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Model Forum / Radio Controlled / Air Models / February 2004Repost: Rudder has negative roll coupling on Arresti?
Sorry to those who've already read this on uk.rec.models.radio-control.air but I've drawn a blank there so far. I'm flying a Hangar 9 Arresti at the moment and notice there is quite a lot of negative roll coupling. That is full left (say) rudder causes a right bank, which the slight dihedral translates into a right yaw. This tends to cancel out the intended direction change. On my other planes with positive dihedral, left rudder usually results in left roll as well as the expected left yaw. On this model it results in a roll in the opposite direction. Could it perhaps be due to insufficient dihedral? No actual dihedral angle is given in the assembly instructions, but is based on one wingtip raised by 1.5" with the other wing placed flat. Anybody have any ideas as to the cause or is it just a feature of this model?  SignatureSteve > Sorry to those who've already read this on uk.rec.models.radio-control.air > but I've drawn a blank there so far. [quoted text clipped - 11 lines] > > Anybody have any ideas as to the cause or is it just a feature of this model? Rudder inputs cause yawing, which THEN results in roll. If the airplane yaws the wrong way, I would suspect too much rudder travel which is stalling the rudder and allowing the tail to swing the wrong way. Rudder travel shouldn't be much more than 20 degrees or so. Too much elevator travel will also cause problems. My son can't get his Four-Star 40 to stall/spin with the 50 degrees up-elevator. He has to use about half that to get the nose up enough to stall it, and further input will result in nose-down tendency. Does this airplane have a relatively short wing with a tall rudder/fin? The usual rudder arrangement does have some anti-rolling tendency but is usually negligible. Left rudder will try to push the top of the fin to the right and roll the airplane the wrong way, but the rudder/fin would have to be very tall in relation to wingspan. Dihedral itself is not entirely necessary for yaw/roll coupling. There are FS airplanes with no dihedral at all that will roll properly with rudder input. Dan >> Sorry to those who've already read this on uk.rec.models.radio-control.air >> but I've drawn a blank there so far. [quoted text clipped - 16 lines] >which is stalling the rudder and allowing the tail to swing the wrong >way. Rudder travel shouldn't be much more than 20 degrees or so. Thanks for the suggestions. Well, initial reaction of course is left yaw for left rudder, but then soon begins a right roll followed by what looks like right yaw. The behaviour is symmetrical. The max rudder movement is quite a bit less than the maximum 2.25" from centre recommended in the instructions. Stall turns aren't pretty either, with what seems like insufficient rudder authority, although I'm still experimenting with throttle blip timing to assist the turn. I have just changed to a different rudder servo arm to increase the available movement to the max 2.25", but haven't tried it yet. If your guess of too much movement is valid, I guess I'll soon find out! > Too much elevator travel will also cause problems. My son can't >get his Four-Star 40 to stall/spin with the 50 degrees up-elevator. >He has to use about half that to get the nose up enough to stall it, >and further input will result in nose-down tendency. Certainly not too much elevator travel. Recommended and current value is up 5/8" and performance is quite gentle with that. It is a fairly generous elevator though for the size/type of plane. > Does this airplane have a relatively short wing with a tall >rudder/fin? 56" wing, here is a link to the plane:- http://horizon.hobbyshopnow.com/products/description.asp?prod=HAN2175# [SNIP] Another annoying trait is the tendency to nose over when taxiing on grass, even with full up elevator. The CoG is bang in the middle of the recommended range (3.5"-3.75" from memory). I've moved it as far back as possible without adding weight and it's a minor amount better in this respect. SignatureSteve > Another annoying trait is the tendency to nose over when taxiing on grass, > even with full up elevator. The CoG is bang in the middle of the recommended > range (3.5"-3.75" from memory). I've moved it as far back as possible > without adding weight and it's a minor amount better in this respect. Sounds like the wheels are too small or the gear need to move forward. The website says the airplane has "neutral" handling qualities, which may mean neutral stability and a lack of yaw/roll coupling. Maybe the airplane is flying as designed. Got to use more aileron. Dan |
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