|
Lou Crane -> RE: Graupner Vs APC prop observations (5/17/2008 3:31:36 AM)
|
Newbie to this forum, but this caught my eye...
Has anyone "pitched" both props on a pitch guage?
A prop with a higher back face angle will put more load on the engine, on the ground, when the airflow through the prop is largely stalled. In flight, the prop will unload more than one with a flatter pitch back face angle. Diameter is more of a load item than pitch. If all else is right, the model will catch up to the 'pitch speed' of the prop pretty closely - RPM x pitch distance... If RPM is limited (mixture, throttle, whatever...) model speed will relate to the thrust needed to overcome net model drag. (Except in maneuver load situations, of course.)
The difference is like the experience in full-size aircraft. The US Army ground support and observation Cessna (?L-19?) had a "take-off" prop - relatively flat, fixed-pitch. Great acceleration, short field take-offs superb, but max and cruising speeds were limited by engine RPM capacity... OTOH, many racing planes of the Golden Age - thru the 1930's - had fixed pitch top end props. Takeoffs were agony; the props didn't start working until lift-off plus several seconds. Of course, top end was the target, and these props did that very well.
I fly CL Stunt, where matching the prop pitch and engine RPM is a major interest. Recent engines have their best power at higher than "traditional" stunt RPM. (Older engines- like the classic Fox 35 - had little RPM change from 4-cycling to 2-cycling on a suitable prop. The extra combustions helped maintain RPM against maneuver loads.) With their extended useful RPM range, the recent engines are relatively puny when loaded to fly around their torque peak RPM. Solution? Gear down to where the engine is in its happy RPM zone - above torque peak RPM - when mixture, prop load and model speed all fit. THEN, load the engine back towards its torque peak as maneuvering loads come on. LESS RPM loss, as in improved vertical in RC, better acceleration (ditto), and more flier confidence and enjoyment. In CLPA, we frequently do that by using flatter pitch props - good combination: Engine revs to where it is happy. and forward speed is easier to live with. (Excessively fast laptimes are wearing on the flier, and harder for the judges to evaluate well.)
Granted, in CL I don't have unlimited vertical lines to prove this out, but remember, CL flight is always less than one mistake high, always in ground chop, always exposed to wind changes from 5' altitude to 45' or so altitude. Also, we have - in CL - maneuver load factors up to 30+ g. Maneuver drags become serious...
In addition to a possible 'rear face' angle difference, the prop blade airfoil may matter for both CL and RC... The blades have airfoils! ...generally a flat bottomed airfoil, which means that the section's "zero lift Angle of Attack" is above the flat back face angle. (Phillips Entry - rounding the bottom of the leading edge up - actually increases the difference between flat, back face angle and airfoil zero lift angle...)
So, I'm curious if the old gray Graupners are essentially higher-load to begin with, and the companion APCs more matched to the computer resolved in-fight unloading condition. Any of this make sense to anyone in here?
|
|
|
|
