Propeller Efficiency Help

[Wagger]

Okay Stoney your the leader.  I'll march 100 miles for you just don't ask me to do the math.  And I thought Geometry was hard.

[Badboy]

Prop Effi @ stall:  15.8%

You should be looking for another mistake, because the prop efficiency at stall will be much higher and probably around the 60% mark. At first glance I would double check your induced drag values.

Badboy

[Badboy]

Brookes' formulas seem to be "rule of thumb" -sort also. They don't take account of the blade numbers, blade width, blade airfoil etc.
The equation 2.2 seems to need some iterative calculation.

The equation for prop efficiency can be derived from f=ma and leads to a cubic which has a relatively simple solution that gives the theoretical maximum thrust possible, similar to the curve in figure 3-17 on Brooke's page.

If any one is interested in seeing how the equation is derived and the solution, with an example, I'd be happy to post it.

Badboy

[Stoney]

You should be looking for another mistake, because the prop efficiency at stall will be much higher and probably around the 60% mark. At first glance I would double check your induced drag values.

Badboy

Ok, to back up a bit...

I computed e, K, and Cdi using the following formulas...

e = 1.78(1-.045A^.68)-.64 with A in this case representing Aspect ratio.  So e for the F6F-5 = .88 (this formulas was presented by Raymer as a more accurate approximation than the Glauert or Wessinger approximation)

K = 1 / pi*A*e or K = pi*5.5*.88 = .066

Cdi = K*Cl^2

Clmax at stall is 1.44 @ sea level, 11358 lbs and 96 mph IAS.  23.57 for dynamic pressure there.

So...

Cdi = .066*1.44^2 = .1363 at stall.

Di = (1/2pv^2)*S*Cdi or 23.57*334*.1363 = 1073.16 lbs. of drag.

One of these formulas must be wrong, I guess, as I've double checked the math manually.  Perhaps Raymer's "e" approximation is bad.  .88 seems a bit high, but the math works.  I know that .7 to .85 is considered the normal range.  According to this link:  http://karoliinasalminen.wordpress.com/category/oswalds-efficiency-factor the e-range presented by various authorities varies between .6 - .9.  Even with a .6 entered for e, the prop efficiency only rises to 22%.

[Stoney]

The equation for prop efficiency can be derived from f=ma and leads to a cubic which has a relatively simple solution that gives the theoretical maximum thrust possible, similar to the curve in figure 3-17 on Brooke's page.

If any one is interested in seeing how the equation is derived and the solution, with an example, I'd be happy to post it.

Badboy

I'd enjoy seeing that.

[dtango]

One of these formulas must be wrong, I guess, as I've double checked the math manually.  Perhaps Raymer's "e" approximation is bad.  .88 seems a bit high, but the math works.  I know that .7 to .85 is considered the normal range.  According to this link:  http://karoliinasalminen.wordpress.com/category/oswalds-efficiency-factor the e-range presented by various authorities varies between .6 - .9.  Even with a .6 entered for e, the prop efficiency only rises to 22%.

Raymer's equation is good.  It's basis DATCOM emperical data.  I'll have a look at your numbers as well.

[dtango]

Stoney- checking your calcs I think your drag is right but for the stall speed power required you're using the wrong amount of power.  2000hp is the power available.  Basis your total drag at stall of 1287 lbs that's 329hp power required.  Using an prop eff of .6 the power required is 549 bhp.  Hope that helps!

[Badboy]

Stoney- checking your calcs I think your drag is right but for the stall speed power required you're using the wrong amount of power.  2000hp is the power available.  Basis your total drag at stall of 1287 lbs that's 329hp power required.  Using an prop eff of .6 the power required is 549 bhp.  Hope that helps!

Yep, well spotted.

Stoney,

Raymer's equations are ok, as are your calculations, accept for the point dtango made. When you equate thrust to drag you imply an equilibrium (or a steady state) condition, and sustaining the 1g stall speed of 96mph requires much less than full power. As dtango has calculated, if the prop efficiency was 60% at stall, you could sustain that stall speed with 549 horse power. Full power at 1g and 96mph will cause the aircraft to accelerate, so you can't equate thrust to drag under those conditions.

What your calculations show, is that if you really did apply 2000bhp and were only able to sustain 96mph in a 1g stall, the prop efficiency would have to be as low as 16.5%, which is well below what it should be at that speed.   

Hope that helps.

Badboy

   

[BulletVI]

Speaking of the prop's in the game why not give us the ability to adjust prop pitch and also put fire extinguisher's in the bomber's engine's so that when the engine catches fire  we can put it out its a thought it may also improve some of the aircraft. Its a thought like

What you think of it HiTech ????

Be a bit more challenging wont it

[Lusche]

also put fire extinguisher's in the bomber's engine's so that when the engine catches fire  we can put it out

They won't help.

Engines do not burn in AH.

[hitech]

Speaking of the prop's in the game why not give us the ability to adjust prop pitch and also put fire extinguisher's in the bomber's engine's so that when the engine catches fire  we can put it out its a thought it may also improve some of the aircraft. Its a thought like

What you think of it HiTech ????

Be a bit more challenging wont it

You can already adjust prop pitch, it's called an RPM control.

HiTech

[hitech]

Stoneys mistake brought a question to my head.

What is the definition of a power on stall speed?

I see 2 possibilities.

1. Full power in climb stall speed.

2. Min power required to maintain level flight.

HiTech

[gripen]

Stoney,
A bit busy right now but do you assume that D=T at stall speed?

Edit: Ah... the others have allready noted this, sorry 

[Stoney]

Yep, well spotted.

Stoney,

Raymer's equations are ok, as are your calculations, accept for the point dtango made. When you equate thrust to drag you imply an equilibrium (or a steady state) condition, and sustaining the 1g stall speed of 96mph requires much less than full power. As dtango has calculated, if the prop efficiency was 60% at stall, you could sustain that stall speed with 549 horse power. Full power at 1g and 96mph will cause the aircraft to accelerate, so you can't equate thrust to drag under those conditions.

What your calculations show, is that if you really did apply 2000bhp and were only able to sustain 96mph in a 1g stall, the prop efficiency would have to be as low as 16.5%, which is well below what it should be at that speed.

Ok, the lightbulb came on with that last sentence.  So, for example, to say eta p at 320mph is 83% is probably close, since my calculation used 2000bhp for that, and at max level speed, power required would be 2000bhp.

[Stoney]

Stoneys mistake brought a question to my head.

What is the definition of a power on stall speed?

I see 2 possibilities.

1. Full power in climb stall speed.

2. Min power required to maintain level flight.

HiTech

For operational purposes, I'd assume stall speed at full power and in climb, since you're mostly concerned with the stall speed in takeoff configuration/attitude with power on.  That's why power-on stalls are part of the PPL syllabus right?  Just like they teach power off stalls in the dirty configuration to simulate stalls in landing operations?  I don't know how that would dovetail into the pure aerodynamic aspect, but that'd be my argument.