" of clearing. On a tractor the distance from a prop placed at the same height to the rotor is obviously bigger, but you need this distance, because when your landings aren't real smooth, the blades will flap down by a considerable amount and may hit your prop.
At last some discussion....

My only point was that all things being equal, if the motor was the same distance ahead of the rotor vs behind it the natural 15 tilt of the rotor will give more prop clearance in front.
Having to have the tractor prop far away from the mast is only if you have the rudder outside the rotor diameter and need to balance it. If you put the tail under the rotor just like the pusher the prop will be the same distance from the mast as the pusher. That being the situation the tractor will have more prop to rotor clearance.
Your point about flapping the rotor into the prop is well taken but if you land tail first the flap can just as easily occur to the rear. I've had many more tail strikes than prop strikes...

Quote:
 Originally Posted by JochenK You're right, the pusher will be taller than the tractor, but only by the thickness of its tail boom plus a bit.
This was my point. But essence of the discussion is that the pusher model ends up taller than the tractor.
Your diagrams illustrate that. A secondary point is that because the rudder is attached to the structure under the prop the rudder tends to end up lower in the airframe without additional structure. This creates the need for a longer main gear to attain the proper takeoff stance. This adds to the model height. This is illustrated by your two models side by side.
Quote:
 Originally Posted by JochenK When you're building a pusher, you have the whole distance from the tail boom to the motor to place your rc equipment and batteries and you can easily adjust the position of the cg relative to the thrustline of the motor without any downthrust. Using upthrust or downthrust with pushers isn't very effective anyway, the motor is too near the cg to create the necessary torque. It's far better to shift the cg or the motor position up or down.
I don't disagree, but this argument applies to the tractor. There's nothing to stop you from raising the gear and batteries up the mast there either.

Quote:
 Originally Posted by JochenK Your explanations of the static and dynamic effects of thrust changes are of course right, as always, but I think you overestimate their importance - especially if your gyro has a little horizontal stab at the rear end. All my pushers have the thrustline near but above the cg to counter the rotor drag. But all of them will still climb when I apply power. It's - like the tractors downthrust - just a matter of adjustment.
I don't think I made any claims as the relative importance of this factor. It's more a risk with full sized aircraft. I agree adjustment is important. The only effect that the horizontal stab can provide to the dynamic case is damping due to it being "waved" by the fuselage. The initial resistance to motion is all based on the additional inertia of the stab, not it's aerodynamic properties. At the instantaneous onset of the torque a weight would have the same effect.
Quote:
 Originally Posted by JochenK Another advantage of the pusher besides the flexibility of the adjustment is that your prop and motor are in a safe position when something goes wrong again. It's only real disadvantage, as far as I can see, is the fact that the rudder is very near the prop.
Prop security is an advantage. The close coupling of the rudder in pushers does tend to make them less stable in yaw, especially with the power off.

Quote:
 Originally Posted by JochenK So what's the real difference bewteen a tractor and a pushher? Nothing much. Jochen
I don't have any motivation to favor one over the other. I've got both with identical rotors, weights and power.
I'm not asserting that the differences are huge, just that there are some differences. I'm just trying to point out what the differences might be and why. I think we differ mostly in the perceived magnitude of the effects. You clearly worked to avoid the typical situations that occur with pusher designs and the result is a good design. But you had to be aware that the problems were worth considering to have done so in the first place.
regards
mick
Latest blog entry: AIrcraft I've built.
Jan 23, 2007, 05:28 PM
Registered User
Mickey,

I'm sorry, but I don't seem to be able to handle this 'quote' system as well as you do. All the qoutes are from your post # 123 and I'll do my best.

Quote:
 If you put the tail under the rotor just like the pusher the prop will be the same distance from the mast as the pusher. That being the situation the tractor will have more prop to rotor clearance.
If you tuck a tractor's rudder under the rotor, it will certainly have more prop to rotor clearing than the tractor. But then you have the fuse in the position of the pushers tail boom because you need the same rotor - rudder clearence for both types of gyros. If you add the landing gear, the tractor is suddenly much taller than the pusher, with its cg much further down than the pusher's. Not a good solution.

Quote:
 Your diagrams illustrate that. A secondary point is that because the rudder is attached to the structure under the prop the rudder tends to end up lower in the airframe without additional structure. This creates the need for a longer main gear to attain the proper takeoff stance. This adds to the model height. This is illustrated by your two models side by side.
I really didn't want to fool you, but I just found that my tractor is 5 mm taller than my pusher.

Quote:
 There's nothing to stop you from raising the gear and batteries up the mast there either.
That's right. But it will not give you more flexibility because that will only raise the cg even more above the thrustline of the motor. With a pusher you can easily have the cg above, below or on the thrustline.

Quote:
 The initial resistance to motion is all based on the additional inertia of the stab, not it's aerodynamic properties. At the instantaneous onset of the torque a weight would have the same effect.
...on both pusher and tractors.

Quote:
 The close coupling of the rudder in pushers does tend to make them less stable in yaw, especially with the power off.
I think that's right. That's why I use a hang angle of something like 10°. This is the way I set my gyros up: I try to get them flying and when I'm comfortable with the situation, I turn the power off and watch the autorotational flight. If the gyro hangs there like a parachute, I'll increase the hang angle until I get a proper descend. If it comes down to steep, I'll decrease the hang angle. After that I readjust the height of the motor position to obtain level flight under power. This process is repeated several times until I'm satisfied with the overall performance.

Quote:
 I don't have any motivation to favor one over the other.
As you may have guessed, I'm certainly favouring pushers, but that's no dogma. Personally I think they're easier to build because you've got nearly all the stuff that deals with forces and power fixed to the mast. The rest - with the exception of the landing gear - is just ballast.

But don't let me interrupt the tale of gyrocopter aerodynamics for too long. I favour reading much more than writing.

Jochen
Jan 23, 2007, 06:02 PM
I'm not as bad as they say.
Quote:
 Originally Posted by JochenK If you tuck a tractor's rudder under the rotor, it will certainly have more prop to rotor clearing than the tractor (mickey: pusher?). But then you have the fuse in the position of the pushers tail boom because you need the same rotor - rudder clearence for both types of gyros. If you add the landing gear, the tractor is suddenly much taller than the pusher, with its cg much further down than the pusher's.
Your first statement is exactly my point.
2nd point, not sure about the reasoning here.
I've drawn your pusher with motor in tractor position
and moved battery to balance. My only point here is that the model now has more prop to rotor clearance. If you reduce the mast height to get the same prop to rotor clearance as the pusher had the model would be shorter.
I think you are reading too much into a rather simple minded observation, that being that if the rotor is tilted back its closer to the ground/prop, etc. in the back than the front. Given that, the model can be shorter with the prop in front as it is the limiting factor on clearance.

Quote:
 Originally Posted by JochenK That's right. But it will not give you more flexibility because that will only raise the cg even more above the thrustline of the motor. With a pusher you can easily have the cg above, below or on the thrustline.
You seem to have constrained yourself to a single tractor layout. I'm speaking in general terms. Again, as per my diagram your statement applies equally well to a tractor or pusher.

Quote:
 Originally Posted by JochenK ...on both pusher and tractors.
No argument, but I wasn't discussing the merits of stab/no stab, I was discussing the differences in pusher vs tractor all other things being equal, not the difference between Minimum and G3PO as an example. But since you went there, a "traditional" tractor with the rudder out from under the rotor that also has a stab will have more pitch stiffness and therefore resistance to thrust/pitch coupling than the "traditional" pusher.

Quote:
 Originally Posted by JochenK I think that's right. That's why I use a hang angle of something like 10°. This is the way I set my gyros up: I try to get them flying and when I'm comfortable with the situation, I turn the power off and watch the autorotational flight. If the gyro hangs there like a parachute, I'll increase the hang angle until I get a proper descend. If it comes down to steep, I'll decrease the hang angle. After that I readjust the height of the motor position to obtain level flight under power. This process is repeated several times until I'm satisfied with the overall performance.
Not sure I understand what this has to do with yaw stability, sounds more like pitch.....
Quote:
 Originally Posted by JochenK As you may have guessed, I'm certainly favouring pushers, but that's no dogma.
Funny, I didn't pick up on that....
Quote:
 Originally Posted by JochenK But don't let me interrupt the tale of gyrocopter aerodynamics for too long. I favour reading much more than writing. Jochen
It doesn't feel like an interruption. Writing in this manner is quite strange, because there is very little feedback.
A little discussion, or request for clarification is really welcome, otherwise it seems like I'm talking in a darkened room not knowing if there's an audience.

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Latest blog entry: AIrcraft I've built.
Jan 23, 2007, 06:41 PM
Registered User
Mickey,

Quote:
 Not sure I understand what this has to do with yaw stability, sounds more like pitch.....
If there's no air flowing by your rudder, you don't have any yaw stability at all. I put the cg forward to get a minmum of forward movement in autorotation without having to work my sticks too much.

Jochen.
Jan 23, 2007, 07:08 PM
I'm not as bad as they say.
Quote:
 Originally Posted by JochenK Mickey, If there's no air flowing by your rudder, you don't have any yaw stability at all. I put the cg forward to get a minmum of forward movement in autorotation without having to work my sticks too much. Jochen.
Aha.
I have to put the nose down with BEGi and G3PO, since they have no stab. They will descend vertically. You can land that way, a vertical power off descent and a blast of power right at the ground to a touchdown. Kind of a figure L landing.
GT17 has a stab and takes a nice nose down glide angle with power off.
I guess I'm kinda spoiled when it comes to different gyro models, cause I have a really short height, long tailed tractor with tons of rudder authority, a pusher and a full fuselage tractor with a stab. Gives you a real feel for the differences to fly them one after the other.
mickey
Latest blog entry: AIrcraft I've built.
 Jan 23, 2007, 07:09 PM I'm not as bad as they say. Hmm, forgot what to talk about next. Flybars anyone? Or does someone else have a burning topic? Latest blog entry: AIrcraft I've built.
 Jan 23, 2007, 08:51 PM High-power Rocket Gliders I have a question about twisted blades. In practical terms, with blades made of solid balsa, how do you maintain rigidity in twist if you give them washin? Ari.
Jan 23, 2007, 09:44 PM
Winging it Ò>
Quote:
 Originally Posted by mnowell129 Hmm, forgot what to talk about next. Flybars anyone? Or does someone else have a burning topic?
One topic you had mentioned some time ago had to do with stability gained by having the rotor blades "droop" at the tips like the PiccoZ. I know it isn't practical for larger rotors, but why does it work?

Dan
 Jan 24, 2007, 12:24 AM High-power Rocket Gliders "Drooping" creates the equivalent of negative coning. Ari.
Jan 25, 2007, 01:53 PM
I'm not as bad as they say.
Quote:
 Originally Posted by leadfeather One topic you had mentioned some time ago had to do with stability gained by having the rotor blades "droop" at the tips like the PiccoZ. I know it isn't practical for larger rotors, but why does it work? Dan
Looks like the PiccoZ has twist, not droop. This is just normal for a heli rotor.
Droop would be negative coning, would probably do the same thing as positive coning just the cross coupling would be negative rather than positive (pitch down with slip to the advancing side rather than pitch up).
Latest blog entry: AIrcraft I've built.
Jan 25, 2007, 02:00 PM
I'm not as bad as they say.
Quote:
 Originally Posted by iter I have a question about twisted blades. In practical terms, with blades made of solid balsa, how do you maintain rigidity in twist if you give them washin? Ari.
You don't get any more torsional ridigity that the material has to start with. So if you take a balsa blade and try to twist it, it will resist the twisting. If you put some permanent twist in and the attempt to twist it, it will resist with essentially the same amount of resistance, just starting from the permanent twist point as the starting place.
I've made twisted blades by steaming/soaking and by laminating. Soaking wood in ammonia makes the cellular material relax until the ammonia evaporates, then it re-hardens in the new shape. So you can soak in ammonia then strap to a twisted board and when dry they will hold the twist.
Latest blog entry: AIrcraft I've built.
Jan 25, 2007, 04:07 PM
Registered User
Mickey,

Quote:
 I've made twisted blades by steaming/soaking and by laminating. Soaking wood in ammonia makes the cellular material relax until the ammonia evaporates, then it re-hardens in the new shape. So you can soak in ammonia then strap to a twisted board and when dry they will hold the twist.
Something learned again.

Jochen
Jan 25, 2007, 04:51 PM
High-power Rocket Gliders
Quote:
 Originally Posted by mnowell129 I've made twisted blades by steaming/soaking and by laminating. Soaking wood in ammonia makes the cellular material relax until the ammonia evaporates, then it re-hardens in the new shape. So you can soak in ammonia then strap to a twisted board and when dry they will hold the twist.
I'll have to try that with Ackus blades. They are 20mm wide and 11% at the thickest point. Do you think they are thin enough to twist with ammonia?

Ari.