BIG PROP.LOW PITCH VS SMALLER PROP.HIGH PITCH
Vestfold Elektronikk Service
by telling me how pitch\diameter is affecting the planes
performance. In my case a TF P-51D 1:7, with a OS .91pumper.
Thanks...
Sandman
David Houston
This should generate some good replies.
Propellers are a tradeoff. Can compare it to buying a car and having to
select a single gear to use. The only way to change it is to drop the
transmission and put a new one in. If you pick first gear, you can
accelerate quickly, but your top speed is 25 mph and the engine is
screaming. You pick fifth gear, your very slow off the line but you have a
good top speed.
In the airplane world, a low pitch prop (and whatever diameter it takes got
keep the engine from overreving) is like first gear. Great for big planes
that have small engines (i.e. a Cub) or smaller planes with lots of drag
(i.e. some biplanes). You won't get much top speed because the propeller
runs out of thrust as the pitch x RPM approaches the planes airspeed. But
you get more thrust from the prop at slow airspeeds to pull the plane
around. Sometimes used on other types of planes to get better take-off
acceleration or vertical performance.
The high-pitch prop (again, with appropriate diameter) is 5th gear. Good for
planes that need to fly fast. Planes with low drag, thin wings, streamlined
shape.
Most of the time, you pick something in the middle. For your combination, a
15x4 is probably 1st gear, a 12x12 is probably 5th gear. Seems like a 13x8
or 14x8 might work well on a clean airframe like the P51, giving you the
speed it needs while still giving enough thrust from the engine to pull
around the 8-10 lbs of plane.
When picking the RPM to run at (adjusting the diameter), you want it to be
somewhere above the engines peak torque (good for low pitch, large diameter
props) and below the peak power (good for high-pitch, small diameter props).
Others, please comment on this (goes without saying on this group, huh :->)
because it is just speculation.
Hope this helps.
David
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In article <86266c$9io$1...@web1.cup.hp.com>,
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Before you buy.
Kevin Kline
K. Kline
Audrey Knowles
Vestfold Elektronikk Service <v...@c2i.net> wrote in message
news:XTYg4.800$WH2....@juliett.dax.net...
> Could anyone give me an insight in the prop.world,
> by telling me how pitch\diameter is affecting the planes
> performance. In my case a TF P-51D 1:7, with a OS .91pumper.
As I understand it, it works like this.
Fine pitch is a bit like low gears on a car, Lots of acceleration at low
speed but not a lot of top speed, High pitch props are slow off the mark but
will give a higher max speed.
For example, take a 11x6 prop doing 10,000 rpm, a six inch prop 'moves
forward' (assume no slip for simplicity) 6inches per revolution, 10.000 rpm
is 5.000 feet per minute which in very found figures is one mile per minute
or 60 miles per hour. If we dive the plane downhill, speed will increase but
soon the prop will begin to slow the model because it cannot, at that rpm
cause the plane to fly at more than 60mph the theoretical maximum.
Change the prop to a 12 x 4, same rpm, which at three revs per foot will
only travel 3.300 feet per minute, much slower than the 11x6.
Change it again to a 10 x 8 , same 10.000 rpm and now it will pull the plane
6.600 feet per minute, much faster than the 11x6.
Even I would admit that this simplfys things a wee bit too far but the
prinicple is right. If you model is not performing and the engine is on
song, it may be that the propellor has reached its max speed and either you
need more revs on the same pitch or more pitch on the same revs. If you
have a slow flying but draggy model, or you want to prop hang, large
diameter low pitch will shift lots of air at slow speed, but even coming
down hill you will soon find that the model is driving the prop instead of
the other way round and the spped will be held back.
Something like a P51 will need a fair bit of pitch (guess 8") where as a
bipe would want less (6"??)
Thats my theory anyway, Anyone like to comment??
Stu,
Doncaster. U.K.
Paul McIntosh
performance. You want to prop the OS to run in the 9-10k range on the
ground, so a 13X8 APC sport prop will likely be a good compromise to
start. A 13X6 would also be a good choice if the plane is a little
heavier.
Vestfold Elektronikk Service wrote:
>
> Could anyone give me an insight in the prop.world,
> by telling me how pitch\diameter is affecting the planes
> performance. In my case a TF P-51D 1:7, with a OS .91pumper.
>
> Thanks...
>
> Sandman
--
Paul McIntosh
Desert Sky Model Aviation
(877) 311-3759 (toll free)
(602) 780-9005 (in AZ)
desertsky.goplace.com
A.T.
Dr1Driver
GIVEN THE SAME RPM. . . a larger diameter/lower pitch prop gives more pull and
less top speed. It's like low gear in a car. The reverse is true for small
diameter/high pitch props. They are like high gear. Your P-51 is probably a
little heavy, but still streamlined, and it's supposed to be fairly fast.
Start with a 6-8 pitch prop, adjusting the diameter for optimum engine RPM.
Dr.1 Driver
"There's a Hun in the sun!"