Best prop RPM speeds

zkvii · #1 11-17-2008, 02:51 AM

Hi,

Things are beginning to settle down with the 7A and I'm now trying to get the most bang for $$ on fuel. How do I find the "best/most efficent" RPM speed?

For example - this weekend I had an hour X-Country at 10K which allowed a bit of data collection, but I'm no further forward with my understanding.

I have a 7496 Blended airofoil Hartzell prop - is there a single 'best' rpm for it - or does it depend on DA and speed for the optimum speed? If so how do I find that and or work it out.

Second linked question - I was able to maintain 21.2" MAP at 11500' DA @ 50F which gives me about 75% at 2650 rpm and 65% at 2350.

If I want to get a good fuel burn 63% seems about right with just under 28 lph and 161 TAS @11.5K DA, but I have to pull the MAP back to 20.8" with 2350. (BSFC of about 0.39

)

Down lower obviously this is magnified even more - so am I better pulling RPM back to 2000 or just the MAP back further? Somehow restricting air into the engine goes against what feels like the best option.

I've got all the raw data, but it is difficult to graph / visualise when I have 3 input variables - MAP, RPM, Fuel F -> IAS.

Thanks for any pointers,

Carl

Steve Sampson · #2 11-17-2008, 03:16 AM

Carl, what are you collecting the data with? If you have an AFS 3400/3500 I have written some software which will help you analyse what you have. Its not so sophisticated, but better than a raw eyeball. Its an Excel macro.

I am finding it really hard. Yesterday I decided to calibrate the ASI using Tascalc, but the air just wont stay still enough to achieve anything meaningful.

FWIW my O-320 160 hp appears to have a very efficient low power cruise at 22.5" and 2050rpm with an MT prop. The fuel comes back to around 21.5lph but then I am crawling around the UK at only 3000'.

zkvii · #3 11-17-2008, 03:41 AM

Hi Steve,

GRT demo recording and one of my other projects - GRTDecode ( http://www.rvproject.gen.nz/grt/ ) - this extracts the raw data very well, but none of the computed data - things like TAS, DA, % pwr are stored. TAS and DA aren't too hard to work into an Excel column but % pwr (which would reduce the MAP/RPM variables by one) is rather more complex. With % then a 3D plot might work.

I already have most of the Excell stuff working and with the FADEC / Hartzell I have a 2350 min RPM for greater than 22".

Crawling around down low was the return trip from the weekend - going was 1.5 hours, 47 litres coming home was 62 litres 1.6 hours, 215 nm.

Interesting that 2050 rpm with the MT works well, I haven't tried below 2350 much - mine is just so smooth there.....

Carl

Steve Sampson · #4 11-17-2008, 04:31 AM

Carl, you have taken a different approach to me.

What I have done is write a macro which first of all charts all the relevant data and then adds a line anytime that I am not changing altitude, where the mph divided by the lph exceeds 6 miles per litre. There is some smoothing of the fuel flow and cutting out of very low power operation also.
(The grey line at the top is the bit that I look at.)

This has the effect of highlighting when it was operating efficiently. Subsequently I mess around in that area on the next flight and have slowly refined what I do. Its a bit empirical I have to say, but it has helped me find this one area, 2050, 22.5 3000' 21.5/22lph which is a good way to cruise in the UK for efficiency in my -4. Curiously, Saturday, I went to 6000 and used that as a starting point but so far have not managed to equal that efficiency! I think I need to make carb heat a variable also. I have one flight a few days ago where I left the carb heat on by accident and looking back it looks as though the fuel distribution improved dramatically and the efficiency also. i discarded the data at the time because it was not what I had meant to do. Another area to explore. (Oh well! Penicillin was discovered by accident!)

Overall, I am finding a lot of wood and few trees. Still air is also very difficult to find!

n468ac · #5 11-17-2008, 05:44 AM

You really need to read this book.

http://www.propellersexplained.com/

Steve Sampson · #6 11-17-2008, 07:46 AM

So before I get a chance, what is the point I have missed? Have I made a howler or what? Please share your knowledge. What is the point you want me to take on board?

thanks.

airguy · #7 11-17-2008, 07:50 AM

From an efficiency standpoint at cruise with respect to the engine, you are better off maintaining wide open throttle (WOT) and controlling power by setting RPM via the prop control. From the standpoint of the prop, lower RPM's are more efficient due to parasitic drag and Mach compression effects on the prop tips.

zkvii · #8 11-18-2008, 11:57 AM

So getting back to the original question....

Prop theory: For any given prop is there a peak efficency point? Does it change with DA or can IAS just be used? Is it just RPM based or is it actually power based?

Where roughly is it on a Hartzell BA or how can I test and find out?

Thanks,

Carl