Thread: Discussion Lawn dart upon launch?
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Old Oct 02, 2012, 12:37 PM
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G_T
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G_T
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Gary,

Ballast up your plane, get it to good altitude, and dive it down steeply in speed mode. Get it going very fast. Level it, then very abruptly pull flaps.

If the compensation curve is off, then there can be a notable pitch response.

If the flow disruption due to dropping the flaps messes with the flow around the horizontal stabilizer, then there can be a notable pitch response.

There can be a difference between the transient response from suddenly dumping flaps, and the normal response from more sedately dumping flaps. Moving flaps slowly keeps the flow closer to steady-state conditions. Very rapid motion though won't necessarily let the flow stay in the steady-state pattern. We see an example of this all the time with DLGs - the vertical tail. It would stall with the loads we are subjecting it to, were the flow constrained to have to fit the steady-state behavior. But the yaw is so quick compared to the distance traveled that the flow never reaches steady state. The flow stays attached better, and much higher lift coefficients can be achieved than would normally be the case.

There is another reason which a tuck can occur. If one uses weak flaperon servos, or the flaperons are not stiff enough, then they give way under the high air pressure. The elevator though wouldn't give way. So with a proper compensation curve for the elevator, rapidly dropping flaperons at high speeds can result in a dive because the elevator is compensating for normal flap travel but the actual travel is reduced in this case.

Most people though don't have the compensation curve very close to correct anyway. It is worth taking the time to fix it... At relatively normal flight speeds, one should be able to cycle the flaps slowly anywhere in the range of travel, or over the whole range, and the glider should keep flying sedately along with no manual elevator correction required. If this is not the case, more work should be done. Landings are so much nicer when flap compensation is properly set up. BTW, a linear mix will not do it. It takes at least a bi-linear mix to come close, and that tends to be close enough in practice for normal flap travel ranges. If one sets flaps up to have extreme travel (for a DLG) then bi-linear won't be a good enough approximation and one needs to use a full curve mixer. The compensation rate is greater in the upper region of the flap travel and lesser in the lower region. The built-in mixers often available with modern programmable transmitters are usually not sufficient for the job. Just set it to zeroed out, and use a programmable mixer.

Gerald
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