Propeller Aerodynamics

[rx20040]

I was experimenting with different propellers on my Mini Funtana X (Great plane by the way!) and the wattmeter is showing that an 11x7 prop is pulling 275W and 26.87A WOT while a 12x6 prop is pulling 253W and 25.9A WOT, however I am noticing that the 12x6 is showing much better performance than the 11x7. In my mind this shouldn't be so as the 11x7 is pulling more amps and watts, therefore it should perform worse in comparison to the 12x6, but obviously my logic is flawed. Would someone please inform me as to why this apparent disagreement is happening?

[vintage1]

It all depends on what you mean by 'performing better'

You will get a lot more thrust out of the 12x6, but it will probably run out of thrust at a considerably lower speed.

[rx20040]

It all depends on what you mean by 'performing better'

You will get a lot more thrust out of the 12x6, but it will probably run out of thrust at a considerably lower speed.

When I say "performing better" I mean it has more power and it seems to "jump" out of a hover better than with the 11x7. It also seems to have a much shorter take off run (~15 ft versus ~25 ft). I guess these could be attrubited to more thrust but I still don't understand why a bigger prop with a shallower pitch generates more thrust than a smaller prop with a steeper pitch.

[raptor22]

Like vintage said, you get better low speed thrust with a bigger prop, so you accellerate faster...but it runs out of speed more quickly as you speed up so your high speed thrust is less.

The more complicated explanation is that the amount of thrust produced by a powerplant is equal to the volume of air it moves times the speed at which it accelerates that air. However, the faster you accelerate the air the lower the efficiency of the prop at that speed, so more power is wasted. Therefore, if you accelerate lots of air a tiny amount you can make a given amount of thrust with a lot less power than you can by accelerating a small amount of air a lot.

Why is this important? Because big, low pitch, props accelerate a lot of air a small amount; while smaller high pitch props accelerate small amounts of air to much higher speeds. As a result, the bigger prop wastes a lot less power and produces more low speed thrust for a certain wattage.

--Alex

[slipstick]

I guess these could be attrubited to more thrust but I still don't understand why a bigger prop with a shallower pitch generates more thrust than a smaller prop with a steeper pitch.
If you're really interested in the details have a look at Propellers in http://www.mh-aerotools.de/airfoils/index.htm but be prepared for some calculations.

Steve

[vintage1]

When I say "performing better" I mean it has more power and it seems to "jump" out of a hover better than with the 11x7. It also seems to have a much shorter take off run (~15 ft versus ~25 ft). I guess these could be attrubited to more thrust but I still don't understand why a bigger prop with a shallower pitch generates more thrust than a smaller prop with a steeper pitch.

Blade area. Lift (thrust) is proportional to wing area.

For a given RPM.

If it suits the model, fly it.

[Tom Harper]

rx,

From one design perspective, the purpose of the prop is to 'absorb all of the power the motor can produce'.

It is possible that the 12X6 operates closer to the motor's peak torque point than the 11X7. Input power can be deceptive.

I agree with Vintage. Fly it!

[groundfx]

rx20040,

Without sarcasm, I'll also add, congratulations for trying out different props (that stay within the motors watt range) and seeing which one flies best. Matching the prop to the airplane and your flying style is always worth doing and yet so few people do it.

[MR.Thinking]

How can I buy the scale planes?
Thank you all:)