This page is dedicated to Compuserve Modelnet C/L forum. (or may be any other)
"Stunt model dimensions" topic on RCOnline ~February 99
There are some lift and moment curves showing lift and airfoil moment jumps of flapped airfoils used on wings and tails of C/L stunt models. Those jumps can cause shape errors of some figures.
Wind tunnel measured lift curve of FX71 airfoil. It is 15% airfoil with 20% flap.
The lift curve shows there is serious problem around 0 deg angle of attack if flap is deflected over 15 deg. It can happen in corner. The flap is deflected and wing angle of attack is from 0 to 5 deg depending on elevator to tail ratio, because of circular airflow. It is evident that more flap deflection makes larger jump.
Theoretical (calculated) lift curve of 20% airfoil like used on C/L stunt models with 20% flap at 20 deg.
Second graph shows both lift (positive values) and moment curve (negative values around -0.2). Both curves explain what is going on. The airflow is separated on negative pressure side at hingeline if angle of attack is too high. The jump height is around 10 % of lift. The flap is around 20% of chord length. If airflow is separated at 80% of chord on one side, the 10% lost of Cy is expected. The same is on moment curve. The value -0.2 (the negative pitching moment of flaps) drops to half at the same angle of attack because of one side separation.
"Engine / Prop Performance Program" topick on RCOnline ~February 99
Some peoples asked me to send prop HP curves. So they are here:
If anybody need some of those graphs in better resolution, let me know.
"Undercambered versus flat" topick on Compuserve ~October 98.
On pictures are two airfoils looking like those used on our props. It only looks, so take it only qualitatively, not quantitatively. First airfoil has flat bottom GO 795, second is undercambered GO 342. Both are measured at RE around 80 000 which is appropriate for tip of thin 12" prop at 10 000 rpm.
First important thing is that both airfoils have zero lift (cy=0) at -3 to -4 deg. As I remember both "eyeballing" and "rpm calculating" used here gave us result around -3 deg in level flight (low wing drag). It is clear from alpha-:-cy curve.
Very important is comparison of cx-:-cy curves of both airfoils around zero lift (cy=0). It looks that GO 795 has vertical curve at higher RE and growing cy with growing cx at RE < 80 000. This mean unloaded prop releases engine to higher rpm. Undercambered GO 342 has totally opposite character. Loaded prop decreases needed torque. This mean after a maneuver, when relative speed to the air causes higher angle of attack moves airfoil to LOWER cx!!! This can cause a kind of accelerating. It is not true acceleration of cause, it is some "catching" to lift. It explains nearly vertical alpha-:-cy curve around zero lift. The same is while unloading the prop … the drag (cx) of airfoil is 10x higher at cy=-.2 (braking) than at cy=+1 (high lift at small speed; only little over stalling). Now I understand why my OS .40 FPS can nicely fly with underchambered prop without 4-2-4 break in hard wind nearly without speedup in loops.
Another very important property of underchambered airfoil is drag at zero lift. It is 3x higher than flat. This is because it needs more power to turn it. But … the drag is nearly equivalent at high lift (cy=.4)! This is answer why some Europeans pipe flyers like large diameter undercambered props. They fly it with loaded engine adjusted to steady 2 cycling. Does not matter how it flies at level flight. While loaded it have enough lift to fly over the figure. It is very important with magic muffler type minipipes without brake effect.
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