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edit: question was too long, and is actually simple, so I removed unneccesary description

Background:

There are many questions here about:

  • propeller efficiency
  • changes to thrust when it is dynamic
  • calculating the prop thrust for a given power
  • etc.

(Equation binding prop thrust, velocity and power)

(Units for prop thrust equations)

(Calculating thrust and required prop size for given engine power)

(Software for prop thrust and required power)

(Prop thrust equation taking into account number of blades)

(Prop efficiency of modern light sport aircraft)

(Correct formula for prop efficiency)

(Engine power vs. prop thrust for a 3-blade 14" prop)

But I would like to find more examples of measured static thrust made by a propeller.

So, if possible, please post some figures or some link with any data on measured static thrust on an aircraft with a piston engine.

Thanks

(example in photo below)

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ivanantuns
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  • Thrust per HP will be somewhat higher if using a constant speed prop, closer to 4+ lbs/hp static. – John K Apr 29 '22 at 15:24
  • Excellent remark I will edit that in the question. thanks! – ivanantuns Apr 29 '22 at 15:26
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    Thrust depends primarily on five factors: prop diameter, prop speed, prop pitch, airspeed and horsepower. Ignoring four to focus only on horsepower isn't going to work, which is why it is hard to find the simple chart you want. Other factors like solidity also start to matter depending on disc loading and altitude, which drives density of air. You are probably best off looking at multiple specific cases to see how thrust changes with variation of one or two factors, like your Cessna chart. – Pilothead Apr 29 '22 at 17:53
  • Dynamic thrust, ofcourse, is a different matter, and altitude/density/airspeed etc are important for those calculations. These can be derived once static thrust is known. However all this is irrelevant to this question, as here the aim is to find examples of all three parameters measured, not calculated. There is no ignoring any factors as there is no focus on HP. The focus is on looking at data where RPM + Thrust (static, ofcourse......... as only this can be measured unless the whole airplane is in the wind tunnel) + kW (or HP) which can be easily inferred from RPM because torque is known... – ivanantuns Apr 29 '22 at 18:37
  • but thanks for the comment let me edit that into the question so it will be more clear what is sought – ivanantuns Apr 29 '22 at 18:39
  • You didnt mention rule of thumb, or? No I do not expect to calculate dynamic thrust. This is not mentioned anywhere. And even less calculating it just by extrapolating from static thrust. Dynamic thrust requires , as you also mention, other parameters... this is just a question to see what (if any) examples can be found of real measurements of static thrust vs RPM with atleast the engine known - a graph already showing static thrust vs RPM vs engine – ivanantuns Apr 29 '22 at 19:18
  • Fine, I was only saying that your rule of thumb may work for some certain instances but is too simple to expect a research paper to substantiate. I didn't suggest that the cases you examine should be calculated. Static thrust only fixes the airspeed input. There are still three other primary inputs that vary. – Pilothead Apr 29 '22 at 19:23
  • i don't understand what are you referring to... firstly it is not my own figure it's a rule of thumb by the general aviation community , as rough an estimate as it is. It approximates a simple ratio which is not easily found (as there are many engines and propellers etc). And the aim of the question is not finding research papers etc but rather simple measurements of thrust ( static ofcourse) but at different RPM settings, not only full throttle, etc.. – ivanantuns Apr 29 '22 at 19:31
  • i see what you mean now, from the other user's answer, in reality there isn't any point in using static thrust for comparison of engine power conversion to thrust, as it shows something very basic. And all that can be calculated from parameters of prop diameter, blade angle, prop efficiency, air parameters, prop L/D etc. So basically, better to pursue the calculations to view thrust at prop and power produced by engine for that thrust? – ivanantuns Apr 30 '22 at 00:21

1 Answers1

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Let's first clear up some nomenclature about "power".

Power into the system, or throttle, or rate of fuel consumption is one meaning. Power as Force × Velocity is another, also known as mV$^2$/s. These can be linked as potential energy/second fuel burn = Thrust × Velocity = Drag × Velocity (at steady state or constant Velocity.

More simply put Thrust = Drag

So, you can work this problem theoretically through calculation of form and induced drag for a given airframe in a range of speeds. But, it is still difficult to calculate oweing to effects of "prop blast", which can affect both lift and form drag.

There is a wealth of data out there for various airframe/engine/prop combinations, which through years of testing have shown what is optimal, but it is really like trying to hit a moving target to develop a "Rosetta Stone" formula because of drag, weight, and desired speed range at what altitude all factor in.

But static test data would not be where I would start, as the relationship of prop pitch, RPMs, and airspeed is crucial. All your Cessna 150 data is showing is that the prop AoA on the bench seems to be more efficient as RPMs increase.

More meaningful data might be airspeed vs RPMs in flight. Thrust could be estimated from drag calculations, but in reality how the plane performs at a given throttle setting is what counts.

An approximation of drag at various airspeeds could be achieved by plotting angle of descent in a power-off glide.

glide ratio × mass = approximate Thrust at that airspeed

Discounting effects of changes in the airstream from turning props, this yields thrust requirement for level flight, which could then be compared to RPMs required for level flight at that airspeed.

POST SCRIPT

So, attaching two props with bicycle chains and sprockets to a Belphagor turbine might improve thrust efficiency. I believe Kuznetsov knows this.

Robert DiGiovanni
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  • thanks for this answer. You hit the nail on the head with the glide ratio! It is the fundamental cause of why I posted this question. Knowing the D/L gives the thrust required for an airspeed, but the issue here is that it concerns a specified airframe. – ivanantuns Apr 29 '22 at 23:22
  • So, in contrast to that, I am not looking to know what thrust is required, and then being able to calculate everything deduced from that (prop rpm, engine kW, etc). What I am very interested to see is, for example, what sort of thrust would be at 2000 RPM for an engine which provides 150 kgF of static thrust at 3000 RPM with an output of 60 kW...? So, regardless of airframe – ivanantuns Apr 29 '22 at 23:27
  • Also, by "power" i mean the power output on the engine's main shaft, the torque it produces at a given RPM. – ivanantuns Apr 29 '22 at 23:34
  • so, by "engine power" I mean what it usually means (engine output at the prop shaft) and not the potential energy of the fuel etc – ivanantuns Apr 29 '22 at 23:35
  • @ivanantuns well, you could work with the theoretical prop airfoil "lift" (thrust) at a given rpm. Even if pitch was not variable, AoA for a given relative wind could yield a theoretical thrust, but, when in doubt, overdesign it and test it safely please. Note that prop RPM is also a function of prop drag. I would stay away from static data. – Robert DiGiovanni Apr 29 '22 at 23:49
  • overdesigning and being conservative, absolutely good advice! Thanks – ivanantuns Apr 29 '22 at 23:54
  • Engine torque = Prop Drag torque, so if you know the lift to drag ratio of the prop airfoil, viola! – Robert DiGiovanni Apr 29 '22 at 23:54
  • So, would that be the parameter which basically "governs" the whole situation. Can it be said that the L/D ratio of the prop blade at a specified blade angle (AoA), at a given prop speed (RPM), would give us the prop drag torque/engine torque requirement (which in turn gives the engine power output) for that RPM and propeller L/D.... and then again by knowing the L/D of the prop blade we can calculate the lift done by the propeller? Thus getting both thrust and rpm / engine power? – ivanantuns Apr 30 '22 at 00:04
  • Yes! But I would compare these calculations to actual results. Big long slow wings are far more efficient than rapidly spinning prop blades for a given amount of drag. Next might be to plug in some horsepower - Newton meters/ second torque values to see how much angular drag the prop is making, then go ahead and compare it to static thrust data. I think the L/D ratios will be much less than a similar wing airfoil. Pursue it. – Robert DiGiovanni Apr 30 '22 at 00:19
  • I see. So basically, both thrust and engine power required for that thrust at a given RPM can be calculated by taking all known parameters: prop diameter, L/D and AoA [relative windflow at speed], prop efficiency [I guess this prop efficiency factor is what you refer to as compared to more efficient big long slow wings?], RPM, air density/pressure/temperature, airflow... etc.? – ivanantuns Apr 30 '22 at 00:25
  • Yes. But blade interference is an issue with static testing, which is why "push" at a given airspeed is elusive. L/D (thrust per horsepower in) for props would be a great field of study. I know helicopters get more out of theirs for above discussed reasons. Good luck with this! – Robert DiGiovanni Apr 30 '22 at 09:58
  • in any case, the fundamental cause behind this question is just that - comparison of real-life examples... Interesting how people tend to answer a question which asks "what examples of measured static thrust are there?" with "it would be very complicated to use these examples for further calculation let me explain these complications and why you shouldn't do that" – ivanantuns Apr 30 '22 at 16:31
  • @ivanantums anyone can tie an engine to a bench and measure static thrust. It's just that the data has very little application to aircraft. Maybe it will help for us to remember props are like spinning wings, and they pull with lift. This is why AoA and relative wind are so important. – Robert DiGiovanni Apr 30 '22 at 21:39
  • I see. Now that is very very clearly put! I realize now why I was confused with this... the torque required for the propeller drag combined with the RPM available from the motor will simply give out a power rating. But across an operative RPM range and across the operative airspeed range (relative wind) at various blade angles the thrust will vary - so I actually need to study propeller efficiency throughout those situations and compare with actual results!! Thank you for all the input and the perseverance to clear this up!!!! – ivanantuns May 02 '22 at 00:10
  • p.s. , what does the "p.s." in your answer mean ? why do you link wright bros , belphagor and nk12 ? :) – ivanantuns May 02 '22 at 00:14
  • @ivanantuns sorry I could not resist that one. Really trying to show how props (especially counter rotating ones) can be optimized. Turbine give plenty of power, but props give thrust, especially at lower speeds. Higher speeds generally not as good, but that NK-12 is does well at 500+. – Robert DiGiovanni May 02 '22 at 01:29