I have found some programs on the net calculating propeller thrust and bhp. Some of them are based on equations from AMA mag Oct 86.

I would like to do some calculations based on these formulas. Does anyone have this article on some file format (scanned, .doc etc) being able to email them to me?

Thanks,

Helge Walle,
Norway.
[email protected]

I don't have your article but here are links to online thrust and HP calculators. Indications are the thrust calcs are like 50% high. HP may be closer. I am compiling reported thrust and HP for RC engines and have enough data to select props for a given engine pretty well. Thrust data is sparse, tho we can all measure thrust with a fish scale - very few do it and report the results. I am building a simple, fairly accurate thrust stand and will run some props at home to see what I get. We don't need super accuracy for our purposes but I would like something better than "it pulls like crazy".


http://pages.sbcglobal.net/limeybob/horsepower3.html
http://www.lcrcc.net/thrust_calc.htm

Hi HW50,

You are looking for the article by George Abbott. You should be able to contact the AMA and get either a copy of the article or access to the digital archive. Article starts on paper page 40, and archive page 42 (cover and inside cover adds two pages to archive count).

Measuring and comparing the propellers' Static Thrust won't give reliable results, as the blades of a given prop may stall, resulting in a bad Static Thrust on the test bench, while it may give excellent performance in flight and even outperform others that give a better Static Thrust.
With a given power, the more thrust you have, the less top speed you get.

maybe --maybe not - been fooled too many times.

The Static Thrust tells you if a plane will be able to hover. But you also need to know the pitch speed to be sure the plane will be able to reach flying speed…

The propeller blades are prone to stall on the test bench because the AOA of the blades is higher in the static condition than when moving in flight.
If you want to increase the thrust and keep the top speed, you have to increase the power.
Power = thrust Ă— forward velocity

that is obvious -
what I meant is that a high static thrust prop just may provide the best forward speed also -for a given plane -
without getting in to formulas (which I detest in explanations ), a very draggy craft may do do best with the best "thrust" prop"
I have set up far too many models to doubt that the dynamic setup can ONLY be setup in dynamic conditions .
to clarify - unless the plane is tested under real world conditions - the
formulas are simply wild assed guesses.
(Tho formulas derived from past tests are in the right direction )--
increasing power is not usually an option.
I made a thrust test setup for smal electrics -which provides info very usuable for 3D electrics - it does not do much for the flat out speed enthusiast tho - that type flying requires lots of fine tuning in both arframe and powerplant - been there - done it .

It only does if you’ve also chosen the right pitch speed (pitch x rpm).
The Static Thrust value you measure on the scale is misleading...
For example:
Take a small electric motor and gear it down enough to swing a 100" diameter prop, and it will produce you several pounds of thrust. No way is it going to FLY your draggy airplane, unless you can get the stall speed down to less than one MPH.....

Interesting website. Hav not tried the program myself:
http://progs.oh7eleven.de/betpat/

And of course, Hepperle's good stuff:
http://www.mh-aerotools.de/airfoils/index.htm

my info is not misleading .
It is practical , learned information.
Thestand shows a thrust - --and I know what the pitch is -so I know from past, real world testing - that the pitch being use is good for the upper speed range of the model or the mid speed range -or the low speed range.
How good it is, is not going to show up in a chart -
It has to be tested in conjunction with the model.
Do you see a flaw in that approach?

What I meant is that static thrust alone is not enough to predict how the plane will fly and even whether or not it will fly at all, since you also have to take into account the pitch speed, which has to be higher than the stall speed.
However, a too much prop pitch speed in relation to diameter makes the prop inefficient at low airspeeds (e.g. during take-off).
That’s why the full scale airplanes often have the facility to change their prop pitch to suit the actual airspeed (like changing a car gear).


Yes, because as you start increasing the pitch it may start showing you a bad static thrust due to the prop stall at the static condition, which may not happen in flight at the same rpm, that's why it can be misleading.
So, a prop that gives you a bad static thrust on the test stand, may in the air outperform others that give you a better static thrust.
Static thrust alone is only useful if you want the model being able to hover (like a helicopter).

Yep - but that is quite obvious
Static thrust is important in that it shows you the worst case condition.
what is that?
That is when you are trying to get the model to perform at critical speeds - This typically means at landing and take off .
Hovering is a real world part of modeling now ( really) and the static test is important at showng how much power is required to produce X amount static thrust.

Again - real world data now tells us that simple 1-1 thrust to weight power does not provide hover capability- tho that may seem to be the case .
So -- the static data -coupled with proven flight data can be extremely helpful.
Countless millions (billions) have been spent by various aircraft manufacturers, trying to duplicate the hover now acheived by high school boys with simple cheap foam /electric powered stuff.
some have now been able to make models which stop-start -in forward flight and actually make controlled reverse (yes, controlled ) flight!
If all of this is a bit out of sync with your knowledge of flight rules - check it out -it is extremely interesting stuff and may even change your attitude about what really really constitutes a flying machine.
Did you know that the full scale aerobatic pilots now haunt the model flight exibitions -to see what new possibilities in flight have developed?

How do you know the profile of the propeller that you have so that you can put it in the calculations?

Maybe, but still you can get the wrong prop diameter/pitch combination for a given plane, if you only rely on the static thrust shown.
A prop that gives you the best static thrust for hovering may not be the best prop for flying by the wing.

again obvious - but you can't calculate prop performance -
you can guess (calculate) but until you actually do it-- nothing is learned.
I never said I rely on static data -I simply use it with other known data.
wing schming -
You really ought to actually try some of this stuff.

Well said Dick.

There is no better test than real-world, real-time, real air.

I've gone to the field with 6 absolutely identical racing props. CNC machined, purpose designed. All identical. One ALWAYS ends up faster than the others.

Why? It could have been those 10 minutes of change in time, or the +0.80667543 deg in air temperature. Often its hard to tell. There are so many variables to deal with once you get into a dynamic situation. Combine those with know variables, and its almost a guess. Sometimes thats the best you can do. Make your best educated guess, try it. Then try to repeat it.

A wise man, and a very estute engineer, once quoted to me "Everything happens once in a row. When it happens 15 or 20 times in a row, come see me."

Bob

Well, that's not the main issue here.[8D]
The issue here is about whether or not the static thrust is relevant to determine how the plane flies (not hovering as a helicopter).
By choosing the highest static thrust you can get, is the same as driving your car with the first gear all the way, you'll never win the race, but of course, you'll get a lot of thrust!

My apoligies then. I missed the point somewhere.

The only thing you actually get to experience is static performance. So in a way, with a fixed geometry prop, it has to be a point of reference.

In many cases with speed applications, the engine is propped up to the point where it can barely turn the prop on the ground.... and initial acceleration is terrible. But as the aircraft and engine accelerate, the prop gets into a rhelm where the prop works its best. On the ground, you simply never see the prop or engine perform.

Static has some corrilation to inflight performance. But its hard to quantify.

With the advent of some of the in-flight telemetry stuff, its becoming a bit easier to see real time performance.

racing props are crazy makers - More theories than you can believe---
FWIW another nifty tool I found -is the ROTO gas engine with it's own tattletale tach built into the ignition.
you shut down the plane after a flight - by throttle --then poke a optical tach in front of a LED atached to the ignition - he led is flashing at last highest rpm rate.
so you read the tach -- multiply by two (usual tach reading setup) and bingo -there is the actual unloaded prop speed!
On my 3D gasser - I prop for about 9000rpm - then look at the tach after flying to see what unloaded ( even tho this is only say under 40 mph) revs are reached . 10,000 happens easily
For a gas racer I would think this setup is of huge value.
We would just listen and guess , years back.

Yes, that's it!
Assuming that you have the same power available: - if you want to increase thrust you have to reduce pitch speed and vice versa.
Since we modellers don't have the privilege to change the prop pitch during flight, we have to choose a compromise depending on the actual model's flight characteristics.
That's why some EDFs with low power to weight ratio can't take-off from the ground and need a special launch catapult because they have to sacrifice thrust for the sake of speed… but once in the air they may fly nicely.
Static thrust only tells you whether your plane is able to hover, nothing more.
The key is: power to weight ratio.
If you have the right power to weight ratio, you can get the thrust and the speed you want.

for your info -varible pitch props are now available for even small electrics .
All of the basic theory you keep repeating is of course true - but you seem to be stuck in first gear -
The static data is relevant -if you know how to use it -
I will give you a clue:
On full scale aircraft engines - they are tested after rebuilding --with a large fan/prop load .
the engine is anticipated to turn this load at X rpm - that proves that the engine is producing the intended power.
We do the same thing with our static test setups but we look at thrust delivered per watts of power input.
carrying this one step further - one can read thrust at various rpm PLUS watts of power to produce this thrust.
This info tells you what power you have availabe at various rpm under load.
Now you can plug in a calculated pitch and diameter of prop -along with slip etc. and get ball bark info.
Having done this sh-t for years -I know about which type prop will work for any given task.
BUT here is where theory simply becomes guesswork: (not uncommon)
the inter-relationship of prop/power and the airframe can and will skew the "best prop theory - badly ,in some casess .
So please stop with the "static tests are meaningless parroting"- they may be to you -but if you could learn how to use the info , you would find it worthwhile.

You may keep using your static thrust for whatever you want, but who wrote the following:
Just check the post #5 in this thread...[8D]

I did of course -
If you read it all you would see that my point is:
you can't calculate the prop efficiency - you must ultimately TEST in actual flight conditions.

I've absolutely nothing against practical or trial & error tests, it’s obvious that you’ve to check whether the calculation works in practice, but you can in fact get pretty close to the best prop for a given plane if you calculate it in the right way.

But, if you do as your following post suggests:
Then you’ll get stuck in the first gear!
Good luck.
[8D]

adam - -do you have any practical experience with his stuff ?