Shuttle pitch up in fast forward fight - RC Groups
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Jul 18, 2004, 10:03 PM
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Shuttle pitch up in fast forward fight

I have a question about trim or heli aerodynamics. Iíve now started flying in fast forward flight and notice that as I go faster I have to add more and more forward stick to keep the heli from going into a climb. This happens in the speed range of 50 to 90 kph. Iíve also noticed that the effects depends on the type of blade I have on the ship. The blades I have are marketed by Heli world in San Jose Ca. The first set of blades are heavy wood blades straight cord with rounded tips I think a have 130 grams of lead in then. ( I can hover for what seams an hour after stopping an auto fall.) I think they were called Samurai. They behave well but are not as efficient as the Ninja set. The Ninja blades are about 2/3 the mass of the wood blades, are made of glass and have a thinner airfoil. The shape is a straight cord with a 45į raked tip that goes behind the trailing edge by about 50% of the cord.. This is a much better performing blade other than the strong pitch up tendency. (I had to lower the hole pitch curve about 15% as shown on a Futaba Super Seven) Both blades look symmetrical in cross section

Now donít laugh too hard, the ship is an old blue head Shuttle with a single solid axle head. The fly bar is above the blades and the blades and tail boom and main shaft are ďlongĒ.
So the question is how do I keep the ship from climbing like a home sick angel in fast forward flight ?
Thanks ,
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Jul 18, 2004, 11:53 PM
It's a really big number.
Ok, what you are experiencing is blade precession (I hope I spelt that right). The incomming blade (swinging into the wind) produces more lift than the one retreating towards the tail. The flybar / teeter assembly allows the incoming blade to feather and at the same time to increase the retreating blades pitch, balancing the lift produced, or should I say mostly balancing it. Any imbalance tries to roll the rotor disk to the right, and the 90 deg transform of force that you get from the bicycle wheel effect makes the disk bend back, and hence you need compensate with forward stick.

1) Friction in the teeter, 2) fore / aft blade chord area and 3) flybar : blade disk mass ratio affect the peformance of the pitch correction as the disk rotates.

as for 1), friction causes the teeter not to beable to repitch the blades fast enough. The faster you go, the worse the problem becomes due to the amount of repitching required. Flybar less helis (like most of the real thing), require more forward stick to be held the faster you go because they do not have a teeter. Since a model's rotor disk spins faster than a real heli, the precession is more noticeable.

2) The less area behind the blade's pivot (bolt hole) vs the front reduces the amount of 'feathering force' applied to the teeter, again slowing the speed of refeathing and causing pitching.

3) heavier blades vs flybar get less pitching. F = MA, so the force applied by the refeathing is reduced if the counter force applied by the flybar has more mass behind it.

Make sense?

Generally, lighter blades require a lighter flybar to get same flight characteristics as a heavy set, which is why the FG blades pitch up more than the woodies.

Jul 19, 2004, 12:50 AM
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It looks like my glass blades have a double hit against them in fast forward flight. One; less mass in the blades, same mass in the flybar
Two; the tip is swept aft giving more area behind the pivot.

Remove mass from fly bar
move pivot aft

Thank you very much.

PS. My ship is dead in roll. Am I correct that less mass in the flybar will also speed up the roll responce?
Jul 19, 2004, 11:25 PM
It's a really big number.
Yup, pulling mass out of the flybar really boosts performance, ditto shortening it. My Ergo is a dead as a dodo on cyclic, but you replace the heavy steel flybar and paddles with a light alloy one (Sceadu) and WAHOOO! roll city.

More area behind the pivot is good on a flybar heli - it increases amount of air pressure pushing the approaching blade into feather. Usually light blades are shaped as you described to get more feathering to compensate for the reduced mass. Take a look as dragonfly blades - they are 90% area behind pivot to compensate for them having very little mass relative to flybar. I'd say it is just the fact they are lighter is your biggest issue, and you need to adjust your flybar to compensate.

Get some Sceadu 50 'saw toothed' paddles (I have no idea what part #, but you'll see what I mean if you get some), they have removable weights to allow you to adjust your cyclic stability / response level to your liking. I'm planning on sticking some on my Ergo when I get comfortable on it (only just got it set up after buying it from a buddy, so it's still not 100%).

Jul 20, 2004, 11:17 PM
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Thanks again. I've got to stop thinking in such a very narrow box. What you are talking about is force balance. What I'm thinking of is mass. The way I'm thinking limits me in seeing that I can adjust the Heli with rate (speed,rpm) and or moment arm (length). Thanks for hitting me over the head with the idea of shorting the flybar! I have my old paddles on the XY table removing some mass with an end mill. One snip of the fly bar would have been a lot easier! I'm a mechanical engineer and I'm some times amazed at how often I can't see the forest for the trees. Thanks again for showing me there is more than one way to skin a cat.