My case is: Old timer with 9" cord and 45" span with a high lift foil 17" between wing TE and horiz stab LE 7" cord and 18" length on horiz stab (which has a lifting foil same as wing) Question: Will the lifting foil on the horiz stab have any effect on the CG?(in ref to wing LE) My thinking is that the extra lift at the rear will cause the tail to fly high which seems to simulate a nose heavy configuration.
snipped-for-privacy@aol.com (broke=not working, retired=not working, retired=broke)
Or, perhaps, compensate for a tail-heavy design? Or provide more automatic stability in case of radio failure?
I dunno. I haven't built any planes like that. Just theorizing.
CG is essentially determined by the distribution of weight through the plane. The amount of lift generated by the wing and the stab contributes to a center of lift, which (I imagine) varies with the angle of attack of the wing, the stab, and the elevator.
If your problem is to calculate where to locate the CG for test flights, I would do it by the standard formula of
25% to 33% of the Mean Aerodynamic Chord for the wing. I imagine it will work for test flying, and once you have it in the air, you can refine it flight-by-flight.Marty
Careful- Some of the Free-flight jobs with lifting stabs had the actual static CG at or aft of the Trailing edge of the wing!
Mart> On 11 Jan 2004 22:27:36 GMT, snipped-for-privacy@aol.com (Brokebob) wrote: >
What the large lifting stab does is the same as what happens when you apply down elevator to a regular stab. Some minor differences can occur if you are flying a pylon job or a cabin job. Basically, especially cabin job, start with a CG in the 30-35% range. Use some downthrust. Note the angle difference in the incidence between the wing chord-line and the stab chord-line. The stab chord line is the true base and reference line, not the fuse. center-line. If the wing is several degrees positive, the engine should be a few degrees negative to stab or the airplane will tend to climb at higher power settings. (REALLY so with flat stab.) Old FFs with lifting stabs were not so prone to loop as the flat stabs, but the CG had to be really aft (70-85%) so as to negate the flying stab's down load on the wing during the glide portion. During the faster climb, the flying stab loaded the wing adequately to offset the rear CG. That is why FF modelers trimmed, trimmed, flew, trimmed trimmed and flew then trimmed some more. Really it was great sport.
For RC, of course the sym. or flat stab makes things much easier, but the flying stab can be considered a flat stab with a lot of positive incidence. Set the wing at more and then your downthrust is built in. Still I start with a couple degrees diff. in stab/wing, some downthrust, and a not so far aft CG.
HC
Telemasters have lifting stabs, and yes, it does affect the CG. The CG on a Telemaster 40 is about in inch behind the "on the spar" balance point, and the stab has a 2 degree negative incidence relative to the wing.
Thanks a bunch guys! Very good answers for a fellow that adhers to the TLAR school teachings.
I think I will start by hand launching with the engine not running to see what happens and adjust accordingly.
snipped-for-privacy@aol.com (broke=not working, retired=not working, retired=broke)
If I may stick my $.02 in here. Be patient if someone has already talked about this. The center of lift on a flat bottom wing like the TM has will move forward as speed is increased and this causes the plane to climb. The lifting stab on the back will also increase in lift with the speed increase and automatically give the effect of applying down elevator. Automatic trim of a sorts. I have never seen a lifting stab on any plane without a flat bottom airfoil wing.
Dan Dan Thompson (AMA 32873, EAA 60974, WB4GUK, GROL) remove POST in address for email
PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.