Props...Is bigger better?

[TXFlyGuy]

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.

[Mike S]

Same pitch?

What is the aircraft mission??

[TXFlyGuy]

Originally Posted by Mike S

Same pitch?

What is the aircraft mission??

Pretty sure the pitch is the same. Mission? Limited aerobatics, fast x-country travel. Air show circuit (Texas)...maybe.

[Glas467]

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.

[TXFlyGuy]

It is a constant speed, 4 blade, hydraulic prop.

[Sig600]

Originally Posted by TXFlyGuy

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?

[TXFlyGuy]

Titan T-51D Mustang, 3.5 J35A4 V6 engine.

[N941WR]

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.

[David Paule]

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

[TXFlyGuy]

The tip speed can be dealt with by limiting engine rpm.