PSRU Gear Ratio vs. Prop Tip Speed

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BBerson

Light Plane Philosopher
HBA Supporter
Metal may be able to handle 1,000 fps, but the propeller may be extremely noisy; my experience in the propeller industry (I did prop aero at a little company that made propellers in Windsor Locks, Connecticut) is that most commercial operators (ATR-42, ATR-72, etc) keep tip speeds down to about 700 fps, with a couple of outliers to about 800 fps. We didn't do propellers small enough to be swung by an auto-derived V-8 when I was there, so maybe small props have different limits.

Advance ratio is simply airspeed (in ft/sec) divided by the product of prop rotation rate (in rev/sec) times diameter (J = v/(nD)); it's one of the four important non-dimensional prop parameters (the others are thrust coefficient, power coefficient, and activity factor)
I think the C-185 seaplane is around 1100 fps at takeoff. Not much choice with direct drive.
But with a gear drive the designer has the option to choose lower tip speed.
Did the aircraft or prop designer recommend an optimal prop rpm so the builder could then choose a proper gear ratio?
I think the gear ratio should be calculated after optimal prop rpm is determined.

Swampyankee

Well-Known Member
I think the C-185 seaplane is around 1100 fps at takeoff. Not much choice with direct drive.
But with a gear drive the designer has the option to choose lower tip speed.
Did the aircraft or prop designer recommend an optimal prop rpm so the builder could then choose a proper gear ratio?
I think the gear ratio should be calculated after optimal prop rpm is determined.
The engine and prop makers always talked to each other, so set the ratio based on prop makers' estimates.

N8053H

Well-Known Member
HWGA

This is from McCauley
"How do you control the RPM
We do it by varying the pitch of the propeller blades. In the sense that we're talking about it the pitch is the angle of the blades with relation to the plane of the rotation. As the blade angle is reduced, the torque required to spin the propeller is reduced and for any given power setting, the airspeed and RPM of the engine will tend to increase. Conversely, if the blade angle increases, the required torque increases. Then the engine and the propeller will tend to slow down. Thus, by varying the blade angle or pitch of the propeller we can control the RPM."

So, torque is what you're concerned with

View attachment 58106
The 2.21 is by far the better choice. The greater the reduction, the greater the increase in torque.
The pilot always has control of the target speed of the CS prop, see what works best for you.

Are you going with a 3 bladed prop?

TXFlyGuy

Well-Known Member
The engine and prop makers always talked to each other, so set the ratio based on prop makers' estimates.
And the people at Titan Aircraft are working in concert with Whirlwind on the development of this propeller. Providing the performance criteria and engine operating specs.

skypuppy

Member
Don't forget to add forward speed to the prop rotational speed in calculating prop tip speed. IIRC, the torque curves on the LS engines are pretty flat (relatively) so you may be spending lots of money for diminishing returns.

TXFlyGuy

Well-Known Member
Don't forget to add forward speed to the prop rotational speed in calculating prop tip speed. IIRC, the torque curves on the LS engines are pretty flat (relatively) so you may be spending lots of money for diminishing returns.
The gear reduction ratio is 2.21-1. That will allow for slow tip speeds, and let the engine run at a good rpm. PSRU not ready for shipment until June, this year. The propeller might be ready by April.

TXFlyGuy

Well-Known Member
The LS3 is undergoing Dyno break in right now, plus the programming of the ECU for max torque between 2500 and 4500 rpm. The 2.21-1 ratio will work just fine.

TXFlyGuy

Well-Known Member
I have been advised that a 94" propeller will be the goal, as it will work better than the 96" version. Here is a photo of an MT 94" prop, on a 3/4 scale plans built Jurca P-51:

Swampyankee

Well-Known Member
We always have the option of throttling back to a slower rpm. And 3600 rpm for the LS3 is only half of what the engine will do, so even at that it is not being overworked.

The question is, at 1630 prop rpm, does 232 hp get you more thrust than 195 hp? It seems obvious to me, but I am not an engineer.

edit: The designer at Whirlwind told me the obvious - more hp always equals more thrust.
As long as the blades aren't heavily stalled.

piepermd

Well-Known Member
A P-51 3/4 scale prop would be 100.5”, not 96”.

TXFlyGuy

Well-Known Member
A P-51 3/4 scale prop would be 100.5”, not 96”.
Yes, you are correct.
Jim Rust, of Whirlwind Propeller, designed my prop as a 90" diameter. It started life at 96", then 94", then 92", and finally 90".
Rust states that is the largest design he could make with all of the constraints and efficiency issues he had to deal with.

check6

Member
What you need to be asking is what is the maximum hub speed of the prop. Once that is established you can pick a reduction ratio that will put your engine’s torque peak near that speed.
The Hartzell four bladed prop used on the Stewart S51 has a maximum hub speed of 2200 RPM. Jim Stewart deigned the PSRU with a 2.13 to 1 ratio making the maximum engine speed limit to be 4686 RPM. The engine in the S51 does not need to use more than 3800 RPM for takeoff, even then you have to retract the gear quickly and pull the nose up in order to prevent a gear overspeed.

TXFlyGuy

Well-Known Member
Our PSRU is 2.21-1 ratio. 4500 rpm max continuous. That puts us at 412 hp, 480 pound feet of torque.
3800 rpm is used for T/O, and max cruise will be 4000-4050, to keep the prop tips at or near .645 MACH.
Yes, you have to get the gear up immediately to avoid exceeding Vle. Or...point the nose straight up.
Econ Cruise 2800-3200 rpm.

Prop Max RPM = 2200

At our max PSRU limit, the prop will only turn 2036 rpm.