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May 21, 2015, 08:55 AM
dusty bible = dirty life
Majortomski's Avatar
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OK friends why do we free wheel our propellers?


First off please be aware that I am an aero engineer and a multi engine pilot. From that I understand that that we 'feather' propellers on multi engine airplanes to minimize the huge drag penalty caused by the wind milling propeller on a dead engine. I've even been told by full scale instructors if you have an engine failure in a single engine plane, if you have enough altitude and time, try to slow the plane till the prop stops wind milling so there wil be less drag.

That being said, why do we work so hard in free flight to MAKE the prop windmill when the rubber winds run out?

Seems it would be more efficient to let the prop eventually stop by spinning the rubber backwards till the air load and the turns balance out.

Then again a 'perfect' rubber flight is where the rubber wind run out as the plane touches down. So it should never have a windmilling prop to begin with.

Your thoughts please.
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May 21, 2015, 09:48 AM
Registered User
"if you have an engine failure in a single engine plane, if you have enough altitude and time, try to slow the plane till the prop stops wind milling so there will be less drag."

I've done this ... deliberately ... on a number of occasions.

It works. The drag reduction is very noticeable, as evidenced by the reduced rate of descent. You may have to bring back the speed to the stall, to get the prop to stop turning, though.
May 21, 2015, 10:21 AM
B for Bruce
BMatthews's Avatar
This was covered in one of the old Zaic yearbooks. Frank Zaic was a keen fan of higher levels of aerodynamics in our model building.

The Reader's Digest version is that for low pitch:diameter ratios found on engine props and some specialty low pitch rubber model props there is less drag to be had by stopping the prop. Better the stick in the breeze than an autogyro like spinning disc. But for higher pitch:diameter ratios above somewhere around 1:1 there's less drag with a free wheeling prop. And pretty well all duration style rubber models for sport or competition have a high ratio value. So they are better off with a freewheeling prop.

Of course the optimum solution to the issue was found in the folding props used originally in the late 1930's and on from there in so many serious duration categories.
May 21, 2015, 10:25 AM
Power Upward
GeorgeG97322's Avatar
There are many competition rubber powered free flight models that will thermal very high and without the free wheeling prop would be much harder to achieve their best performance. Several events like P-30 have a limited amount of rubber available to power the model and after only a minute or so of time the winds are expended and the prop will free wheel until the model either comes back to earth gently under glide or quickly under some form of dethermalization.

George
May 21, 2015, 04:52 PM
B for Bruce
BMatthews's Avatar
I know that this came up many years ago for fixed props on RC electric old timers and sailplanes. The props on those tend to have lower pitch:diameter ratios than rubber models by quite a lot. The electric stuff tending to use from 0.6 to 1.0. It was well recognized at the time that a brake function to stop the prop from freewheeling was a big help to lowering the drag.

But competition rubber models tend to use higher ratios of well over 1.0. Typically the props tend to be from 1.3 to 1.6. And that's where free wheeling works to lower the drag over a stopped prop.
May 21, 2015, 05:16 PM
Power Upward
GeorgeG97322's Avatar
See, this place has all the answers...

Bruce,

You going to be able to get down this way this year?

George
May 21, 2015, 09:35 PM
B for Bruce
BMatthews's Avatar
For sure at least the August and September meets. Might even come down for the later Fall Annual as well if the weather looks like it'll cooperate.
May 22, 2015, 12:19 AM
Registered User
With respect to full-sized ... the turning prop does not "free-wheel"... it turns over the engine, which requires quite a bit of effort.
May 22, 2015, 03:42 AM
Diesel Danny
danny mz's Avatar
I found that a model with a 'mandatory' free-wheeling prop (such as a P-30) is slightly more resistant to gusts than a pure folding prop on other classes of model.

Maybe this is because a folding prop moves the CG rearwards, but I suspect that the plastic gyroscope spinning around at the front of the model helps to stabilise things a bit.

Agree that the drag of a spinning prop may be greater than a fixed one - all that energy to spin it has to come from somewhere!

Anyway - that's my 2 worth.

* Danny M *
May 22, 2015, 08:44 AM
dusty bible = dirty life
Majortomski's Avatar
Thread OP
Quote:
Originally Posted by bogbeagle
With respect to full-sized ... the turning prop does not "free-wheel"... it turns over the engine, which requires quite a bit of effort.
With respect, please note I used the term 'windmill' the long accepted term for what you describe.

http://www.datwiki.net/page.php?id=8...&searching=yes
May 22, 2015, 10:10 AM
Registered User
Quote:
Originally Posted by Majortomski
With respect, please note I used the term 'windmill' the long accepted term for what you describe.

http://www.datwiki.net/page.php?id=8...&searching=yes
I'm not having a dig at anyone. ... just saying that the full-sized situation is different from that of the rubber-powered model, owing to the lack of the freewheel.

I've also noticed that my RC models often have a better glide when dead-stick, than they do if the engine is idling.
May 22, 2015, 11:17 AM
B for Bruce
BMatthews's Avatar
Quote:
I've also noticed that my RC models often have a better glide when dead-stick, than they do if the engine is idling.
Very much so. You can hear the engine being sped up to well over the idle speed. And that takes energy to do. At that point the prop is acting somewhat like an autogyro rotor to develop far more "lift" which would be noticed in this case as drag than a fixed prop.
May 23, 2015, 07:16 PM
I eat glue
I do it because I was told to.
May 23, 2015, 08:30 PM
B for Bruce
BMatthews's Avatar
Quote:
Originally Posted by baldguy
I do it because I was told to.
Your mother flew free flight rubber models too!?!?!?

.... nice.....
May 23, 2015, 11:22 PM
Registered User
Another explanation for freewheeling: FAC rules don't allow feathering or folding props, though obviously they're authentic for the time period FAC is supposed to be nostalgic for (1930's, mostly). Same deal about gears. Lots of scale models would be feasible with gears that aren't now. Unless you're putting a big ugly prop on the nose where it doesn't always belong.


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