Jeremy Sebens
Posts: 91
Joined: 4/22/2002
From: Champaign,
IL, USA
Status: offline
|
Pilot here... Paul, what you say about monowheels is true - luckily, this plane seems to have enough tail to hold her straight, even before flying speed. While taxi-testing, I gave it a few hard bursts - enough to lift the tail slightly, and at that point, the plane was light enough on its feet that I was afraid of an inadvertent takeoff from the parking lot! We had very straight, stable tracking throughout. Certainly, flight testing will be the final authority, but I'm pretty confident that it'll do OK. Actually, the tipskids act as a stabilizing influence as well. I was going over the testing in my head last night, and in fast straight runs, the plane tended to rock back and forth gently between the two skids. What seems to be happening is that when the plane starts to veer in one direction, the wing to the outside of the incipient turn dips. The skid drags, which produces a resoring moment out of the turn. Angular momentum takes over and we veer slightly to the other side. The process repeates in reverse - lather, rinse, repeat. What this boils down to is what looks like a stable LCO (limit-cycle oscillation). The really neat thing is that I think it should be speed independent. With increasing speed (until the tail takes over completely) the instability of CG-behind-wheel will become stronger. However, so will the centripetal acceleration as a result of the veer, resulting in a harder push-down on the outside skid, and a correspondingly larger restoring moment. I think all these dependencies are linear, resulting in an LCO that doesn't change noticeably in amplitude with speed, but rather increases its frequency. So long as we don't end up with some sort of resonance between the LCO and the structure, this should not cause any real problem. In fact, it might explain why Europa owners think that their monowheel airplanes taxi better than any other airplane they've ever flown...
As to the excessive tail area you mentioned earlier, there's no such thing as too much tail! At least, not within reason. With heavily loaded airplanes like DBF'ers a lot of tail is needed to keep the dynamic modes in check - the single worst flight characteristic I see at DBF competitions is lightly damped modes from too-small tails, particularly a bad dutch roll (or "Bonanza boogie"  from too little vertical area. I looked over the shoulders of the guys doing S&C calcs on this plane and while they were conservative, the extra tail area only adds an ounce or two, and the modes look really good. Well-damped dutch roll, critically damped short period (I think they got zeta to something like .715 - .707 is ideal), lightly damped phugoid , and slowly diverging spiral mode. Hehe - I think I might have said this in this thread last year... I'll go back and look later.
And boy, is this thing light. Remember, this baby is going to lift 8.8 lbf of water - the operational empty weight of 9.5 lbf is amazing! Good work, guys! Too bad you didn't quite make that 50% payload fraction you dreamed of, but 60% is still stellar!
Sadly, I'm no longer a member of the team, just an advisor and pilot, but it's still a lot of fun to be associated with these guys!
_____________________________
Landing: Contest of strength between planet and aircraft.
NOTE: To date, planet remains undefeated.
|
Printable Version
All Forums >> RC Airplanes >> Aerodynamics >> RE: UIUC AIAA Design/Build/Fly Project 
RE: UIUC AIAA Design/Build/Fly Project - 
