whee wrote:1. If a constant speed propeller is removed from engine A, installed on engine B and the same max static rpm is achieved do engines A and B produce the same horsepower at that rpm?

whee wrote:2. Will a 88" long propeller turning 2600rpm produce more or less thrust than a 84" long propeller turning 2800rpm.

We tested all the propellers past 2700 rpm. We found the propellers produced the best static thrust between 2600 and 2670 rpm. Static rpm higher than this produced less thrust due to the lower blade angles required to run the higher rpm. The maximum thrust achieved for each propeller is listed below using 2600 rpm as the base rpm.

akschu wrote:

whee wrote:1. If a constant speed propeller is removed from engine A, installed on engine B and the same max static rpm is achieved do engines A and B produce the same horsepower at that rpm?

I don't think so, because the pitch will probably be different. I would think the governor would let each engine hit whatever RPM they are setup for.

Mark Y. wrote:Question one - I would say yes, if the prop is going to the same minimum pitch as the first engine. Theoretically same comparison as fixed climb prop on two different engines.

Question two: Same pitch on both props, the larger diameter would have more thrust at the same rpm as the smaller diameter.

Battson wrote:About point 2.

Because they are constant speed, you have to factor in the engine horsepower I think. Are you assuming both props are running at those speeds on the same engine and if so, what is the horsepower loss at 2600 vs 2800 RPM?

shorton wrote:Lots of variables to consider but if you remove a prop from A and install it on B and ALL other factors remain the same IE pitch settings then yes it would require the same power on both engines.

On the second question I don't see that you are asking about controllable pitch props or solid props. Others have spoken about adjustable so I will confine my remarks to solid props. I would have to do some research to see how close each of your examples are to going super sonic at the tips, but again all things being equal, such as the airfoil of the blade, the profile of where the curves come in and so on the longer prop at the slower speed should deliver more static thrust.

This is my off the cuff opinion and may or may not have a real strong relationship with the truth. [emoji47]

whee wrote:Couple questions I've been thinking about:

1. If a constant speed propeller is removed from engine A, installed on engine B and the same max static rpm is achieved do engines A and B produce the same horsepower at that rpm?

2. Will a 88" long constant speed propeller turning 2600rpm produce more or less thrust than a 84" long constant speed propeller turning 2800rpm.

CamTom12 wrote:

whee wrote:Couple questions I've been thinking about:

1. If a constant speed propeller is removed from engine A, installed on engine B and the same max static rpm is achieved do engines A and B produce the same horsepower at that rpm?

2. Will a 88" long constant speed propeller turning 2600rpm produce more or less thrust than a 84" long constant speed propeller turning 2800rpm.

1) it depends. Is engine A on the fine pitch stop and engine B governing to (selected) max RPM? CS props are weird, can’t think of them like a FP prop unless you’re on a hard stop which is a pretty small part of the operating range.

2) it depends. Engine horsepower available at 2600 and 2800, blade design, etc etc etc. There’s a ton more variables here than you can eliminate successfully for a “rough swag” answer that trends towards correctness.

A1Skinner wrote:

CamTom12 wrote:
1) it depends. Is engine A on the fine pitch stop and engine B governing to (selected) max RPM? CS props are weird, can’t think of them like a FP prop unless you’re on a hard stop which is a pretty small part of the operating range.

2) it depends. Engine horsepower available at 2600 and 2800, blade design, etc etc etc. There’s a ton more variables here than you can eliminate successfully for a “rough swag” answer that trends towards correctness.

I think when you say static RPM with a CS prop, it is on the stops. When you go full power with the prop knob all the way in, the governor won't be in the CS range yet and the prop should be right against its stops. Least that's how my feeble mind unsterstands it...

Troy Hamon wrote:The props will adjust, so you cannot compare them this way. A Cessna 185 with an IO-520 has a redline of 2800 with an 82 inch Mac on. With an IO-550 and a Hartzell, the redline is 2500. It is clear when you push in the throttle that the latter has a lot more pull, but the Mac is widely regarded as the prop with more pull.

I expect the props are really only at the stops when the engine is at reduced power approaching to land, with the prop full forward, or when the engine is older and gets a bit tired...other than that the props are adjusting to limit engine rpm, including in static and on takeoff.

hotrod180 wrote:Is this a purely theoretical question, or does it have implications for your BH?

I suggest that you go back to Ponk's tip speed calculator and see what punching in different temps gets you.
Tip speed goes up as temp goes down.
A prop that's well within the sweet spot on a 59F degree standard day might bust mach when it's colder.