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Props...Is bigger better?

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TXFlyGuy

Well Known Member
A case of two identical propellers (composite, hollow blade), only different diameters...

84"

T/O rpm = 2480
Climb rpm = 2260
Max Cruise rpm = 1800
Econ Cruise rpm = 1580

96"

T/O rpm = 2350
Climb rpm = 2260
Max Cruise rpm = 1620
Econ Cruise rpm = 1450

With the above numbers, which would be the better performer?

HP = 325 @ 5500rpm (2480 prop rpm)
lb ft torque = 290 @ 4500 rpm (2000 prop rpm)

gear ratio 2.21 - 1

Whirlwind 100-4-84 Prop

This is all the info I have. The 96" 4 blade prop (Whirlwind 100-4-96) is in the design phase now.
 
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Defining your mission for the aircraft will help you define "better performance."

To some better performance means faster cruise speed, to others it means short field takeoff and climb ... with a fixed pitch prop, often "performance" means tradeoffs between these two regimes, while constant speed gives you more flexibility to tailor desired performance.

Based on my experience, I would expect slightly better cruise performance with the 84" prop but more low speed thrust, i.e. better takeoff performance, with the 96" prop.

Assuming all other parameters being equal, same pitch, HP, blade airfoil design, etc. then it comes down to the prop arc frontal area (less = reduced drag = higher cruise) vs. thrust potential (longer blade = more prop airfoil = higher low speed thrust). As airspeed increases, drag increases exponentially negating the effects of thrust potential over prop arc area.
 
It is a constant speed, 4 blade, hydraulic prop.

Are you talking diameter or pitch? Constant speed props you don't normally refer to in terms of pitch, and you'll never fit a 96" prop on any RV. (Let alone keep the blades subsonic at the speeds you're talking)

What kind of airplane are we talking about here?
 
I have very limited experience with props but I did talk to Craig Catto once.

When I was selecting my fixed pitch prop when I re-engined my RV we discussed my desire to maximize my cruise speed.

His suggestion was to go to a 2" larger diameter prop than "standard". His reasoning was that the extra disk area would help with takeoff thrust.

That may or may not apply to your application but it is something to think about.
 
Compare tip speeds: the smaller prop has a static tip speed of 909 feet per second, about .83 Mach, and the longer one has a static tip speed of 984 feet per second or about .9 Mach.

The shorter one will be noticeably quieter and more friendly to the airport neighbors.

It's possible that the smaller prop will be faster cruising but slower climbing, and the net effect on the time spent going somewhere will be the same as if you had the bigger prop with its better climb but slower cruise.

Considering both noise at take-off and the overall effect on trip time, I think I'd go with the smaller prop. That's the choice I made with my Cessna 180, choosing an 82" prop instead of an 88" prop. Looking back on that choice now, many years later, it was the right decision.

Dave
 
Yes, bigger is better

Just had to get my feet wet here with OP's interesting question about an interesting project that has tech interest for RV's as well.

First, a question about the Honda V6:
The engine that you ref has a rating of about 250 in the 5500 rpm range. It isn't clear to me where the extra 30% torque stated is coming from since the engine is highly refined in stock form. It seems that it would have to be boosted to get the stated 325hp. Is that the case?

OK. With a given gearing, slowing down the engine to accommodate the larger prop will reduce HP available. But the bigger prop will tend to offset that and the thrust might well be about the same at lower air speeds, and lower fuel flow as well.. I would go for the bigger prop if it was mine.

Since a 3/4 scale Mustang prop would be about 100 inches, the 96 inches prop will be close to scale and look much better. Little props on scale warbirds look a little dorky to me, suggesting that the Merlin was replaced by a Briggs & Stratton. Esthetics says bigger.

Performance. A bigger, slower turning prop will give a more lightly loaded propeller disk, and potentially lower tip speeds and so, will be more efficient at all speeds on both scores; this notwithstanding a previous ref to "prop frontal area drag", a misconception, I believe.

Of course practicality comes into the picture. It usually means more complexity and longer landing gear to gain the big prop advantage. But, you have that covered already with your combination of airframe and engine. So I say, take maximum advantage if you can. 250hp or so on a 96 inch prop will make a real stump-puller, guaranteed! And propulsive efficiency at cruise will be higher as well.

Max prop tip speed on the Merlin P51 at 3000 rpm is 836 fps, if my arithmetic is correct. Your 96" setup would duplicate this at about 4430 rpm on the Honda with 2.21 gearing.

And good luck with the vibration survey. :D

Ron
 
Prop me up...

Since a 3/4 scale Mustang prop would be about 100 inches, the 96 inches prop will be close to scale and look much better. Little props on scale warbirds look a little dorky to me, suggesting that the Merlin was replaced by a Briggs & Stratton. Esthetics says bigger.

Performance. A bigger, slower turning prop will give a more lightly loaded propeller disk, and potentially lower tip speeds and so, will be more efficient at all speeds on both scores; this notwithstanding a previous ref to "prop frontal area drag", a misconception, I believe.

Of course practicality comes into the picture. It usually means more complexity and longer landing gear to gain the big prop advantage. But, you have that covered already with your combination of airframe and engine. So I say, take maximum advantage if you can. 250hp or so on a 96 inch prop will make a real stump-puller, guaranteed! And propulsive efficiency at cruise will be higher as well.

Max prop tip speed on the Merlin P51 at 3000 rpm is 836 fps, if my arithmetic is correct. Your 96" setup would duplicate this at about 4430 rpm on the Honda with 2.21 gearing.

Ron

Ron,
Welcome to VAF, as a Jurrasic contributor it's refreshing to see an Engineering inspired thread! However comma, I'm sure this entire thread is in the non-RV category, but hey it"s Monday and my friend and VAF creator DR is 2 hangars away! :)
Theodorsen"s propeller theory consistently showed lthat for a given HP, max propeller diameter was critical in the thrust vs drag equation. As you mentioned, size matters...
Agreed, a P-51 replica needs to look reasonably like the original sitting still, or more scale. If turning a big prop slower still keeps Tex's performance acceptable, I say go with it. Even the original P-51 went through several prop iterations ending with the big Ham Std 4 blade paddle. Of course the C-130-J has composite Six-Blade props to achieve amazing efficiency on a 50+ year old design...

V/R
Smokey
 
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your right about the 130J smokey, but its in the blade design, the 5 blade rotol on the P51J and H worked like cra....

bob burns
RV-4 N82RB
 
Well...5 gears later, here we are. The final prop is 90 inch diameter. Hyper-efficient design from Whirlwind. 4 blade paddle prop. Looks like a 1942 Hamilton-Standard.

And the engine is not the original Honda, but the LS376/495. Able to cruise with 300 to 350 horsepower on tap.

It?s my understanding that the ?sweet spot? for efficient cruise rpm is to have the tips at .65 Mach, or about 750 feet per second.

Are these figures legit, or just BS?

I have also emailed Whirlwind about this.
 
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