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Static Thrust Question

G

Gary Ward

SPONSOR
Lincolnton, GA
Assume that you have an engine and prop mounted on a test stand where you could measure static thrust. Also assume the prop is fixed pitch and the engine is running at full throttle and turning 2500 RPM under no wind condition.

Assume the static thrust under these conditions is 500#.

How would the static thrust change with a headwind or a tailwind. Just say 15 KTS for example.

Obviously, if there was a 15 KT headwind blowing into the prop, the RPM would increase slightly and the engine would make more power so the thrust might go up. And the reverse may be true if there is a tailwind?

What if the RPM were kept constant at 2500? The power would have to be reduced to keep it there with a 15KT wind into the prop.

What if it were a constant speed prop and the RPM was governed at 2500 RPM?


Gary
 
Static thrust is just that, static. It is the measured thrust when the engine is at a certain rpm and not moving. As soon as you introduce the winds, you no longer have a static condition. Therefore the winds have no effect on static thrust.
 
phdigger123 said:
Static thrust is just that, static. It is the measured thrust when the engine is at a certain rpm and not moving. As soon as you introduce the winds, you no longer have a static condition. Therefore the winds have no effect on static thrust.
Click to expand...
An accurate answer, yet conveys no meaningful information. Have you considered a career as a lawyer? :smile:
 
For the sake of Cub type airplanes I'm more interested in what a prop's thrust is in 15, 30, 100mph air than when sitting still. Acceleration to takeoff speed and rate of climb at Vy or Vx are more important to me than what a scale says with my airplane tied to a wall.
 
If you and your buddies have a pull test just pull hard, let it go slack and then blast the power again and record the jerk. You can then go home saying you won. 8)
 
Static thrust is important. The more you have, the better your climb, cruise, and accelerate, assuming your weight remains the same. So, if you have 500 pounds of pull at full throttle sitting on the ground, you will have 500 pounds of pull in level flight at full throttle. Your airplane will continue to accelerate until the drag equals the thrust. If you are climbing at full throttle at a constant speed, you are pulling with the 500 pounds of static thrust at the maximum climb rate possible with the available power. You can increase your speed and decrease your climb rate, and you are still using the 500 pounds of static thrust. I know this is a simplification, as your engine power will vary with air density, but in general it is true and why static pull is important.
 
Let's take a pickup truck, and put it in granny gear (it's an old truck), and hook a scale to the hitch. It would pull some impressive numbers! Now let's check that pull at 85 mph, as a fixed pitch prop is essentialy like a one speed tranny, it would be " wound out", and not nearly as impressive in its thrust. Static thrust is just that, and offers little information as to the overall performance of a given prop to a given airframe. I learned this 38 years ago flying ultralights, max static thrust is good for max thrust WHEN YOU'RE NOT.MOVING, I prefer to move when I fly, as such it's a pretty much meaningless way to rate a prop to an airframe. No engineer here, FWIW.
 
I have a friend that built and flew a 2T1A Ranger powered Great Lakes. I flew it also and it was an awesome ride. It had a wooden prop that would never stay balanced as humidity changed. It cruised about 95 mph at 80℅ power. He got another prop installed and it staticed about 50 rpm more then the other prop that never reached redline. It seemed like a perfect fit. He kept the Lakes on his 1100' one way strip. When he went to fly it it pulled great the first couple hundred feet but didn't get above 50 mph. He had already used up 700' so had to continue or wreck it trying to stop. It got airborne the last 100' and he flew it off the end of his runway as the ground dropped away. It was 100rpm over redline and had trouble maintaining 60 mph while level on an airplane that stalls at 55mph. He did a 2 mile flat turn WOT and came back and landed. Static doesn't always give up the true performance.

Glenn
 
Steve Pierce said:
If you and your buddies have a pull test just pull hard, let it go slack and then blast the power again and record the jerk. You can then go home saying you won. 8)
Click to expand...

The only manly way to settle a bragging contest would be to hook the airplanes tail to tail with spiked chocks at the rear of the wheels. Loser gets deflated ego and 2 deflated tires!
 
I think too many are reading too much into the question I posed. I was merely trying to determine what effect wind flow in the front or rear of the prop has.

And to the comments about static thrust

"Static thrust is just that, static."

As an engineer myself (at least by training but not by practicing), I probably used the wrong terminology. I should have just asked what would be the effects of thrust on a stationary engine/prop with varying wind conditions. Sorry, it's been a long time since engineering school. And I don't recall taking a course in prop efficiency anyway.

I didn't want to mention this as it is sacrilegious in this group but I started taking helo lessons last week in a R44 and there is a bad situation known as LTE or Loss or Tail Rotor Effectiveness. I'm sure there are some sleeper Helo pilots in this group. LTE can be caused by several factors such as turbulence from the main rotor. But one situation seemed contrary to what I thought and that is with a strong cross wind from the left during hover or slow speed maneuvering (American helicopter), when tail rotor effectiveness is lost, instead of weather vaning into the wind, the tail actually goes into the wind.

So there you go, I have come out of the closet.
 
The thrust curve for propellers decreases with velocity, although the power available curve will increase until it reaches a peak. So in theory peak thrust for a propeller is at zero velocity.

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Sorta - - How do you suppose the prop makes the plane go 85? Oh yeah, thrust. What the engineers here have been trying to point out is that props are non-linear, unlike gears.
 
Yes, I just used a poor simile. What I referenced from my ultralight days in the early 80s was a certain prop maker and UL builder whose main demo was the static his props could pull. "Not so fast," (ha ha) said Jack McCornack, head honcho of the brand UL I was flying, Pterodactyl. What followed was a lengthy public battle, some in print, between the two, they never did like each other to start with! Decades later, Jack seems to have been spot on with his overall explanation as to how static thrust relates to overall prop performance. No engineer, but a real sharp guy with a gift for simplifying complex subjects. To this day, Everytime I hear static thrust mentioned as a main, or sometimes the end all be all, performance parameter in evaluating a prop. I raise my non engineer eyebrows.
 
Personally, i believe static thrust gets me moving. That is important.

Almost everything after that becomes a compromise.

Also, let us not forget that a propeller is an airfoil. .....and airfoil cross sections are a compromise.
 
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