Low speed propeller efficiency too optimistic in DCS?

[USARStarkey]

OK, good to hear that you agree about the expected difference in climb rate. I plan to do some more tests later this week to see if I get similar results as in the tracks I posted in the OP.

 

Concerning how I estimated climb rates I have two methods for this: One is using a C++ program I developed which can calculate speed, climb, turn, dive and acceleration performance etc. and another simpler Excel spreadsheet for ballpark calculations.

 

I post data from the spreadsheet below so you can see what assumptions I made for the 175 mph case. I could not paste in the table but I think you can see the assumptions made in the text anyway. Granted, Specific Excess Power (SEP) climb rate is only valid for small climb angles which is not really true for WW2 fighters but it is close enough to get the ballpark numbers for comparisons I think, at least to compare relative performance between 120 and 175 mph climb speeds.

 

 

P51D Mustang

 

Mass (Kg) 4445

Span (m) 11.26

Wing area (m*2) 21.79

Cdo 0.0176

Aspect ratio A 5.818614

Pi 3.141593

Cdi 0.01972

Cl 0.537017

Oswald factor e 0.8

Mach 0.229142

vTAS (m/s) 78

a (m/s) 340.4

ra (Kg/m*3) 1.225

q (N) 3726.45

loadfactor n 1

g (Kgm/s*2) 9.81

Propellerefficiency n 0.8

Engine power (hp) 1670

Tprop (N) 12606.36

Texhaust (N) 681.6327

Ttotal(N) 13287.99

D (N) 3030.397

 

IAS (m/s) 78

SEP climb estimate (m/s) 18.34845

SEP climb estimate (fpm) 3611.899

 

Propeller diameter (m) 3.39

engine rpm 3000

reduction ratio 2.088

Propeller rps 23.94636

Propeller advance ratio J=v(n*D) 0.96085

Cp=P(ra*n^2*D^5) 0.163208

 

Wait a minute, why is the CDo so high? The mustang had a CDo of .0163.

[BitMaster]

Why do you need Horsepower in your equation ?

 

The energy involved in your theory is already included by propeller data and rpm in correlation to atmospheric conditions. You could actually calculate the Joule from this.

 

In the professional Excel sheet we use to calculate climb rate you either need the RPM for a given propeller or the wattage and propeller and you get the rpm. Both seems to me a wrong introduction into the equation.

 

Also, the 0,8 efficiency for a pitch-changing propeller seems very very optimistic to me, especially

when you consider the shape of blades changes dramatically if you optimize them for either speed, pull low or high altitude. A constant rpm prop can hardly have a shape to fulfill all the requirements.

 

Just some concerns I have how close your equation can be.

 

Also, you don't need to have x,xxxxxx and then x,xx

As a rule for physic equations: the closest you get is the most uncertain number you have and that is load factor 1 without any comma. How do you calculate in terms of physical conventions ?

You can skip any stage behind..say x,xx cause many of your numbers are only two digits behind the comma, some have none or only one digit behind the comma.No need to have others with 5+ digits behind the comma, useless and a Prof would mark it red.

 

I am not arguing, just trying to understand you equation based on conventions of how equations

are presented.

 

 

Bit

[BitMaster]

...I reread your OPand follow-ups, the 0.8 eff. only seems to apply for certain conditions. That I can accept.

 

Most props don't get 0.8 unless at optimum rpm and altitude ( barometric pressure ) I guess.

 

It would be easier to recalculate and proof the efficiency if PITCH was labelled not in degrees but how far the prop would wind forward in 1 turn.

Then, you can easily recalculate your efficiency by rpm x PITCH : IAS = EFF

 

 

I am no aerodynamic engineer, just trolling around :) Tell me to shut up if I am of no use for your topic...I can accept that haha :)

Maybe I should add that I own 100+propellers and fiddle with them a lot, though it`s R/C I basically know what you are talking about

and we use Professional ( means, the club paid for it to have the right to use them ) excel sheets to calculate engine-prop combos.

 

Bit

Edited August 26, 2014 by BitMaster

[Pilum]

Wait a minute, why is the CDo so high? The mustang had a CDo of .0163.

 

First of all, while the Cdo has a big impact on top speed it does not impact climb rate that much. Secondly, where did you get the 0.0163 figure from? Sounds on the low side compared to other estimates IIRC. Anyway, In addition to propeller drag I take exhaust thrust into account which has a substantial impact on top speed. Maybe the estimate you mentions comes from only accounting for propeller thrust?

[Pilum]

...I reread your OPand follow-ups, the 0.8 eff. only seems to apply for certain conditions. That I can accept.

 

Most props don't get 0.8 unless at optimum rpm and altitude ( barometric pressure ) I guess.

 

It would be easier to recalculate and proof the efficiency if PITCH was labelled not in degrees but how far the prop would wind forward in 1 turn.

Then, you can easily recalculate your efficiency by rpm x PITCH : IAS = EFF

 

 

I am no aerodynamic engineer, just trolling around :) Tell me to shut up if I am of no use for your topic...I can accept that haha :)

Maybe I should add that I own 100+propellers and fiddle with them a lot, though it`s R/C I basically know what you are talking about

and we use Professional ( means, the club paid for it to have the right to use them ) excel sheets to calculate engine-prop combos.

 

Bit

 

The spreadsheet data I posted data from above uses both calculated and manually input data hence the different number of digits so don't worry about that.

 

I don't use the advance ratio or Cp in the calculation either. I enter the prop efficiency manually in the spreadsheet. My C++ simulation however uses a NACA dimensionless parameter Cs called the prop speed coefficient so it takes different speeds, power loadings, atmospheric conditions into account.

 

I appreciate that R/C props is most likely also an art but you have to realize that the aerodynamics for a WW2 prop is quite different due to Reynolds number effects so as I said before, read the NACA report I linked and I think you will get answers to your questions like why you can use 5 blade prop data to estimate 4. NACA engineers think you can and I think we can trust them too ;)

Edited August 28, 2014 by Pilum