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Oct 23, 2005, 10:20 AM
Thread OP

engine thrust and propeller geometry relationship and effect on stalling speed

hi everyone
im new here..a noobie as well.. ive been browsing aroun for some time and i thought that i couldn't find the answers i require, so here goes

1. does anyone know the relationhsip between the thrust produced and the geometries (pitch and diameter..what missing anything?) of the propeller. Im doing an experiment using a rig ive created for this project. The rig is as show in the pics below. [id be glad to post the details of the rig upon request..] Id like to compare my results with you guys' data if any if possible

The test rig basically enables an engine of various sizes to be mounted on it, and started. when engine is started, servo is used to control the throttle. forward thrust values are read from an attached 2kg spring scale of 10g increments. Ill be collecting test data from several propellers for this projhect. Ive just done one run on it, using an FS-52S OS 4stroke engine with a master airscrew 11x7 propeller, more props will be tested this week

2. What would be the effect of different propeller geometry [pitch and diameter] towards the stalling speed..After a thorough intenet search, i believe that stalling speed is only affected by the attack angle and the load [weight?] do correct me if im wrong

hope u guyz can elp me ere
Last edited by panaroL; Dec 12, 2005 at 02:13 AM.
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Oct 23, 2005, 10:38 AM
Don't spend too much time testing props on a stand. All you get are static measurements, props behave differently when moving into the air. There is no direct relationship between prop diameter, pitch, thrust and stalling characteristics (gliders don't have props, yet they fly). As long as your engine/prop combo is able to fly the plane over the stall speed, it will fly. Factors governing stalling characteristics are : CG, wing incidence (attack angle), wing airfoil and speed moving through the air.
Oct 23, 2005, 10:39 AM
Ascended Master
Sparky Paul's Avatar
Your test rig is certainly up to the task.
Looks a lot more sophisticated than anything I've used.
Static thrust determines how much each propellor can put out at zero airspeed, but it's the flying that determines which prop the airplane needs.
For normal flying.. non-3D, the prop doesn't have much to do with the stall speed, that's all airplane geometry and weight.
Oct 23, 2005, 10:53 AM
Thread OP how do we [email protected]@find out the stall speed without actually fylying da thing to its stall speed
"the prop doesn't have much to do with the stall speed, that's all airplane geometry and weight"
is there any mathematical equations or graphs i could refer to?

about the static thinggy..its not a problem here as i am actually trying to find out the static thrust..later when im done ill put it in somekinda wind tunnel and walla~ we are simulating flying conditions
Oct 23, 2005, 11:22 AM
If you insist on going technical about it, do a web search on : "NACA airfoil". You should be linked to technical articles explaining airfoils, lift coefficient, airfoil wind tunnel testing and Reynolds number.
Oct 24, 2005, 07:38 AM
This article might give you some ideas...
Oct 24, 2005, 07:39 AM
cubber nailed it with the stall characteristics. The prop has nothing to do with it. A good example is that a glider will stall if you exceed the AofA. As long as the aircraft has relative wind, it will fly. The wing doesn't know if it is in the air or not. This is why they have to be tied down. If wind is moving over the wing with enough force, it will create lift. It's Bernoulli's law of venturi (buy it if you want, if not, don't =) )
You can also stall from going to fast. But in you have to be pretty much super-sonic for this to happen. Unless you are flying rotory of course.

All the prop really does is provide relative wind to the airfoil and a whole lotta thrust, except in the case of a j3 of course =).

After sitting in a class room for 6 hours a day, 5 days a week, I FINALLY get to use this crap.
Oct 24, 2005, 02:13 PM
Registered User
Originally Posted by norcal tom
After sitting in a class room for 6 hours a day, 5 days a week, I FINALLY get to use this crap.
Uhum.... And after that you never get to use it again. Been there done that for over 30 years in the aerospace business. I remember designing parts for a certain airframe (doors). Our Chief Designer kept walking around with the drawings of another airframe from a competitor and asking around if it looked that already, or almost. Simply because he knew that the other design worked. Go figure.
Oct 24, 2005, 03:06 PM
yeah.... but MAN it's fun when you get to
Oct 26, 2005, 11:14 AM
Thread OP net connection is soo slow~~

hey thanx guys
preciate da infos
nyone has nytin to add
pliz feel free to do so
thanx in advance..again
Oct 26, 2005, 01:24 PM
Registered User
Originally Posted by panaroL
nyone has nytin to add
Wonder where you are located. Your picture shows you in a certain posture. A position btw that not many people can do comfortably except when you are from SE Asia maybe....
Nov 13, 2005, 03:03 AM
Thread OP
me is in malaysia
anyone ahs to add summore plizz?

btw does anyone know the effects of thrust on speed?
Last edited by panaroL; Nov 13, 2005 at 03:09 AM.
Nov 15, 2005, 03:18 AM
Thread OP
help pliss..
Nov 15, 2005, 05:56 AM
Actually all of this brings up some good points that it takes a while for most of us to realize- static thrust vs pitch and theoretical speed is very complex but critical to performance!

Most of us start out with a 40 size trainer like an LT-40 and a 10x6 or 11x6 prop on a 2-cycle engine. In theory this will generate around 6 lbs of thrust and a speed of over 65 MPH. But an LT-40 has tons of drag and won't fly near that fast. So some of us learn that if the prop is replaced with a 5 pitch it takes some of the load off the engine allowing to spin faster which generates more thrust at the expense of theoretical speed. In this case the additional 1/2 lb of thrust is well worth the loss of a few (5-10) theoretical MPH because the drag of the trainer wing won't allow it to go that fast anyway. In fact the plane may actually fly faster because of the increase in thrust. As for the stall speed of the trainer wing, its a non-issue.

Then we try 3-D and learn that a really low pitch on a 4-stroke engine is the hot ticket. A 4-pitch long-blade prop on an engine that makes its power below 10,000 RPM gives tons of thrust and who cares if the theoretical speed is only 35 MPH... the stall speed of the wing is even lower than that on the trainer.

Then on to warbirds and it gets a lot trickier. Try a 6-pitch prop on a 4-stroke on a P-51 and see what happens (been there done that). You have to carry speed to make a warbird fly right because the stall speed of the wing is high. So we go to an 8 pitch or 10 pitch or more and find we have to reduce the prop diameter to keep the RPM up. Much less ground thrust but we need the speed to keep the plane flying. And in this case ground thrust readings become less meaningful because most of the drag on the prop is caused by the pitch. When in flight at speed there is much less resistance on the prop and the engine unloads (speeds up), which increases the thrust... it gets very complex and prop selection becomes a trial and error exercise.

I am sure that there are formulas for all this and the effects could be plotted but I've never seen it done. Thrusthp is about as good as it gets.

So if panaroL (or anyone else) wants to take it on as a project, more power to them. I'd like to see the results of the study!

Nov 15, 2005, 09:28 AM
Registered User
downunder's Avatar
One of the things with props is that they're actually a high speed wing which is why they have an airfoil section and some have better sections than others. But being a wing they also have a stall speed above a certain angle of attack. If the prop has a certain pitch along the length of the blade then this angle of attack has to increase as it gets closer to the hub. At some point on the blade this angle of attack is going to be above the stall point for that airfoil section and from there on inwards it's not producing much thrust at all. It's not quite as bad as it sounds though because there's a relative inward airflow (even ground running) which tends to reduce the effective angle of attack. In flight, the airspeed adds to this inward flow and reduces the angle of attack even more so more of the prop length begins to work as a wing.

If you start with a very high pitch prop then probably most of the blade is stalled on the ground giving low thrust (and prop drag) but as airspeed picks up more and more of the blade unstalls, reduces prop drag, adds thrust and revs until prop thrust equals airframe drag and you've then reached max level flight speed.

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