Plane Won't Climb Upwards?

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Re: Plane Won't Climb Upwards?

Post by bjt4888 » February 5th, 2016, 3:19 pm


Congrats on completing a couple of wing sets and getting better flights. The wing incidence I suggested you start with produces 6.15 degrees of decalage, but, of course, you knew this already, right (zero degrees on the stabilizer and arcsin .25/2.33 = 6.15 degrees). This decalage angle is slightly excessive and would produce a slightly stalling flight based upon the center of gravity (CG) you indicated in your previous post (just in front of the wing TE).

If you reduced the weight of your airplane 4.2 grams entirely by lightening the wing, you have effectively moved the CG several inches back (double check this) and by doing so you have most likely moved the CG too close to (or beyond) the center of pressure (or neutral point). When the CG is too close to the neutral point, the airplane becomes very unstable in the pitch axis (nose up/nose down). When unstable in the pitch axis and very tail heavy the airplane will "wallow", this is the non-technical term for a combination of yawing and rolling, which is what I think you are describing. If this is what is happening (check your current CG; did you change anything else besides the wing?) you will need to fix the CG either by making the tail lighter, adding weight to the nose (bad idea, as you are already overweight in total) or moving the wing back).

If you go back to my original post and make a list of all the specs I requested and send them, then I can give you very specific help on how to fix the CG (if it needs to be fixed). You are very close to a solution to this difficult event. Please be very specific in your observations and notes and, if you haven't read last year's posts yet, you will benefit greatly from doing this. In last year's posts you will find very detailed descriptions of winding, adjusting and many other aspects of this event. It may look like a lot of material to read, but there is a lot that can be skimmed till you see posts from me, Jeff Anderson and a couple of others that have experience in the event.

Your description of winding does not mention backing off. This is a critical step in the winding process for the typical gym ceiling height. If you do not know about this step, definitely read last year's posts about winding.

Glad to hear that tests are behind you,

Good luck,

Brian T.

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Re: Plane Won't Climb Upwards?

Post by fifty_missions » March 14th, 2016, 9:27 pm

I believe there might be another issue here that has not been thoroughly covered or understood, the bowing motor stick. If the motor stick bows under high torque motor load, it will deflect to point the nose down adding down thrust and also force the trailing edge of the stabilizer down as well. Both issues will keep the model from climbing and may induce a dive.

In addition, as the rubber unwinds, the motor stick may begin to straighten but without more power the model still refuses to climb. This condition is an infinitely changing variable and very difficult to overcome with trim adjustments. I would check to see if there is a bow in the motor stick. It should be easy to see when the wound motor is hooked onto the model. Check the motor stick from a side view.

I also recommend using a motor stick made from stiff balsa, 1/4" X 3/8" cross section or larger.

Good Luck,
50 Missions


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