In the scenario suggested by FF, I believe he is saying the engine "slips" on the ground, so the engine is artifically unloaded to a degree and rpm is higher. Once launched, as the airspeed increases and the angle of attack of the relative wind on the prop blades becomes less, the prop would then bite as you say, the load will increase suddenly, and the engine may load down again. With a slippery airframe he says the airspeed may increase further to allow it to unload again. In a regular scenario, the engine launches slower, then unloads in flight as the angle of attack decreases on the prop.

Now, I thought the drag was higher with stalled blades, but perhaps this is not stalling but what, cavitation? I dunno, I'm no fluid dynamics expert.

MJD

Ever covered the hose (inlet or outlet) on a vacuum cleaner? It speeds up, less load....the impeller blades are stalled.

Stall a wing on a plane? less lift, less wing loading.

Cavitation..... happens on props for boats only.

I still like your analogy's.

Your correct, and then there are those special planes/engines that scream on the first staging on the ground and stage even higher again in the air, that's when it sounds like the world's coming to an end.[sm=tongue_smile.gif]

Oh yeah, I guess cavitation has to do with liquid phase and vapor phase and air pockets and compressible versus noncompressible and... arrrgh my head, back to sanding.

Good enough, I can visualize it. Never had it happen to me, but I am relatively new into running higher pitch stuff so have not seen the situation. But I'll know what is up when I do! Good to know.

MJD

I only can imagine that the props slip when they work on very thin air, like high altitude areas (Denver or areas like that) where it is know that engines present loss of power.

Hey guys!
This discussion has nothing to do with the OP question.
Since we do not use oversquare props, blade stalling is a non-issue here, and a can of worms better left closed. All analogies don't help much in understanding, and just try to explain what happens without exactly knowing what happens. There is good software (free) out there that shows what higher pitched blades mean to prop thrust and prop disk airspeed relations. It certainly is not an analogy with a clogged vacuum cleaner (with a radial pump). That device follows quite a different set of rules.

PS,
Check out Gylesaero for a nice calculator that lets you see how thrust (and power demand) deminishes with airspeed
http://www.gylesaero.com/aeronerds/s...or/index.shtml 
Power demand is a bit misleading, because at very low lift coefficient, when plane speed exceeds pitch speed, the flatbottom foils tend to have a very lousy lift/drag ratio.

I don't think the plane would take off if the prop was stalled, unless only part of the prop is stalled. When the prop is turning on the ground the air speeds up and has velocity before entering the prop. It would likely only be stalled when revving up quickly from idle.

The load decreases as the plane speeds up, that is why the engine RPM goes up in flight, not down.

The fan speed increases because it is not allowed to move air. So a small amount of air recirculates between the blades and around the edge of the fan. The fan increased the pressure on the outlet side and decreases the pressure on the inlet side, the fan blades are now operating in a partial vacuum, even if blocked on the outlet side, so there is less load. It has nothing to do with a stalled condition.

Prop slip is defined as the difference in aircraft speed and prop wash airspeed. Thinner air decreases the load, but it slips at all speeds especially on the ground.

I have a nice example of foil stall.
When extending the AoA of the blade, the lift does not drop all that much, and there is sufficient lift to propel a plane. At certain aspect ratios, there even is a second lift peak! However, the drag at these high angles becomes very large, and a lot of engine power is wasted in just stirring the air. The realm of low drag is the region before the first lift peak has occurred. That is where the engine can unload, and produce best power/thrust ratios.

For the most part I thought the OP's question was answered and was happy at post 17 so I threw in a stimulating subject. But back to the OP's original post, he stated he was turning a Top Flight 15X6 @ 9150 and he was getting 7 pounds of thrust with his fish scale, however the calculator shows 10.49) pounds static thrust (#3 bulky plane). That's quite a discrepancy, do you think he may need a better fish scale?


We do use a lot of square and over square props, APC sells a heck of a lot sizes of those in pattern, pylon and sport props. Granted, not as many of those are sold as say an 11X6.


The Gyles calculator looks like it's exactly what I was looking for but it doesn't seem to work, at least on my computer, does anyone else have the problem? There may be a problem with that site.

Yes a vacuum cleaner (axial/centrifugal impeller) follows a different set of rules, but it got my point across to MJD...I think anyway. I have momentarily stalled ducted fans with my hands blocking intakes or outlets on both IC and electric, they too speed up.

The engine is pulling a S.E.5a WW1 biplane, the first test was done with wing assembled.
Lately I have done a second test with out wings, same engine, same prop but the fishing scale is now reading 8.4 pounds of thrust. Then I believe that sticking with the results of the Pe Reivers original chart is not a bad idea.
The fuse with out wings represents a cleaner air flow but the big radiator at front and weight of the fuse represents a lot of drag which can not be determinated in an uniform equation because the shapes of all different airplanes so there is no way to say that this method is close to the real facts, but it is a good method to have an idea when testing different props. that is why I will build my test stand with the fisher scale fashion.

You would be correct if you were pertaining to something like a 11X6 prop. But not a 10X10, I guarantee you the engine load is greater when the plane is at 60 mph than it is static. As (if) it gets to say 150mph it will unload and the rpm will increase. It's very obvious to the ears with two totally different type engines and planes using that prop.

That may have something to do with your lower #'s if it's huge in relation to your prop. But I think you have way more thrust to weight than the full scale had.

Lots of info, search: stalled axial centrifugal impeller

Yeah it did, when you mentioned that I said "oh yeah.." to myself. I was unsure of what was going on because I started thinking about the drag increase when stalling a wing, but wasn't worried enough about the nitty gritty to query it further. I decided to settle for an analogy and move on.. eager to get back to the bench.

MJD

This is a front picture of the S.E.5a.

Draggy isn't it?

In your case, static performance will be significant..

Nice bipe, I hear those are fun. Does it live up to Dynaflite's usual reputation for lightness?

MJD

Yeah, nice! A lot of work into that one.

The full scale takeoff weight was about 1950 lbs and the 200hp ("a" version) engine probably produced about 900 lbs of thrust, so I think you are way ahead with the thrust to weight your model has.
The full scale was no slouch either.

Thanks for your comments guys.

Not really light, it is around 14 pounds, I will recheck the weight with someone else s scale. Manual stats around 12 lbs.

Test flight will be in around 12 days. I do believe that wing loading will help a lot with this particular biplane.

The original ones were powered by a small 4 cylinder engine, they were build with wood, fabric and little metallic sheeting, mostly at front.

Propeller was bigger and I do believe that rpm were low.

I looks like they may have all had 8 cylinder engines, even the prototypes.

http://en.wikipedia.org/wiki/Royal_A..._Factory_S.E.5

The plane of yours has a very large and draggy frontal surface. It would need two extra scales in my calculator
1-racer
2-clean midwing
3 trainer
4 parasol wing/clean bipe w/struts, butw/o flying wires
5 dirty ww1 bipe

If you have thrust that is half the plane weight, it will fly OK if prop pitch is at least 0.5xD

You are confusing load with power I think. If you use a brake dyno you increase load by putting more friction or brake to the engine, it slows down, that is increased load. But best power is developed when the load is relatively light.