I'm trying to figure out which size engine to put on a large plane I am building. I have noticed some major descrepancies between published thrust figures and the calculated thrust using the same parameters that were in the published review. I'm using the formula: prop diameter squared times pitch times RPM times a constant of .000011127. I got this formula from an article in Model Aviation. I have an MDS .68 that I'm thinking of using. In an RC Report review of this engine, they claimed 176 oz of thrust using a 14x5 prop at 10,400 RPM. According to my calculations, 14x14x5x10,400x.0000111127 equals 113.4 oz of thrust. Why is there such a dramatic difference in the two numbers? I'm sure the constant, .000011127, takes prop inefficiency, etc. HHow can I tell what the real thrust is? Any help on this would be greatly appreciated.

thank God for calculations -without them my day would be dreary indead.

The calculations are not facts --unless they are proven in actual use
Miss one period and - well you know -- things change dramatically.

Engines do not make thrust. Propellers make thrust. Same motor, 2 different props at the exact same RPM, 2 different thrusts.
Don't believe all you read.

Get rid of that formula. It is grossly simplified, beside the fact that it is dead wrong!
general dependancy of thrust data:
diameter ^4
rpm^2
pitch^1
Then throw in a constant for prop pitch design and or prop blade geometry
another constant for blade airfoil and zero AoA lift
AoA changes at speed
another way to calculate the effect of the amount of power for a given prop disk

Then you are olmost done.
Did I mention pressure altitude and fuselage resistance?

Cynics please note, there is no room for prop efficiency here. This is about thrust, and nothing else.

Kitchen cynics OK?
I like your chart -good ballpark info
Where I see problems in getting best actual thrust reading -is that the airfame - cowl- prop proximity to cowl etc., all affect the "raw" prop data. Here is my "better " setup suggestion on getting good prop efficiency - move the prop as far as possible from any cowl points.

Guys, thanks for the responses, but I still don't understand why the calculated and measured thrust of a single engine and a single prop can be so different. I'm not worrying about any complexities, like angle of attack, air density, prop efficiency, or stuff like that. The formula used in the calculations was published in Model Aviation magazine and written by a knowledgeable person. I'm sure the formula is somewhat simplified and thats OK. The measured thrust figures were from an engine review in RC Report magazine. There is just too much difference between 113 oz and 176 oz. The engine under review was mounted on an engine test stand, so there shouldn't be any fuselage interference, air density altitude, etc, to worry about. Thanks.

It will probably be easier to make your calculations match reality than the other way around.

The reason that the calculated and actual are different is the complexities you are not worried about. Pe Reivers has it correct. Your formula is simplified and does not take into account the different performance and geometry numbers for your prop versus the test prop in the article. The thrust calculation is sensitive to ALL of the variables you want to ignore. As long as you do not correctly and accurately quantify all the variables involved, you will not get close in your calculations.

BTW, Pe has built the best thrust calculator going and he has made it available free to all who want to download it. Use it correctly and you will get very good thrust predictions for your engine.


Mark

As others have pointed out, thrust calculations as described provide ballpark figures at best.
Another important point is that static thrust is just about useless as an indicator of flight performance, unless hovering is your main interest in life.
As an example one of my old OS46SF engines provides about 4.5 lbs of static thrust using an 11x7 APC prop. The same engine with an APC 12.25x3.75 propeller provides an extra pound of static thrust, i.e., 5.5 lbs. In hovering flight the 12.25" propeller is the best choice, but for traditional aerobatics it is just about useless compared to the 11x7 propeller.

Propeller selection is still very much a trial-and-error business.

/Red B.