Static thrust

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Pilot-34

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Like a lot of things it’s a matter of balance. Static thrust is useful knowledge.
So is torque
Both are more useful as part of a series of information.

Perhaps a 50,000 hp engine that turns 25 RPM Doesn’t sound very useful.
But if that’s turning an appropriately sized and pitched propeller on a large ship it becomes a thing of beauty.
 

Dan Thomas

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Like a lot of things it’s a matter of balance. Static thrust is useful knowledge.
So is torque
Both are more useful as part of a series of information.

Perhaps a 50,000 hp engine that turns 25 RPM Doesn’t sound very useful.
But if that’s turning an appropriately sized and pitched propeller on a large ship it becomes a thing of beauty.
 

davidb

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Here’s a prop pitch question as it relates to altitude. I have a Rotax 914 (turbo) with a ground adjustable prop. I am happy with the takeoff, climb, and cruise performance with the current pitch setting. However, I recently did some high altitude takeoffs and discovered the engine would exceed redline at full throttle. Would increasing the prop pitch allow full throttle without over speeding max rpm? Low speed thrust is important too so I would not want to go too coarse. Thoughts?
 

Vigilant1

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Here’s a prop pitch question as it relates to altitude. I have a Rotax 914 (turbo) with a ground adjustable prop. I am happy with the takeoff, climb, and cruise performance with the current pitch setting. However, I recently did some high altitude takeoffs and discovered the engine would exceed redline at full throttle. Would increasing the prop pitch allow full throttle without over speeding max rpm? Low speed thrust is important too so I would not want to go too coarse. Thoughts?
Yes, going to a coarser pitch would load up the prop more and allow you to use more of the engine's available HP (by not exceeding redline). But you mention that your concern was in getting more HP for high altitude takeoffs, and under those conditions it would seem that choosing a longer prop (rather than more pitch on a prop of the same length) would be a better way to use the available HP at redline to get more takeoff thrust. Compared to increasing the pitch, increasing the diameter will move more air during the low-speed takeoff roll and your blades will be un-stalled earlier, which will increase the prop's efficiency.

But, with your ground-adjustable prop, changing the pitch is easy and cheap, going to a longer prop is neither. :)

Either answer (more pitch or more diameter) might have detrimental effects if/when you return to thicker air for takeoffs.
 
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trimtab

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This is why the constant-speed propeller is the best form of propeller propulsion. Its pitch changes to get the best efficiency for any speed and power setting.
With a caveat. The twist ratio is still a compromise for efficiency at multiple speeds no matter what the pretty cartoons that show the CS prop efficiency curves riding over the crests of a series of fixed pitch prop efficiencies show.

A CS prop that accommodates from takeoff through, say, 180 knots will likely slow down an airplane designed for the same engine and 260 mph, although the faster plane might break ground a bit sooner with the slower compromise CS prop. The distribution of AOA changes with forward speed, with more of the blade further away from optimal AOA as speed rises for a prop designed for lower speed performance, similar to higher speed compromises giving up low airspeed performance.

Considerable effort was put into aeroelastic blade effects to eak out higher airspeed performance. The idea was to design a prop with the right response to flex at high pitch/high airspeed to gain a few to several extra percent prop efficiency when it would normally fall off. It was successful, and there are NASA tech briefs on this from the 80's. I still don't know why the efforts never really went anywhere.
 
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Dan Thomas

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With a caveat. The twist ratio is still a compromise for efficiency at multiple speeds no matter what the pretty cartoons that show the CS prop efficiency curves riding over the crests of a series of fixed pitch prop efficiencies show.

A CS prop that accommodates from takeoff through, say, 180 knots will likely slow down an airplane designed for the same engine and 260 mph, although the faster plane might break ground a bit sooner with the slower compromise CS prop. The distribution of AOA changes with forward speed, with more of the blade further away from optimal AOA as speed rises for a prop designed for lower speed performance, similar to higher speed compromises giving up low airspeed performance.

Considerable effort was put into aeroelastic blade effects to eak out higher airspeed performance. The idea was to design a prop with the right response to flex at high pitch/high airspeed to gain a few to several extra percent prop efficiency when it would normally fall off. It was successful, and there are NASA tech briefs on this from the 80's. I still don't know why the efforts never really went anywhere.
Yes, a really good CS prop would have more pitch change at the tips than at the roots. Pretty hard to achieve without an awful lot of mechanisms inside the prop, and the blade would have to be either really flexible, or in numerous sections. An engineering nightmare.
 

opcod

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So is there any real number that can corelate with one of the 3-4 formula found on different topic ? As a real plane with a 'fish scale' at the tail will give quite the real number.
 

proppastie

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Another misguided idea. HP is the result of torque times RPM.
If not torque x rpm, so how does one measure hp of an airplane propeller/engine combination? Is the HP number only a "brake HP" from the factory?
 
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proppastie

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hp of an airplane propeller/engine combination
How about this....get the straight and level full power red line airspeed.....Put the airplane in a wind tunnel, (take the propeller off ?) and measure the drag at that airspeed, calculate the force/thrust/hp from those measurements?
 

TFF

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For fixed pitch props, what you have to consider is max static thrust means when your plane moves forward just a bit, it’s loosing performance. You want good static thrust, but when the plane moves forward it’s gaining thrust. That’s why static RPM is a check. You want the prop to make more lift once moving not still. It’s not a fan. What you do with static and thrust is tune the performance. Climb performance, remember this is fixed pitch, you want a good bit of static thrust, but once in the air it’s going to fall flat on its face. Cruise prop, static will be low, but once the plane starts moving thrust keeps going up into a different range. Fixed pitch props you have to pick what you want.
 

Vigilant1

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So is there any real number that can corelate with one of the 3-4 formula found on different topic ? As a real plane with a 'fish scale' at the tail will give quite the real number.
-If a person is using static thrust in order to know the HP of an engine, he would be much better off to measure it using a calibrated test club using the Eiffel method. Costs less than a fish scale.
- If a person is using static thrust as a proxy for something that matters (thrust in cruise, climb, or during the takeoff roll), then he's setting himselfvup for a bad conclusion. A fixed pitch prop that allows the engine to reach full power at zero fwd velocity and which produces a lot of thrust at zero fwd velocity is not the same prop as one that produces the most thrust at 50 knots or 100 knots.
But, if we are using our airplanes to pull fences tight or haul trucks out of ditches, static thrust is what we'd want to know. And our props would be long and very flat.
 

Dana

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Dan Thomas said:
Another misguided idea. HP is the result of torque times RPM.

If not torque x rpm, so how does one measure hp of an airplane propeller/engine combination? Is the HP number only a "brake HP" from the factory?
I think you misunderstood Dan... he's saying it IS torque x rpm.

There is brake horsepower (BHP), which is torque x rpm, and there is thrust horsepower (THP), which is thrust x airspeed. THP/BHP equals propeller efficiency.
 

Vigilant1

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How about this....get the straight and level full power red line airspeed.....Put the airplane in a wind tunnel, (take the propeller off ?) and measure the drag at that airspeed, calculate the force/thrust/hp from those measurements?
If we want to know the thrust at airspeeds up to about 70 mph, we could put the prop, engine, and stand on a flatbed trailer and measure the pull it can produce while going 70 mph. Then, add to it the aero drag from the engine, stand, etc as appropriate (or, build a streamlined cowling and call the drag equal to the aircraft's scrubbing drag in the propwash).
 

henryk

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THP/BHP equals propeller efficiency.
P=U*I ...simple power measuremant, aspatially DC !
LEM200, NO gear .

=possibility to examine influence od propellers distance into the thrust.
 

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blane.c

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I think what the problem here is, is that no one wants to invest (spelled waste) the time and money building several propellers for their airplane in order to tune performance. With airplanes that have gone before it is easy to figure out the propeller you wish for your particular needs based on the empirical data. However for a new design of almost any combination of new engine or new airplane or some modification of either or both the time and money to try several propellers is necessary to optimize the design/modification or else you just settle for whatever you get with the first propeller design you try. Without trying other propellers you may never know if you have the optimum one or not. It depends on what that knowledge or performance edge is worth to you. The constant speed propeller takes the mystery out of it for high performance planes but for the fixed propeller faction of flying swapping propellers is likely the only way to be sure you have the best propeller for your needs.
 

BJC

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Yup, but for those who are not familiar with CS propellers, it is important to note that a CS propeller must be selected for the HP, RPM, and speed of the aircraft. One example, below.

Many years ago, a Glasair III builder installed a 300 HP Cherokee 6 FWF package, including the propeller. The G-III was about 45 to 50 knots slower that other 300 HP G-III’s, until he installed a different CS propeller with a blade designed for the correct speed range.


BJC
 
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