Prop pitch v thust.

[birdy]

A quiz for you blokes how can use one o them calculater thingys.
If a prop gives x amount of thrust at a given rpm, how much more thrust[%] is produced if the pitch is doubled and same rpm is reached?
A SCG would say 100% more, but i know phisics ain't that symple.

[CLS447]

Hey Birdy ! I would say alot more ....but the blades could be stalled with that much pitch.

I have gained 350 rpm's on my EA81 by installing dual port heads & intake....so I hope to need at least 2 more degrees of pitch to get it back to the 5000 rpm level it was at before the new heads went on.

I don't have a thrust checker anymore but I am hoping to gain quite a bit of thrust.

On your wasa RAF.....What diameter prop & pitch are you running?

Do you know what your average gross weight is ?

[birdy]

....but the blades could be stalled with that much pitch
Wot much pitch??? Riteo, we'll say we stared with 8* and doubled to 16*, but then stalling will depend on rpm too, so we'll say at 5000rpm ona 4banger rotax with wotever reduction they run.
Just a couple of facts to make it more real, this prop did stall once, at 5300 engine rpm with 18*, ina virtical decent, so we could say that its safe from stalling at 16* full tap and flying, and greater than 15% less power.

The wasa has a 3bl 62" ivo mag set at 18*.
Wouldn't have a clue how heavy it is Chris, but itd be less than 260kg empty.

[CLS447]

How could you tell the prop stalled ?

Why did you go with the Magnum Ivo & not the medium ?

Why such a small diameter prop ? Was it all in the name of lowering the thrustline ?

So , your 914 Rotax spins a mag Ivo set at 18 degrees up to 5300 rpm ? I would think that the turbo is kicked in at that point, Eh ?

[birdy]

How could you tell the prop stalled ?
Thats easy, and you can't ignor it.
I was at bout 100'agl and 'hovering' ona small breeze at near full power when the small breeze got smaller. Not wanting to decend [ coz there was big trees just below] i opened the 914 to full tap, and it did start to reduce the decent rate for a sec, but then all hell broke loose when it stalled. I don't know wot rpm it went to but it didn't sound good and the thrust dissapeared.
Like i said, you can't ignor it.
I'v also stalled a 68" light ivo with a little 462 rotax 2banger, believe it or not.

Why did you go with the Magnum Ivo & not the medium ?
62" was all the prop room i had, and ivo was the only one with a chord that could handle a 914 at that size.

Why such a small diameter prop ? Was it all in the name of lowering the thrustline ?
A lowered TL was a side bonus. The engine frame on the machine i bort with the 914 only allowd for a 62" prop when fitted to the wasa. So instead of the hassel of buildn another frame, i found a prop that suits.

5300 rpm is at full tap, and its the max pitch i can get on the adjuster.


Anyone got an answer to the origional question?............ please.

[Doug Riley]

Birdy, on paper a prop blade is just an ordinary wing. Wings, prop blades, rotor blades, fan blades, H-stabs and other similar devices all work the same way.

In all of these gadgets, a graph of lift plotted against angle of attack is theoretically a straight line. Double the AOA at the same airspeed, and you double the lift. In a prop, lift takes the form of thrust. Your SCG physics is correct. On paper.

Many things mess up the simple picture. Depending on how well twisted the prop is, you may get more inboard stalling when you crank up pitch. The whole prop may stall, too, especially at low airspeeds, as you describe.

Short, stubby airfoils (IOW, ones with low aspect ratio) don't really have straight-line lift curves. The lift curve sags down at higher AOA's, thanks to increasing tip losses. So your thrust won't quite double unless your prop blades look like sailplane wings.

Different prop pitches put your powerplant's "sweet spot" at different airspeeds. An airfoil is most efficient at a pretty low AOA -- 3 or 4 degrees. The actual AOA that a blade of your prop "sees" DEcreases as the gyro goes faster. The prop will hit an efficient "groove" at some airspeed.

For easy cruising, you'd like the "groove" to be at a speed that's a sensible one for the aircraft. An extremely high-pitched prop has a "best" airspeed that is too high for an open gyro. If you're limited in prop diameter and you need to load your engine more, you're apt to be better off adding blades than cranking more pitch... especially if you're doing mostly low-level maneuvering rather than cruising.

[birdy]

Thanx champ.
So, the ferel could have nearly twice the thrust as before, bloody ell, no wunder it wants to do loops when i open the tap. Even if its only 70 poocent more, its made a big difference.
Yeh Doug, if you took into account the props profile, the increase in fed air diasturbance coz of the extra 'suction' and a million other facters, i can see why it wouldn't be as symple as 100 poocent.
Mind you, it still ain't noisy at full tap. So i'm gess'n its still pretty efficiant with the wide chord and added pitch.

[LARRYEBOYER]

I have found that 1 degree of plus or minus pitch will add or subtract 200 rpm.
My Ej-22 with the hot cam setup is getting broke in now and it dooooo run!. I have a 3 blade warp 68". I am turning 5650 at 13 degrees. i am going to add 1 degree in hopes of reducing my RPM to 5450. At that, I will be right on the cam. Tons of thrust!!!:)

[Aussie_Paul]

Birdy, prop stalling, due to too mkuch prop pitch, for the average recreational gyro jocky occurs during the take off run untill there is enough airspeed to reduce the AoA of the relative airflow to the prop.

All you clever people, another question leading on from Birdys original in post 1. How much extra power would be requires to maintain the same rpm at double the prop pitch.

The wasa weight would be on the ASRA certificate of registration and the 2 seat gyro compliance form.

Aussie Paul.:)

[birdy]

PB
I know
Don't know
I know, just can't remember.

[LARRYEBOYER]

I added 1 degree and the engine now turns at 5350. interesting stuff!