prop speed; when does it create drag?

[Amir Neshati]

It is likely  that if our airplane speed is greater than approximately 1.2
times the
pitch speed, then there is a good chance the prop isn't making much thrust
and  it's starting to  create drag.....The larger this ratio, the more drag,
which
means at idle on a down line, the prop has more drag at the bottom than at
the top/start of a down line.

The pitch marked on our props is measured from the bottom surface of the
prop and not the camber, it would be like measuring our wing incidence
from the bottom surface of the wing...So, calculating speed by the marked
pitch will yield slower than actual flight speeds, hence the 1.2 times.

I've flown a few planes with very accurate telemetry on board.....One of
them
with a 15x8  APC turning at 8000 on the ground. It was a 20 lbs. machine
with
about 850 squares (Yes, there was once a plane heavier than Wayne's Prophecy
;-)
Calculated pitch speed will only yield 60 MPH top speed, assuming the engine
didn't unload.
However, actual speed was always around 75 MPH.....This airplane was fairly
clean.
The engine unloaded another 200ish RPMs in the air...... I even flew a 12'
span Telemaster
with struts and that sleek flat bottom wing and it was going 15%+ better
than pitch speed.

This was also tested on a buddy's race plane with the same results.

It is my understanding from our guru airplane fellow, that actual speed is
about 20% higher
than the pitch speed. Give or take 5% depending on how clean or dragy the
plane...So an
airplane with a 12 pitch prop in front of it turning 8500 is capable of
doing around 120 mph
at full throttle (unloaded rpm counts as added speed too).

At 1800 RPMs, with a 12 pitch, the prop has a pitch speed of 20 MPH. Add
that 20%
and an actual speed of approx. 24 MPH.....If the plane is going faster,
you've got drag.
In other words, if ya'll are boogi-yin' along at 100 mph and chop the
throttle down to 1800
rpms, you've got breaks!

So, what's the best RPM for breaking on the down lines? Heck if I know....

Amir






> Correct. Many variables to consider. The speed of the model and if the rpm
> you are it still causing thrust or turning slower than the speed your
plane
> is flying through the air. For models it is very hard to get below an rpm
> that still isn't producing slight thrust on landing when you have slowed
the
> model but while flying downlines I find that rpm's in the 1700-1800 on the
> ground do become noticeable on my downlines. Just upping the throttle trim
> to 2500 makes a significant difference. These are ground rpm figures as I
> have never invested in onboard tach. system. Not necessary to do.
>
>      del
>
> ----- Original Message ----- 
> From: "Wayne Galligan" <wgalligan at goodsonacura.com>
> To: <discussion at nsrca.org>
> Sent: Wednesday, December 03, 2003 3:57 PM
> Subject: prop speed; when does it create drag?
>
>
> | Great question John.
> |
> | Is something like an 1800rpm idle slow enough to create drag while
> landing?
> | I guess would depend several things like actual airspeed, prop size and
> | pitch # of blades, blade width.
> |
> | WG
> |
> | ----- Original Message ----- 
> | From: "John Ferrell" <johnferrell at earthlink.net>
> | To: <discussion at nsrca.org>
> | Sent: Wednesday, December 03, 2003 5:39 PM
> | Subject: Re: Winter / Focus / Fuel/Electric.
> |
> |
> | > As I am sure you are aware, full scale pilots have been taught for
many
> | > years that a windmilling prop creates more drag that a stopped prop.
> | >
> | > How do you determine what rpm generates the most drag?
> | >
> | > John Ferrell
> | > 6241 Phillippi Rd
> | > Julian NC 27283
> | > Phone: (336)685-9606
> | > johnferrell at earthlink.net
> | > http://home.sprintmail.com/~johnferrell/dixiecompetitionproducts/
> | > NSRCA 479 AMA 4190  W8CCW
> | > "My Competition is Not My Enemy"
> | >
> | >
> | > ----- Original Message ----- 
> | > From: "Doug Cronkhite" <seefo at san.rr.com>
> | > To: <discussion at nsrca.org>
> | > Sent: Wednesday, December 03, 2003 4:28 PM
> | > Subject: RE: Winter / Focus / Fuel/Electric.
> | >
> | >
> | > >
> | > > Well.. you fix that by setting a good idle speed before you take
off.
> | You
> | > > never let the motor stop in flight for a few reasons.. 1) as you
> | > > described, it looks weird.. 2) a stopped prop is not the best
solution
> | for
> | > > downline breaking.. 3) the start/stop is hard on the gears..
> | > >
> | > > -Doug
> | >
> | >
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> | > #
> | >
> | >
> |
> |
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>
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