How relevant are static thrust measurements?

[e-geezer]

I do static thrust checks and I am inclined to believe that if a motor/battery/prop combination gives me a high thrust figure, then that combination will give me more thrust when the model is flying. Is that necessarily true?

[BillH]

More thrust yes, but are you figuring that more thrust also means more speed?

To go faster, you want a higher prop pitch speed.

[steve lewin]

Why do you ask ? Thrust doesn't tell you very much about the actual performance in any particular flight mode (except perhaps hovering). It's just one of many numbers.

Steve

[e-geezer]

I take thrust to be the pulling force the propeller applies to the plane - pretty fundamental I would have thought.

What I was wondering was how that force changes as the plane moves faster through the air.

I imagine that as the plane approaches the pitch speed of the rotating prop, that force would approach zero.

[Ollie]

The pitch number stamped on props isn't very useful in calculating the pitch advance because of differences in prop measurement methods, airfoils with different zero lift angles of attack and, differences in induced angle of attack stemming from blade aspect ratio. Comparisons of inflight performance with props of various pitches and diameters are the most practical way to deal with prop selection.

[Dave Hederich]

Static thrust testing accurately predicts hovering and low speed, high angle of attack (high alpha) flight performance. The situation does change in normal flight. A wind tunnel would be required to replicate in-flight conditions. A combination of one of the *Calc programs plus comparitive in-flight testing is probably the most effective solution for the average person.

[Ollie]

If static thrust testing is done outdoors in high wind conditions, where the wind speed exceeds the stall speed of the model, you have conditions that approximate flight conditions. If the range of wind speeds is close to the model's range of airspeeds, the thrust measurements will be close to the range of actual flight conditions. Of course the prop must be pointed directly into the wind.

[banktoturn]

Measuring static thrust can only tell you so much about what will happen in the air. In particular, a prop which is ideal for generating static thrust is not the right prop for generating thrust at high speed. That static thrust test is probably really useful if you are interested in hovering, or some other flight regime that requires high thrust at low airspeed.

The important thing to realize here is that this is not a tradeoff between thrust and speed. Thrust is thrust, whatever the airspeed is. You need high thrust for high speed. The tradeoff is between thrust at low airspeed and thrust at high airspeed. At low airspeed, you generally want low pitch and large diameter. At high airspeed, you need high pitch, and will need to reduce diameter to allow you motor to spin the prop fast enough.

banktoturn

[Dick Huang]

Originally posted by e-geezer
I do static thrust checks and I am inclined to believe that if a motor/battery/prop combination gives me a high thrust figure, then that combination will give me more thrust when the model is flying. Is that necessarily true?

e-geezer,
The static thrust measurement is useful;however it decreases with increasing airspeed until it reaches zero at some velocity.
meanwhile the drag is increasing with air speed. Where drag (thrust required) and thrust available cross at some velocity is the maximum velocity;at the velocity where thrust available minus thrust required is the greatest is the maximum climb speed. All this info can be plotted on both Moto-cal and Elec-calc. You can use these data plots to select the optimum propeller for your requirements and compair the numbers with your static test.
Dick Huang :)

[e-geezer]

So if static thrust and rpm measurements don't apply for a moving plane, what use are they?

Could it be that the prop that looks best in static tests (thrust and pitch speed) isn't the best one in flight?

Assume I want maximum flight time for a given ambient wind speed and flying skill level.

[banktoturn]

Originally posted by e-geezer
So if static thrust and rpm measurements don't apply for a moving plane, what use are they?

Could it be that the prop that looks best in static tests (thrust and pitch speed) isn't the best one in flight?

Assume I want maximum flight time for a given ambient wind speed and flying skill level.

e-geezer,

It is not that static thrust and rpm measurements don't apply for a moving plane. It is that they are most relevant for flight conditions which are most similar to static conditions. Hovering or slow flight, for example. The prop that looks best in static tests is very likely not the best one in flight, unless your flight is limited to low airspeed. If you want maximum flight time, I would assume that you would want a prop which is pretty efficient. This likely means something with a fairly large diameter.

banktoturn

[Martyn McKinney]

On another thread I was asked to relate pitch in inches to pitch in degrees.

I came up with the following approximate formula.

Pitch in degrees = pitch(in)/diameter(in) X 18.25

The interesting part about this is that for a prop with a pitch/diameter ratio of 1 to 1, the pitch angle is 18.25 degrees.

Most airfoils with aspect ratios of 6 to 1 or so (including propellers) stall at pitch angles above 12 degrees.

This means that any prop with a pitch/diameter ratio greater than 0.66 or so will be stalled when static thrust measurements are made.

It isn't until these props are moving at some speed that their true performance will be realized.

Static measurements made on props with pitch diameter ratios less than about .66 should be fairly valid.

For these props the thrust at optimum efficiency will be approximately 1/2 the static thrust.

Most airfoils with aspect ratios of 6 to 1 or so have their maximum lift/drag ratio at approximately 5 degrees.

Props with lower pitch/diameter ratios (less than 0.3 or so) will be extremely inefficient because at zero speed their angle of attack is already close to optimum and any increase in speed will only make it worse.

One author's comment was that they were good only for stirring paint.

[Dick Huang]

Hi All,
I am adding a typical plot of Thrust and drag Vs. Velocityfrom M-cal.
Dick Huang:)

[RSands]

I just joined rcgroups tonight but love the change from RCU! You guys are great! E-geezer, IMHO and based on similar experimentation, static thurst doesn't mean squat. The prop that on the ground produces the most static thrust is not going to be the prop you want to fly with (unless all u r doing is torque rolls). As already pointed out, you have thrust, drag, the airspeed envelope that the plane flies in (and where you will fly it), where the motor operates most efficiently, and your expectations for flight time and performance. Answer? Grab a handful of props, fly em, and decide which one is right for you!

[Dave Hederich]

RSands is correct. Static thrust doesn't mean squat ..... if you are looking for maximum performance at higher speeds.

RSands is incorrect. Static thrust is everything ..... if you are looking for maximum performance in hovering or lower speeds.

You see, it is possible to have it both ways. ;)

[Gary Warner]

This thread has me thinking...

This morning as I'm considering the thrust on a new r/c plane (8.5 pounds for a 16 pound airplane), I wonder why is it that my full scale Cessna 172 doesn't publish the thrust in the op manual? Watts per pound (power loading in hp) and wing loading are represented. Yet, in jet powered planes, it's common to publish the pounds of thrust.

Why would thrust be a pertinent statistic for one type of plane while not for another?

[Gary Warner]

I did find the thrust for my 172. It has about 525 pounds of thrust static (.218 to 1 @ 2400 pounds). Food for thought... Would you fly your latest creation on .218 to 1 thrust? Here's a link and pix.

Data link. (http://www.allstar.fiu.edu/aero/BA-Form&gra.htm)

[ScottS]

Originally posted by Gary Warner
This thread has me thinking...

Why would thrust be a pertinent statistic for one type of plane while not for another?

I think the answer is that it is not the type of plane, but the type of power source used.

With a prop driven by some type of engine or turbine it is pretty easy measure the shaft horsepower. The thrust produced by the prop can vary depending on many conditions, so reporting the horsepower makes more sense.

For the various types of jet engines the production of thrust becomes more integrated into the engine and it is harder to measure the shaft horsepower. For example how many horsepower is an afterburner worth? How about the horsepower produced by the inlet spikes on the SR-71 at high speed? You can of course use physics to backcalculate a horsepower based on the thrust - often a huge number on large jets.

[hoppy]

Originally posted by Dave Hederich
RSands is correct. Static thrust doesn't mean squat .....

Right on Dave!
hoppy

[e-geezer]

Apart from hovering, wouldn't a high static thrust be a good predictor of reliable take-off - where the velocity of the prop with respect to the air is low?

[e-geezer]

Here are some things you can't measure once the plane is flying:


motor current
battery voltage
rpm
thrust


Is there any point in measuring any of them on the ground?

What extra would I have to know to be able to predict flight performance from measurement of these parameters on the ground?

At the moment, I measure all of these for different props and batteries - on the ground, hoping that that will help to select the best prop and battery for flight.

The trouble with actual flight tests (the "just try it" approach) is that there are too many variables - variations in wind speed and direction, pilot skill, air density(?) and probably others. For example, how many flight test would I have to do, to be sure one prop was better for a given plane than another prop?

Or is model plane flying just an art, rather than a science?

[hoppy]

e-g
I measure static amps to make sure I'm not overloading the battery or motor taking into consideration that the amp draw will be somewhat lower in flight and I can always use reduced throttle. It helps to tell you if a prop is way too small or too large.
But flying and the finger on the pack and motor after flying is the best test.:) I like the "how many vertical rolls can you get" test and "how long will it fly the plane" test. Very scientific stuff, don't you think?
hoppy

[vintage1]

Originally posted by e-geezer
Apart from hovering, wouldn't a high static thrust be a good predictor of reliable take-off - where the velocity of the prop with respect to the air is low?

Yes. But planes like to fly at something like 2x stall speed. Thats way above static!

The moment the plane starts to move, the thrust drops away from static.

The ideal is to have the thrust maximum at or around STALL speed, and still well maintianed at up to tow or three times stall speed. Hence Motocalcs rule of thimb that teh pitch speed (aproximately the speed at which the thrust drops to zero) should be around 2.5-3 times stall.

The reaosn static thrust is quoted is because, given a particular propellor, its an easy measure of goodness for comparing motor/pack combos.

RPM is also another one. These are things you CAN measure that give you an inkling off inflight performance, and are also very useful to feed back into e.g. Motocalc to get at least one verified real world data point with which to adjust e.g. prop consants etc.

[steve lewin]

Originally posted by e-geezer
Here are some things you can't measure once the plane is flying:


motor current
battery voltage
rpm
thrust


Is there any point in measuring any of them on the ground?

What extra would I have to know to be able to predict flight performance from measurement of these parameters on the ground?

At the moment, I measure all of these for different props and batteries - on the ground, hoping that that will help to select the best prop and battery for flight.

The trouble with actual flight tests (the "just try it" approach) is that there are too many variables - variations in wind speed and direction, pilot skill, air density(?) and probably others. For example, how many flight test would I have to do, to be sure one prop was better for a given plane than another prop?

Or is model plane flying just an art, rather than a science?
I don't think I'd say "JUST" an art but it's a fact that no-one has yet managed to become a World Champion JUST by measurement and theory. All the figures you quote have some value, none are perfect predictors of flight performance.

Just think what you yourself said...flight tests are no good because the world keeps changing round you. But isn't it exactly that changing real world in which you want the plane to fly as well as possible ?

The list of things you can't measure also includes just about every indicator of performance. We can't get accurate figures for air speed, rate of climb, turn rate and all the other things that a real aeronautical engineer would need to measure to tell if the performance of a plane was right. About the only parameters we can measure are take-off weight and flight duration. Nowhere near enough :(.

You say you're looking for the best prop, battery etc. Do you want to define what you mean by best ? Best for speed, duration, hovering, aerobatics, slow flight, short take-off, load carrying ? For one plane that's 7 potentially different "best" prop/motor/battery combinations right there. We're all looking for different things from our planes. Unless you have a single simple goal, typically "best" for a particular competition, no-one is going to be able to tell you what will best predict a plane that flies as you like it.

I sometimes wish the world was simpler too but I'm afraid it isn't ;).

Steve

[Ollie]

Very well said, Steve! Your comments are directly to the point. I couldn't agree more.

[Dave Hederich]

Hoppy, so how are your hovering and high alpha flight tests coming along? Oh, I forgot, you don't fly that way. So your prop advice would not apply to those who are more focused on low-speed, high-thrust flight regimes, right? ;)

[Martyn McKinney]

e-geezer:

QUOTE:

"Here are some things you can't measure once the plane is flying:


* motor current
* battery voltage
* rpm
* thrust



Is there any point in measuring any of them on the ground?"

Most of these measurements may be determined with reasonable accuracy by indirect methods.

MOTOR CURRENT

If you have a low voltage cutoff on your BEC, fully charge your batteries and measure the current and the time it takes before the low voltage cutoff cuts in while on the ground.

Then recharge your battery and fly the plane in level flight until the low voltage cutoff trips while measuring the time.

This has given me reasonable estimates of my current draw while in flight.

RPM

Put a prop on your plane which has a pitch/diameter ratio of less than 0.7.

It will not be stalled at zero speed. Measure the RPM and the current

Measure the no load RPM of your motor.
If you can't measure it, it will be approximately (battery voltage X Kv).

The in-flight RPM will be somewhere in between these two values and may be estimated from the previously determined in-flight current.

At pitch speed the current will be approximately Io, the no load current.

The RPM will be increasing approximately linearly between the zero speed value and the no load value and may be determined from the in-flight current.

THRUST

Put a prop on the motor which has a pitch/diameter ratio of less than 0.7.

Measure the thrust.

The thrust decreases approximately linearly with aircraft speed, approaching zero at pitch speed.

From the previously measured flight current the thrust may be determined.

Most propeller systems matched to the plane will have an in-flight thrust of approximately one half the static thrust, if the pitch/diameter ratio is less than 0.7.

Hope this helps.

[Ollie]

Unfortunately the number stamped on a prop can't be used to accurately determine the pitch speed. The difference in angle between the tangent line to the bottom of the airfoil has to have the angle to the chord line of the prop airfoil added and the zero lift angle of attack of the airfoil added. Near the tip of the prop the omission of these two corrections may put the stamped pitch number in error by a large amount, perhaps in the range of 20 to 30%. It's a lot easier to try various props than to try to accurately determine the pitch speed.

[Martyn McKinney]

You're right of course.

I have a friend who gets great delight in measuring these things as accurately as possible. Anything worse than plus or minus 5 per cent and he is not happy.

He laughs at my 10 to 20 per cent world.

I am happy to stick my fuselage out the window of a moving car and measure the motor current.

[PhilLin]

Originally posted by Martyn McKinney
You're right of course.

......He laughs at my 10 to 20 per cent world.

I am happy to stick my fuselage out the window of a moving car and measure the motor current.

Martyn, you just reminded me of the above. I've been considering a means for testing dynamic load on ducted fans. A few days ago, some of us got into measuring RPM & air speed using doppler shift on the ground in this & in the e-jets forum. I've been thinking about a means to test unloading without a wind tunnel. Sure can measure in-watts (or volts & amps) at full throttle through the moon roof of my car with the model or fan tied to the roof racks. :D :D
The data would clear up the issue for the EDF crowd.

[Martyn McKinney]

Great idea !

May I suggest that you have someone else doing the driving while you are taking measurements.

[e-geezer]

Is that it? To know what the model's doing at 40 mph you have to be travelling at 40 mph?

Is that how far aerodynamics has got?

Any recommendations for a good book on aerodynamics?

[Martyn McKinney]

Please see post number 27.

[e-geezer]

I think my replies are being filtered at this point. Maybe my imagination. Let's see.

[Ollie]

Model Aircraft Aerodynamics by Martin Simons devotes Chapter 14 to propellers.

[steve lewin]

If you haven't already seen it, Martin Hepperle's website has some very useful info on propulsion by propellers with a section on static thrust including "why the static thrust of a propeller is not such a terribly important number". http://www.mh-aerotools.de/airfoils/index.htm

Steve

[banktoturn]

Originally posted by Martyn McKinney
e-geezer:

QUOTE:

"Here are some things you can't measure once the plane is flying:


* motor current
* battery voltage
* rpm
* thrust



Is there any point in measuring any of them on the ground?"

Most of these measurements may be determined with reasonable accuracy by indirect methods.

MOTOR CURRENT

If you have a low voltage cutoff on your BEC, fully charge your batteries and measure the current and the time it takes before the low voltage cutoff cuts in while on the ground.

Then recharge your battery and fly the plane in level flight until the low voltage cutoff trips while measuring the time.

This has given me reasonable estimates of my current draw while in flight.

RPM

Put a prop on your plane which has a pitch/diameter ratio of less than 0.7.

It will not be stalled at zero speed. Measure the RPM and the current

Measure the no load RPM of your motor.
If you can't measure it, it will be approximately (battery voltage X Kv).

The in-flight RPM will be somewhere in between these two values and may be estimated from the previously determined in-flight current.

At pitch speed the current will be approximately Io, the no load current.

The RPM will be increasing approximately linearly between the zero speed value and the no load value and may be determined from the in-flight current.

THRUST

Put a prop on the motor which has a pitch/diameter ratio of less than 0.7.

Measure the thrust.

The thrust decreases approximately linearly with aircraft speed, approaching zero at pitch speed.

From the previously measured flight current the thrust may be determined.

Most propeller systems matched to the plane will have an in-flight thrust of approximately one half the static thrust, if the pitch/diameter ratio is less than 0.7.

Hope this helps.

Martyn,

I think you are using the word "thrust" in a different sense than I have ever heard it used. Thrust does not go to zero at the pitch speed, and for most props, it does not decrease monotonically as airspeed increases from zero. I think it would be useful to define your terms in some of the statements above.

banktoturn

[Martyn McKinney]

banktoturn:

You are correct, but I was hoping to provide a simple means of estimating in-flight values from static measurements.

Some have posted graphs of thrust vs. speed and a straight line is accurate enough for my purposes (for props with pitch/diameter ratios less than 0.7).

On another thread there is a significant discussion of the meaning of the words pitch speed.

The final consensus was that it is the speed determined by multiplying RPM X PITCH, where there is zero thrust.

At speeds faster than pitch speed the motor will act as a generator.

In one of Bob Boucher's books he talks about pitch speed in another context.

He is using the term to refer to the speed at which the propeller was most efficient.

In this case he sometimes refers to it as "design pitch speed".

As I have mentioned previously, I have friends who enjoy measuring things and they laugh at my 20% world.

There have been comparisons made between Enrico Fermi who was able to determine the output of the first atomic bomb by throwing some pieces of paper in the air and Heisenberg who, working for the Germans and fastidious in his mathematics, was unable to achieve the same goals.

Fermi is my idol.

[jrb]

Lots I could write about thrust measurement, been doing it on propulsion systems (mostly full scale) for over 20 years; static and dynamic.

Static is very useful and quite important; though it doesn’t answer all the questions.

For the nay sayers even Astro Bob thinks “static thrust is useful in looking at the aerodynamic efficiency of props”.

Here’s a photo from a few years back of a test setup someone posted here; never heard about their results.

[RSands]

MAILBOX!!! :) Thanks, that'll make a good "caption this" pic!