Thurst and its meaning

[Frederick Fielder]

I have a question on thrust and what it means. I did a test using a Fox .25 with a Master Air Screw 9 X 6 prop and it pulled 26.5 oz and a when I changed to a Brodak Y & O 9 X 5 prop it pulled 27.5 oz. Does this have to do with speed or power or what?
Frederick

[Richard Oliver]

It is like you shifted to a  lower gear to pull up a hill.
RO

[Paul Smith]

It sounds to me like you are measuring static thrust, which is about all you can test within the limits of resources.

It's probably a valid way to compare several different engines of the same design, or different fuels, or head clearance.

I'm not so sure about props.  The issue with props is how they work at flying speed, not zero speed.

[Bill Little]

Hi Frederick,

I think it is useful to a degree to check out different set ups.  Paul has it right that some propellers do not function as well as others "in flight".  I do not know all the reasoning behind that (I don't have time for a lot of 'science experiments' ) but it's true.

Bill <><

[Jim Thomerson]

Thrust involves three elements: engine horsepower, prop efficiency, and things I don't know about.   I suspect if you put a 10 x 3 on your engine you will get more static thrust.  As said,  relationship to thrust in flight is not as predictable as one would like. 

[Alan Hahn]

What makes static thrust, especially with its relationship to propellers, is, as several people have already pointed out, a tricky business. If you are only interested in how short a takeoff roll you might need for takeoff, then the prop with the largest static thrust is probably a good indicator (depends a little on how much airspeed you need to lift off!).
When you are holding the plane, the propeller is biting into the air at more or less its stated pitch (~angle of attack). Once the plane is moving, the angle of attack decreases due to the forward component of the relative airspeed. In straight and level flight, the plane is no longer accelerating (tangentially at least!) so at that point, the thrust just equals the drag (F=ma, with F= Thrust-Drag, m=mass, a=acceleration=0 at cruise). At this point the prop you chose for maximum static thrust may be flying you too slow in the air.
Since you  are never flying at zero airspeed during the pattern (ignoring Bi-Slobs!), the zero speed prop choice most likely won't be the best choice in the air. My own GUESS, is that the best prop, for a given engine is one that provides the best thrust at the lowest airspeed of the pattern. I am also guessing this is somewhere in the vertical climb after a square corner. That prop will keep the plane accelerating the best(or from decelerating the least) and thus have the best line tension. At this stage as far as I know, this will be an empirical excercise with lots of prop selections if you are using <$5 props.
I believe Al Rabe may have made measurements with motors mounted on a moving car (at least I think he did for wings). A working man's version of a wind tunnel.
My 2 cents!

[Jim Thomerson]

Back when I was semiserious and flying Fox 35's, I started out with the required 10 x 6 Wide Rev-Up.  I got the impression that the airplane was not flying squares like I wanted it to, because it was slowing down as it went through the maneuvers.  I decided that I needed more acceleration out of the corners.  So I tried a Zinger 11 x 5 Wide.  I was flying off a stooge and the first take off was a revelation.  I flew like I was used to with the previous prop and was very quickly flying 10 or 15 feet behind the airplane.  In other words, acceleration from stand still was much greater with the 11 x 5 than with the 10 x 6.  The squares improved noticeably and the speed through the maneuver was much steadier. 

[Frederick Fielder]

OK gang, thanks for responding for now I know that a good prop choice can only be made while flying. Frederick

[john e. holliday]

Now you have hit the nail on the head.  As you go along in flying these things you will also find that two identical airplanes built at the same time by the same person will not fly the same.  DOC Holliday

[RC Storick]

Things like this can be found on the Stunt Hangar complete links page. Try this link for thrust calculator,

http://www.bmaps.net/software/thrusthpv20d.zip


Here is a direct link to the links page, also found at the top in LINKS

http://stunthanger.com/stunt_hangar_complete_links_page.htm

[Jim Morris]

Alot comes into play when choosing the correct prop for a given set up. The weight of the plane, find a starting point for a given engine. Example /  I run Stalker motors now. I know they will all be pitched around a 6. Thats where I start. Then go out and burn some fuel experimenting. trying differnt tupes of props with the same pitch, but I always try to run where the motor is happy.

[Paul Smith]

You could understand this a lot better if you had flown full size airplanes with both fixed pitch and variable pitch props.

An airplane like the Cessna 172, with a fixed pitch prop is truely efficient at only one speed. 
At zero airspeed the prop is over-pitch and doesn't work very well.
At high airspeed, it acts more like a brake than a prop.
Someplace around 100 MPH it hits "design speed" and does the best it can do.

Models are like that, hence, zero-speed thrust doesn't prove much.

Cessna made a plane called a "Hawk XP" which was pretty much a 172 with a variable pitch prop.  Controlled by a flyweight governor, it hunted for the best pitch at every airspeed.  Compared to the basic 172, it accelerated like a rocket, sailed right through 100 MPH and went a lot higher and faster with the same HP, just the proper pitch.

[Phil Bare]

Just a slight correction, the standard Cessna Sky Hawk has a 0320 Lycoming of 150 hp, while the Hawk XP has a 0360 Lycoming of 180 hp......
40 cubic inches of displacement and 30 more hp do make a difference.....
But any who, static thrust numbers translate directly to verticle performance. It is my opinion, and only my opinion that for c/l stunt we should prop our planes with the largest diameter and flatest pitch that the engine will handle and give us the speed that we want..........Phil

[roger gebhart]

Thurst. When everyone around you has a cooler full of beer and you don't have even one.   Meaning?    Your not very well liked.   Just thought I'd clear that up.