PSRU Gear Ratio vs. Prop Tip Speed

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BBerson

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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

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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

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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?
:beer:
 

TXFlyGuy

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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

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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

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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

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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

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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:
IMG_1490.jpg
 

Swampyankee

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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.
 

TXFlyGuy

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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.

E6893B0B-F790-4332-A6B4-A7DB561B6B3B.jpg
 

check6

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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

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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.
 
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The rpm range will be limited, for sure. It (ECU) is being programmed for peak torque from 2500 to 4500 rpm. Doubt if we ever go above that.
Similar engines are in use, a V8 & V12. They employ a 1.9-1 reduction. We will go with the 2.21-1 ratio. I have no idea why the 1.9 ratio was ever decided on.

As has been pointed out on this forum, the manufacturers test their engines at WOT, for many (hundreds?) hours. Running at 3600 rpm all day will not hurt the engine.

The new 96" prop is being designed by Whirlwind Propellers in El Cajon. It will be a very efficient design. This is from the designer:
Objective is to design the most efficient propeller with constraints of 4 blades and 96 in dia-using power train and flt conditions, which boils down to proper chord length to achieve max L/D for propeller.

In the end, does the propeller care what the engine rpm is? As long as it is turning in it's designed sweet spot, should make zero difference.

Dyno at WOT? Sure. But the data taken at 3,000, 3600, 4000 rpm is still valid. Those are points that you pass as you get to peak hp/torque and rpm.
300 hp / 300 lbs torque at 3400 rpm is valid performance number, even if you stop the acceleration at that value.
They probably had resonant vibration problems at a 2:1 ratio.
 

TXFlyGuy

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Required HP and max prop rpm are the main deciding factors in selecting redrive ratio as hp is the measure of work that can be performed per unit time.
As most on this forum know, our's is the new Autoflight-New Zealand Heavy Duty PSRU, rated for 600 hp. Max continuous RPM is 4500. And this gearbox has been discontinued, after only 3 were built. Titan Aircraft decided they were too heavy. The weight is not a problem in our aircraft.
 
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