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.
 
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