How much of an effect on performance?

[VNugget]

In bascic ground shchool they teach us that, with a constant pitch prop, the faster you go, the less of an AoA the prop blades get, which means less thrust, which is why we have variable pitch props to keep the AoA constant, blah blah blah. That's all good.

But, partly prompted by the other thread about windmilling, I got to thinking... there is another factor that reduces thrust in prop planes (both constant AND variable pitch), no? As the airspeed increases, the relative wind on the prop blades steepens in angle, as usual. And, since lift is perpendicular to relative wind (let's ignore induced AoA for now), it seems the thrust vector would also tilt accordingly, thus tilting opposite the direction of rotation, and therefore reducing its forward component?

So why isn't this in any of the basic books... does it have too little effect on performance to matter, or did I draw a wacko conjecture to begin with?

Thanks.

 

[Lrjtcaptain]

I just leave all that deep thinking up to the propeller manufactures and aircraft manufactures to put the most efficient prop on the airplane, and then tell me exactly what to do to make it most effective. What ever is in the book usually is sufficient in teaching. Any more in depth and you might as well work for Mccaully or Hartzell. Unless you like all that trig **CENSORED****CENSORED****CENSORED****CENSORED**, then have at it.

 

[Mongolikecandy]

"As the airspeed increases, the relative wind on the prop blades steepens in angle, as usual. And, since lift is perpendicular to relative wind (let's ignore induced AoA for now), it seems the thrust vector would also tilt accordingly, thus tilting opposite the direction of rotation, and therefore reducing its forward component?"


Holy Crap! You have just sucked my enjoyment for flying out of me for the next.... well let's see, 7 hours and 59 minutes.

 

[crash-proof]

Totally off-topic (which seems to be the general consensus here), but seeing your avatar, nugget, I just watched the thunderbirds take off right over my head at FLL today - in formation. What a blast. They're performing over the beach tomrrow. yeehaw!

Back to the topic at hand...this is probably a topic archer could help you with. Oh oh, here it comes..

 

[VNugget]

Sweet! I get to see the Thunderbirds at SNS this year for the first time, and can't wait.

 

[Flying Illini]

you twisted my mind around like a pretzel just with your question! It's good you question this stuff but...jeez! Unless you plan on building a prop, just accept what happens and what is stated in the books that you have. Of course, if you still NEED to know the answer, you can always call AOPA's pilot hotline or even one of the prop manufacturers.

 

[VNugget]

By now, I've figured that it doesn't make that much practical difference (and I don't care enough to start phoning prop companies) but I was just curious.

 

[Cardinal]

VNugget, I certainly think you're right in principle. Imagine the bunch on this board decrying excessively deep thinking, after all the talk about CASM and RASM, and 4 pages devoted to tipping the van driver.

It seems that in the "normal" realm of operations the problem of a low thrust vector is fixed by high prop rpm. This breaks down, though, when close to the hub. Check out this picture of a RR Trent, I believe: http://www.airliners.net/open.file/343092/L/ The portion of the blades closest to the hub can't be making much forward thrust, at least not very effieciently.

For a given prop rpm, at some high airspeed the blades would eventually go so high pitch that they would essentially feather, just to keep the RPM near that which was selected. At this point the thrust vector would be almost entirely sideways, the powerplant could put out all the torque it could produce but not generate any usable forward thrust.