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Model Forum / Radio Controlled / Air Models / July 2004Symptoms of CG being too far back.
Hello all, I have a plane that is hopefully going to take its maiden flight in the next couple of days. The CG is set according to the plans, however, because of the design of the plane I expect the CG to be very attitude sensitive. (The engine is on a pod above the plane.) What kind of behavior should I be watching out for? Will it tend to be very pitchy? Thanks, Douglas Kaip An aft CG causes pitch sensitivity... touch of up... Woops!, down.. Woops! quite unpleasant, actually. I had a brain fade today with newly renovated Midwest Electric Hots and the CG was about 1" too fa back (new brushless setup was much lighter than the old can motor an car NiCd's which caused the nose to be MUCH lighter)... anyhow, it i very fresh in my memory how hard it is to hang on and land -- Lomceva On 7/26/2004 4:02 AM Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge: > Hello all, > [quoted text clipped - 7 lines] > > Douglas Kaip Do to the sensitivity of the CG I would suggest, for the maiden flight anyway, move the CG forward about 1/4". Yes, this will make the plane nose heavy and sluggish. For your maiden flight though, you will be expecting the sluggishness, however, after trimming it it out and insuring that the engine pod thrust lines are OK, you can move the CG back. I would suggest moving the CG back in 1/8" increments. A nose heavy plane flies sluggishly, a tail heavy plane flies once. If you find it is tail heavy in the air, just keep the speed up and try and land it hot. The speed helps out in a tail heavy situation. > Hello all, > [quoted text clipped - 7 lines] > > Douglas Kaip Hmmm, I've found the opposite to be true. Overcontrolling on a tail heavy plane is more likely with more airflow over the elevator. Or I misunderstood what you are saying. If it zooms up, drop the nose to keep it above stall speed, cut the throttle and go easy on the elevator movement while you get it on the ground. Having the engine above the wing will have a pitch down tendncy with throttle and a pitch up as you remove power. Before the maiden flight, if you're concerned about it, add more weight to the nose for the maiden but make it removeable so you can move the CG back to the plans location as you test fly the plane. I mean, why risk the plane. If YOU think the CG is too far back, move it forward for the maiden flight and then move it back as you fly it. A nose heavy plane flies poorly, a tail heavy plane flies once. Your mileage may vary. Don > If you find it is tail heavy in the air, just keep the speed up and try and > land it hot. The speed helps out in a tail heavy situation. [quoted text clipped - 13 lines] > > > > Douglas Kaip you are both right, but you are talking about differeent things. yes, a rearward CG makes the airplane more pitch sensitive. but airspeed is the only thing that might keep the tail from tucking, so flying fast, especially on landing, may be the only prayer. for example: a few years ago a guy came out wiht a .40 ARF something, the field experts told him it was really nose heavy and he proceeed to put lots of weight in the rear. everything was more or less okay until he throttled back, at which time the tail basically pointed towards the ground, and that was that. Never experienced a "tail tuck" due to weight being in the back and pulling back on power. Sounds like the guy's thrust line was waaay off. We're the field experts wrong... Did he make it tail heavy and just forgot to get the nose down when it pitched up (got complacent since things were going "more or less okay")? Don > you are both right, but you are talking about differeent things. yes, a > rearward CG makes the airplane more pitch sensitive. but airspeed is the only [quoted text clipped - 4 lines] > okay until he throttled back, at which time the tail basically pointed towards > the ground, and that was that. Good points. You can go fairly severe with nose heavy before the plane becomes unflyable. When in doubt, start at about 20-25% from the leading edge and work back. -- Paul McIntosh http://www.rc-bearings.com > Hmmm, I've found the opposite to be true. Overcontrolling on a tail heavy > plane is more likely with more airflow over the elevator. Or I [quoted text clipped - 35 lines] > > > > > > Douglas Kaip |
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