Some good reading

[mnowell129]

Found the Burkham paper online. Of special note is this quote:

"either the teetering motion (blade tips going up and down alternately) or the feathering motion (stabilizer bar
weights going up and down alternately) or both could be locked out. The model could be flown as I) a rigid rotor (unstable), 2) a
teetering-only rotor (unstable), 3) a feathering-only rotor with feathering controlled by the gyro bar similar to the Lockheed rotor
(stable), or 4) a Bell rotor (stable)."

From
http://members.cox.net/arrio2/historic/HistModHeli.pdf

[iter]

I've read some of John's other articles, they are both very informative and entertaining; this one is no exception. Thank you for posing this. I've also built his Penni from an Ackus kit, and it flew very well. I tried some of the experiments, including replacing Hiller paddles with weights, and it was interesting to see the differences in behavior (Actually, I didn't see much difference between Hiller paddles and weights, except for sightly reduced lift, which I attributed to loss of paddles' lifting area - on the Penni, paddles are set up at 9 deg positive).

If my understanding is correct, on helicopters, paddles are set up at zero-lift AoA when collective is centered and do not contribute to lift, only to control. Could one build a version of G3PO with weights instead of paddles? (you'd have to change the control linkages, of course, to move weights up and down instead of rotating them)

I have another question about the Penni that you might know the answer for. I know this isn't quite the right forum, but since you brought John up :=) The teetering hinge is at 85 degrees to the feathering hinge, not 90. What effect does this have? is it similar to delta head designs, where flapping translates to changes in blades' AoA, or is it something different?

Ari.

[mnowell129]

I've read some of John's other articles, they are both very informative and entertaining; this one is no exception. Thank you for posing this. I've also built his Penni from an Ackus kit, and it flew very well. I tried some of the experiments, including replacing Hiller paddles with weights, and it was interesting to see the differences in behavior (Actually, I didn't see much difference between Hiller paddles and weights, except for sightly reduced lift, which I attributed to loss of paddles' lifting area - on the Penni, paddles are set up at 9 deg positive).

If my understanding is correct, on helicopters, paddles are set up at zero-lift AoA when collective is centered and do not contribute to lift, only to control. Could one build a version of G3PO with weights instead of paddles? (you'd have to change the control linkages, of course, to move weights up and down instead of rotating them)

I have another question about the Penni that you might know the answer for. I know this isn't quite the right forum, but since you brought John up :=) The teetering hinge is at 85 degrees to the feathering hinge, not 90. What effect does this have? is it similar to delta head designs, where flapping translates to changes in blades' AoA, or is it something different?

Ari.
Helicopter paddles are set to 0 all the time and don't move with collective. And yes, you are correct, they contribute to control (and stability).
I have considered the weights instead of paddles idea. It is difficult to do and the reason model heli's don't do it. The problem is that you can't just push the weights up and down. You have to make them precess up and down. For example for right roll (CW rotation) you'd have to push on the weight in the back, it would precess up on the left, etc. The difficulty is that when you push on the weight in the back with the servo it doesn't move and when it precesses 90 degrees later and does move but the servo in that position hasn't moved so it just destroys the servos. You have to push on the weights with springs to make this work and then you get a whole host of other problems like resonance, etc. The bell helicopter solution was a little lever with a shock absorber in it (called a viscous coupling) to transfer control to the weights, this also has its problems in models. The reason the flybar is so prevalent is that the cyclic pitch of the flybar (contolled by the swashplate) is independant of the tilt of the swashplate (which controls the cyclic pitch of the main blades). So the servo only sees the load of tilting the flybar paddles. And they are symmetrical and completely balanced. So the servo sees almost no load whatsoever (unless you do bell hiller mixing, another story).
I think this topic is completely appropriate. Gyrocopters and helicopters have exactly the same control problems. There's no reason gyroguiders shouldn't have knowledge of the problems and solutions just because the rotor is not being turned by a motor.
You could do a weighted bar head version just like the penni on a model, you'd just need to steer with rudder and elevator. As an alternative I'm fairly convinced you could do a penni style head and then tilt the shaft. The servo loads would be low if you let the rotor teeter freely and it would be very stable. This just might be the best indoor gyro solution.
I think you could make a very nice rudder elevator controlled gyro by fitting bigger blades to the head off this toy:
http://www.tycorc.com/us/product.asp?category_type_id=19&id=11793&category_id=7836&nostore=0
I think you'd have to put droop stops in but otherwise the thing would be very stable. If you were real ambitious you could put a one way bearing in the last gear and use the whole mechanism for a prespin motor! I bet David Ramsey is capable of doing this (yes it is a challenge).

As to the 85 degrees. It's something else. I looked at that a hundred times and tried to figure the theory behind it and finally had it dawn on me. The 90 degree precession of a gyroscope is only when there is no resistance to precessing i.e. a free moving gimbal. If you restrain the gimbal the precession angle gets less and less. I read about a rigid rotor lockheed cheyenne model that had a precession angle of only 68 degrees. The swashplate had to be rotated to compensate. So the 90 degrees is a perfect world kinda thing. I think most models have something less than 90 but the pilot just subconciously compensates. John wanted to fly the penni with a rigid rotor but controlled by the bar (the lockheed rotor) as one of the demonstrations. When he locked the rotor down from teetering (case 3 in the article) the precession angle became < 90. What would happen if you left the bar 90 degrees from the blade but the precession angle of the blades was 85 is that the bar would apply a stabilizing control in, say nose up. The blade would apply the correction but be 5 degrees early, so the correction would be nose up and a little roll. The stabilizer bar would apply the correction for the roll and there would be a little error nose down. Etc. etc. The net result is that when set for lockheed rotor motor the thing would just wobble itself out of control at a very high rate. So the 85 degrees setting is to make the phase angle between the stab bar and the blades the same as the precession angle of the main blades when locked into the rigid (lockheed) rotor position. When unlocked (the bell rotor position) the angle went back to 90 degrees but the feedback from the bar to the rotor tended to damp the oscillations rather than amplify them. I think if you built it as a teetering only rotor you could just do the 90 degrees. Remember that penni was built to show people at a helicopter aerodynamics conference how different rotors behaved. If you look at Johns other models he didn't use this angle.
Another real interesting note if you look at the plans (http://members.cox.net/arrio2/historic/penni.pdf) is that it has CG correcting weights in the blades!. And this is a stab bar controlled rubber powered model!. John was a good rotor aerodynamicist from all the evidence he left behind and I trust his judgement. I've been an advocate of CG correcting rotor blades on gyros from the beginning due in large part to studying John Burkham's work. I know for my models the use of CG correct blades is very important.
By the way I actually have the AIAA Student journal with the model helicopter article written by John in it. I was a aerospace engineering student at the time and received those journals.
Sorry for the huge post. This is a very interesting topic to me.
Thanks for the questions.
mickey

[David A Ramsey]

this toy:
http://www.tycorc.com/us/product.asp?category_type_id=19&id=11793&category_id=7836&nostore=0
I think you'd have to put droop stops in but otherwise the thing would be very stable. If you were real ambitious you could put a one way bearing in the last gear and use the whole mechanism for a prespin motor! I bet David Ramsey is capable of doing this (yes it is a challenge).




You're a bloody instigator! The prespin mechanics are intriguing.

The input to this thread was enjoyable reading this morning. It's curious to me, why the Robbe/Schluter Whopper never gained more popularity. The only trick to a successful Whopper flight was recognizing headspeed and the use of rudder. After reading so much of your input, I find I'll look at the Whopper mechanics to help understand your theory explinations.

David

[mnowell129]

You're a bloody instigator! The prespin mechanics are intriguing.

The input to this thread was enjoyable reading this morning. It's curious to me, why the Robbe/Schluter Whopper never gained more popularity. The only trick to a successful Whopper flight was recognizing headspeed and the use of rudder. After reading so much of your input, I find I'll look at the Whopper mechanics to help understand your theory explinations.

David
I feel like the whopper came along when helicopters were still difficult enough that if you were going to invest the time building something that complicated you might as well have it hover. My recent experience says indicates that the average helicopter flier finds the gyrocopter amusing but not worth investing time in. The average airplane guy finds the gyrocopter cool but much more complicated than what they are doing. The whole pre-spin business adds as much complexity as a helicopter. And after the fact with experience with my little gyro, the pre-spin is totally un-needed for sport flying. The reality is that if you've flown helicopters, a little cyclic controlled gyrocopter like mine is pretty easy to fly. If you've done only airplanes most single rotor gyrocopters are a real handful. The whopper appealed to neither camp (except for you and me and few others. I never owned one.).
My little G3PO is about as simple as it gets and seems to have lots of appeal, but even the existing gyrocopter community is REAL iffy about those "heli parts".
I think this is the typical gyrocopter person. They don't feel like they have the energy/patience/desire or don't think they have the expert help available to start helicopters. A gyrocopter looks cool and simple, so heck try one of those. Then the holy grail to get the bloody simple looking thing to fly takes over and the quest begins. It is really a dedicated group of folk who will spend years trying to get a model to fly. How many fixed wing guys would spend a year trying to get the same model to just takeoff and make one lap around the field?
The problem as I see it is that gyrocopters, while appearing very simple, are very complicated to get right. In a heli if you want to go up you just push the throttle, to turn you just whip it around. Even with my G3PO you have to stay on the ball about how much power, how much rudder, needed to keep the turn going and the rotor speed up, and because the rotor speed is controlled by the speed its even slower spooling up and down than a fixed pitch heli. You have to think way ahead of it. No wonder they are not more popular. What's needed is a gyro that an experienced fixed wing pilot can take out of the box sit on the runway, take off and fly around. I think the whopper was just way over the complexity limit for a guy used to looking into a kit box and seeing a few sticks and some ribs.

Regarding my last post, I have been corrected. The 85 degree thing may have helped the rigid rotor, maybe one day I'll find out but the correction came by the man himself. The 85 degree thing had a more subtle useful purpose. From hightime.pdf from the website is this little useful quote:

Sweep the main rotor
blades forward about five degrees
ahead of the pitch axis.

This is done so that an upward (or
downward) force on the blade will exert a
tetering moment on the stabilizer bar. The
bar will precess or tilt 90 degrees later in a
revolution, giving the blades just enough
cyclic pitch change to cancel or
withstand the original up (or down) force.


So it turns out the 85 degree thing was a stability augmentation device. Smart guy that Burkham.
Does anybody know if he passed away or is he still alive somewhere reading our stuff and shaking his head?
mickey

[iter]

You could do a weighted bar head version just like the penni on a model, you'd just need to steer with rudder and elevator. As an alternative I'm fairly convinced you could do a penni style head and then tilt the shaft. The servo loads would be low if you let the rotor teeter freely and it would be very stable. This just might be the best indoor gyro solution.You didn't think I was bringing this up from idle interest did you? :=)
I recently came into some Ackus Penni blades and was thinking precisely that. Watch this space for my attempts. Ackus blades, BTW, are different from what is shown on the plans. They have an 11% Clark-Y-type, flat-bottom, cambered airfoil machined from balsa blanks.
Another real interesting note if you look at the plans (http://members.cox.net/arrio2/historic/penni.pdf) is that it has CG correcting weights in the blades!. And this is a stab bar controlled rubber powered model!.Do you think it might have to do with the fact that John's original blades were rolled form sheet and might not have been rigid enough, and the weights added stiffness through centrifugal tension? From the article:
"My own blades, made from 1/32 balsa, were very flexible, would not track properly, and had to have the weights."
Thanks for the questions.I should be thanking you for your answers! With any luck, and this forum's help, I might have a flying autogyro one day.

[mnowell129]

[QUOTE=iter]
Do you think it might have to do with the fact that John's original blades were rolled form sheet and might not have been rigid enough, and the weights added stiffness through centrifugal tension? From the article:
"My own blades, made from 1/32 balsa, were very flexible, would not track properly, and had to have the weights."
QUOTE]
I'm sure the blade type contributed to needing the weights. But even stiffer blades need CG correcting. It's not just the blade rigidity that is the issue. When the blade is tail heavy it "leads", that is the centerline of the blade angles slightly forward of the feathering axis. This is the exact same situation as an airplane with swept forward wings. Any little disturbance tends to make the blade increase its pitch and go further out of track. If you go look at this post http://www.rcgroups.com/forums/showpost.php?p=3221268&postcount=18
there is a diagram showing a blade leading.
The blade rigidity plays into the going out of track but so does all the mechanical slop in the head, linkages, etc. Even if the blade were perfectly rigid the other sloppy parts would come into play. The CG correcting of blades is not to make the blade stable all by itself like it were a flying wing, it is to make the blade run straight out the feathering axis with no lead or lag.
This is a common misunderstanding, you are not CG correcting the blade for the blade's sake, but for the orientation of the blade with the head due to centrifigal force.
mick