# Calculating "thrust" of an engine

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I've been reading a lot about the thrust of different glow engines recently and I'm trying to figure out how it is calculated. I see mention of an engine having more thrust than the weight of the plane for example. If you know the prop diameter, pitch, and rpm can you calculate the thrust from that?

I am trying to figure out (roughly) how much power a certain plane needs based on it's weight.

Thanks, Steve

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Steve,

Pardon me but I just want to avoid a lot of arguments as there is a lot of wrong information out there. One site in particular has a thrust calculator but it does not include pitch. Unfortunitly, pitch is a factor. Take 3 props , same diameter and manufacturer but different pitchs, and run them all at the same RPM, the highest pitch will develope the greatest thrust and the lowest pitch will develope the least thrust.

The thrust calculated is static thrust, that is the aircraft is stationary. No information is given on how thrust varies with airspeed. I have not found any information on this in the model literature BUT there is considerable info in the old NACA literature from the 1930's and 1940's. I found a lot of this during the

1950's but have not been able to locate it again since NACA was reorganized into NASA as their interest changed into jets, rockets, and space.

I'm running out of time as I have to get to work. Let me know if I can help.

Ray Shearer

----- Original Message ----- From: Newsgroups: rec.models.rc.air Sent: Thursday, May 05, 2005 3:37 AM Subject: Calculating "thrust" of an engine

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Not always that simple. A high pitch can sometimes actually produce less static thrust than a low pitch in the static condition because so much of the prop blade is stalled. A stalled airfoil produces much less thrust (or lift), and a high pitch produces more thrust once the aircraft has forward speed and the angle of attack of the blade decreases enough to put the whole thing to work. This effect can be seen in any full-scale aircraft; a "cruise" prop, which has a higher pitch, has poor takeoff and climb performance, and only part of that is due to the slightly lower RPM. My own FS airplane (a Jodel) has a cruise prop, and as the takeoff roll speed increases the RPM will drop a little as the blades unstall and start producing increasing thrust, and as tip vortices diminish. A low-pitch prop might allow the engine to run up faster and produce lots of thrust, but it limits the airplane's speed since AOA drops off too soon. Propeller science tends to be more educated guesses than anything. At least with models we can afford to buy a few props and find the ideal for a particular airplane; I can't afford to do that with the real thing. A metal prop for a 172, for instance, can cost \$5K.

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Ray -

You're correct, the thrust does vary with pitch. The canonical formula for propeller thrust is T=(rho)(n^2)(D^4)Ct , where rho is air density, D is prop diameter, n is rpm and Ct is the thrust coefficient. The parameters are all readily available except for Ct, and therein lies the rub. It is, or was in NACA's day before computer modeling, normally determined empirically from small-scale model tests (it's independent of the size of the prop, and that is its utility). Ct varies with blade angle, but the relationship involves a non-trivial integral equation and some other not readily available parameters like the L/D of the blade section. As the question asked for an approximation of thrust, I think a thrust calculator, like the one from Barry Hobson,

, is useful to get you in the right ballpark. Start with prop a little smaller in diameter than the largest practical for the model, and then if needed an increase in thrust can be had by reducing pitch and going up to the next diameter. As for variation in thrust with airspeed, that's determined from simple laws of mechanics. Thrust = rate of change of momentum of the air ingested by the prop at flight velocity Vo and ejected at the higher slipstream velocity Vs, = (rho)(Vo)(A)(Vs-Vo). You can estimate the slipstream velocity from the product of rpm and pitch.

Abel

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I'm not looking for anything hghly scientific, just a ballpark figure. I've seen plenty of quotes about this or that engine having so many pounds of thrust and was just trying to figure out what a couple of mine might be producing. The only factors I really have to work with are pitch, length, and rpm. I really only want a rough idea so that I can compare it to the weight of the plane and get an idea of how it'll fly.

Thanks...

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To answer your question of how it will fly, then start with the power to weight ratio. Prop selection can come later and will involve a compromise between shorter takeoffs and max top speed.

To be fair, "how it will fly" depends in no small part on the aircrafts design itself and how it is loaded and how you intend to fly it. For example, I've got gliders that fly very well with little to no motor at all. I've also got model rockets that do "extreme verticals" and nothing else on the other end of the spectrum.

I think you are specifically trying to figure out how well it will accelerate and climb under power and that equation is dominated by the power to weight ratio.

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Steve-

With the prop parameters and rpm figures you have, the thrust calculator from Barry Hobson I cited in a previous post will give you the ballpark thrust figures you're looking for. It is a freebie download. As for cites of some engines producing so much thrust, that doesn't compute in the real world. Engines don't produce thrust, they produce torque and shaft power, and to relate that to thrust and propulsive power (thrust X velocity), you need to determine how fast the engine can turn a given prop. That could be calculated if you had realistic power figures for the engines, but that is rarely available. The bhp figures cited by distributers are usually at some rpm far above what is a practical range, and usually imaginative at that. Look at the figures for an OS 40 LA in the Tower catalog for example, 1.0 bhp at

16,000 rpm IIRC. If you could coax it to 16K with enough nitro and a small enough prop load, even if it didn't blow apart the tiny prop wouldn't be moving any more air than you frau's hair dryer.

Abel

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Yes, that's more or less what I'm looking for. Whethere I can expect the thing to zip around the sky or roll out for 1/2 mile before it finally manages to pry itself off the ground. The more I read about this the more inexact I realize it is. Time to just finish the thing, throw a prop on it, and go from there....

Steve

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Buy an decent yet inexpensive fish scale from the local sporting goods store. Tie one end of it to the ground and the other end to the tail of the plane. Fire the sucker up, crank it to full throttle and read the scale.

Take note of the particulars, engine, fuel, prop etc. Especially the weight and type of plane. Then make note as to how well it flies. This should give you a starting point for future projects.

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I have a small program called "PropCalc" which does this for you. Let me know if you want it, and I will e-mail it to you.

-- Bob

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