prop selection methodology

[swoeric]

Is there a scientific procedure for determining prop selection and if not scientific is there an acceptable run of thumb

for a given length you have some factor of efficiency you would think this could be mapped for a given rpm range and is dependent on torque

for a given prop pitch you have a level of efficiency tide to this as well as given torque.

this would leave us to believe that a finer pitched long prop would be the same as a coarse pitched short prop

the peak torque of a motor is the most efficient rpm or best mpg do we then prop for a given MP at cruise for this rpm for the best cruise prop

but then how do we decide the best climb prop. we could do the efficiency if it was linear for a range of lengths against the pitch number X rpm to get a volume of air moved?

so after the food for thought has been digested please educate me.

[magman]

Re: prop selection methodology

I just look at thr Spec/TCDS.

[flyguy]

Re: prop selection methodology

Your idea of fine pitch long vs coarse pitch short is accurate if the engine/aircraft is bolted to the ground and you are just trying to load the engine a certain amount. As soon as the aircraft/engine moves everything changes. That is caused by the difference in the angle of attack of the propeller's airfoil to the oncoming airstream. Generally a longer prop turning at a lower RPM and HIGHER torque is more efficient. All kinds of exceptions though.
This could turn into a pretty interesting discussion.
Darryl

[jim cooper]

Re: prop selection methodology

I had a wooden prop on mine when I bought it. It would not climb very well. I called East Coast Propeller (recomended by Bill Pancake) Told the boss Randy what I had currently. He said it was for a cub. He told me the McCauley metal prop I needed (I wanted a metal prop) Gave me a good trade in and the McCauley is MUCH better in climb and cruise is 90. Very Pleased. The MCCauley is a 74/43.

[Edwin Otha]

Re: prop selection methodology

Maximum manifold pressure at rated RPM will be the best theoretical prop selection. But many more things come into play, aspect ratio and blade design are some of the variables.
EO

[astjp2]

Re: prop selection methodology

Climb is a longer flatter prop, Just like a borer. Tim

[n6346m]

Re: prop selection methodology

Tribe,
I've got a metal prop that came with the C-90 I just installed. I always switch to a wood prop for winter flying and the wood prop that I had on my A-65 is not legal for the new engine. I am looking to buy a wood prop and I wonder if anyone has wood prop experience on a C-90 and any recommendations they can make.

[astjp2]

Re: prop selection methodology

You can just call Sensenich, they can send you a new one for 1300.00. Tim

[swoeric]

Re: prop selection methodology

ok so rule of thumb is longer prop better climb. do we have any idea of ideal mp or amount of rpm lost or gained per given pitch change.

I know for fuel burn stress etc I want my motor to cruise about 2300-2400 take off i could care less i would also like to see the 100 mph mark at this cruise. ideally it may mean i need to put another 10 lbs on the tail because mine is a rocket with a loaded baggage compartment.


anybody know how 83 inch prop made it on the lower power machines if 74 is off limits to the 85hp

[astjp2]

Re: prop selection methodology

It depends on blade profile, horsepower etc for performance per inch. McCauley is different than Sensenich. I usually guess about 50 -100 per inch. You have a total of 8" of life in a prop. You can go longer, a cessna 150 on floats is approved for a 75" prop, on wheels its 68" has to do with clearance with a flat strut and tire. A Taylorcraft f-19 runs the same prop as a 150, 75 for floats and 68 for wheels and ski's. Tim

[swoeric]

Re: prop selection methodology

you said 8 inches


this meaning there is only a range of 8 inches of pitch for a particular engine



or that i can only trim a prop down 8 inches

[astjp2]

Re: prop selection methodology

8" of total pitch change during the life of the prop, if you increase 4, you can decrease 4 and that is it. You can do 8 in one shot, you have 0 left. you can do 3 inches now, 40 years from now you still have 5" of change available. Tim

[Hank Jarrett]

Re: prop selection methodology

Unless of curse if the prop is wood, on which case you get fire kindling with the first re-pitch.
Hank

Sorry, couldn't resist. I am enjoying the discussions. My professor in Aero used to say prop design was at least half black magic.

[magman]

Re: prop selection methodology

It seems this is an outdated discussion.

It seems Propellers are now to be called FANS.

The intent is to keep the pilot cool.

When they stop turning he starts sweating!

[N96337]

Re: prop selection methodology

I was hoping Mike Redpath would join in with some of his actual knowledge on the subject..
JH