Thrust formula?
 

Hello all,

I have read in a few threads people talking about the amount of thrust their engines put out. What is the formula for figuring this out? Is it just the prop and RMPs?

Thanks

Thrust formula?
 

This is not a cut and dry subject... as far as I know.
Thrust will depend on engine, prop, airframe, and what you are trying to accomplish.
You can think of thrust as a column of air the prop produces.
A short high pitched prop will move a long slender column of air while a long low-mid pitch prop will move a shorter wider column of air.
The short high pitched prop generally has low static thrust and only produces it's maximum thrust at speed. Just the opposite for the other prop.
You can figure out static thrust with a soft but strong tether, a pull-scale, and your aircraft.

What have you got and what are you trying to do?

Thrust formula?
 

Here is a link to a thrust calculator , have fun..

http://www.bmaps.net/software/thrusthp.html

Thrust HP calculator
 

I use Thrust HP myself, but only for estimates and relative differences between different RPM settings. ;)

I have found Thrust HP to be very optimistic. It seems to assume every prop is 100% efficient. In reality, none are. The best are only 75% - 80% efficient.
Also you should notice in Thrust HP if you change only the pitch setting for a given dia prop & rpm, the speed output changes, but not the thrust output. We all know this is not the case in real life.

The only way to accurately know the true thrust of your engine / prop combination is to have some sort of mechanism setup to measure the amount of pull it is generating. This can be done with an accurate spring or digital fish type scale, or by having the engine lift weight via cable and pulley.

If anyone is interested, post a reply and I will post a detailed description of what I have setup.

Thanks...
 

Thanks for the replies. I am not looking for the EXACT #'s, just a guesstimate for fun. I found the web link to be what I was looking for, plus the idea of hooking up the plane to a scale, I thought of that, but it couldn't be that simple... :D

RE: Thrust formula?
 

i like the calculator

RE: Thrust formula?
 

Drew,


My HP calculator; the PropPower from Pé Reivers, is a MS Excel spread-sheet.

The formula it uses to fill the thrust field is HP x 6...

This is not right.
Output has no direct relationship with force, which is only a component of it (the other is speed).

...Or we would not need to have helicopters...

A pylon racing prop, extracting the same amount of HP, at outrageous RPM, would replace the rotor...

RE: Thrust formula?
 

Nothing like a set of springs ... you know the saying as about proof of the pie ...

RE: Thrust formula?
 

anybody know the prop factor for mejlik?

RE: Thrust formula?
 

Dntmn,


Since Mejzlik props turn somewhat faster than the sport/pattern APC props, I would place my bet on 1.00.
They are, however, more efficient than Master Airscrew props that have the same prop factor number, due to their carbon-fibre rigidity.

I would also place the excellent Bolly C/F props, at about the same number.

RE: Thrust formula?
 

That sounds right I think I have my BME 50 overloaded.

At the field I tacked a DA 100 with Mejlik 23x8 at 6450 rpm
A Different BME 50 with mejlik 21x10 at 7200
and My poor BME 50 with a BME 22x8 at 6150 RPM

I am thinking about switching to a mejlix 21x10 or 22x8, that is if the mejlik 22x8 would give me at lest 7000

RE: Thrust formula?
 

Dntmn,


BME states sizes between 21x10 and 22x10 as suitable for this engine.

The specifically mention Mejzlik (pronounced 'Ma'Iz'Lik).

Other maufacturers of similar displacement engines, list props up to 24x8 as suitable.


I don't think you should necessarily target 7,000 RPM, but the prop on which your model will fly best.
It may even be a prop spun at only 6,700.


Isn't this supposed to be in the 'Gas Engines' forum?

RE: Thrust formula?
 

guess it should be but one more statement a BME is a short stroke engine a DA is a long stroke. What this means is that the DA has more torke and can swing a larger prop a BME has more rpm..Basically the make them equalk you would need a da to swing a Mejlik 23x8 at 6450 and a bme to swing a 23x8 at 7100

thanks

I didn't realize where this thread was I just did a search for thrust calculator

RE: Thrust formula?
 

Dntmn,


The term, or should I say, buzz-word, 'Long Stroke' went out with flared slacks and elevator shoes...

An under-square (i.e. long-stroke) engine does not produce more torque than an over-square engine.

Ring drag is a negligible factor and it is about the only thing, which is lower in an under-square engine that could contribute to torque.

With given event-timing (port opening numbers in degrees), an over-square (short-stroke) engine will have larger port openings, which will allow a more complete exhaust scavenging and a better cylinder filling, compared to an under-square engine.


With a given displacement, the under-square engine will have a longer stroke, for more leverage, but also a smaller diameter piston, which results in a lower combustion force, so no torque improvement will result.


Back to glow engines; some of the engines considered 'torque paragons' like the MVVS .91, are over-square, short stroke engines.

If an engine is designed with a longer stroke, the engineer would typically time it for spinning a larger prop at lower RPM, to prevent excessive piston speeds. So it will readily spin larger props at lower RPM numbers and at this lower RPM regime, it will have an advantage, but it is very unlikely to have an absolute torque advantage, compared to a similar, over-square engine.


Most engines are very close to being square anyway.