Best Speed for a given Fuel Flow

petehowell · #1 03-09-2008, 04:27 PM

Hello,

I am wondering if anyone has taken and analyed data on the how to get the best speed for a given FF or best FF for a given speed. With a constant speed prop, obviously RPM is a variable.

When I can't or don't want to run high(7500+) I set my engine up for a LOP cruise at 20.5 MP and target 130kts TAS on 5GPH. I was running my engine at 2270 as this is where I get my best LOP cruise up high. I played around a bit when taking some data today and found that I could consistently get 132 KTAS at 20.5MP using 2100 RPM and working the mixture to 4.9-5.0 GPH. This was at 5500 ft indicated/3200 dens/-13C OAT. If I varied the rpm above or below this, I'd drop a bit of speed. The engine is very smooth at 2100 and almost spooky quiet. I did not take enough data make any solid conclusions.

Just wondering if anyone has looked into this in detail and what is the most effcient RPM to run the prop. To be honest, the speed diff is not great at diff RPM, but if you can be more effcient, why not?

Any thoughts?

Details:
RV-9A
ECI O-320/carb/dual EI
TAS on Dynon calibrated with 3 way GPS and NTPS spreadsheet
Dynon FF consistently within .3 gal at fillup

randylervold · #2 03-09-2008, 04:43 PM

Pete, you think like I do. While I guess I'm too lazy to record reams of data so as to arrive a rock solid conclusions with which I can defend to all comers, I have definitely noticed some trends while operating both my O-360 RV-8 and IO-320 RV-3B. I too play with different variables to test the results and I've noticed one clear trend... efficiency improves the slower you can turn your engine. I too like the power settings of say 21" MAP and 2100 rpm, and comparing that with a power setting such as say 18" and 2400 rpm (computes to the same % power) that fuel flow will be less with the "square" low rpm setting. With the RV-3 I can easily look at the mpg readout on my EFIS as an efficiency metric. This makes sense really, pumping losses should be less with lower engine speeds. Add electronic ignition and LOP operation and you can really boost the efficiency, I love it.

BTW, my normal 55-65% LOP cruise power settings usually yield about 28 mpg (statute). I can push it to 30 mph with 19x19 or with a slight tailwind (I know, that doesn't count). So I guess I'm not the only one who hates it when the fuel flow reads anything over 7 gph? ;-)

videobobk · #3 03-09-2008, 05:01 PM

Pete,

My 9A seems to like higher altitudes and will do better in mpg around 8500. Like you, I haven't really done enough to graph anything out, but that is how it appears. I can also get higher top speeds, but that is another story, and I need lots of data there. Since I have a Catto, I can't play with rpm, only altitude, throttle setting and carb heat. 21" does seem to be one of the best settings, and I get very similar results. Pulling carb heat evens out my EGT's and allows more leaning.

BTW, when you going to make it down this way (OVO?)

Bob Kelly

steveKs. · #4 03-09-2008, 05:05 PM

A couple days ago after discussing fuel efficiencies on this forum so..while on a short cross country, I pulled back to 2100 and 19 inches at 5500' and was seeing 6.2 at 150 knots.

It looked to be a good tradeoff at approx 4 gal/hr flow for 20 knots, if I am not in any particular hurry...

Mike S · #5 03-09-2008, 05:26 PM

I hope this will help---------maybe giggle someones memory. Finish out the details I cant quite pull out of my poor tired brain.

I remember reading in a book by one of the WW2 greats-----Hoover, Yeager, Boyenton, Anderson----about ferrying fighters IIRC.

Anyway, the method was WOT, keep on adding prop to min RPM, and lean, lean, lean.

As this was a ferrying activity, speed wasnt as critical as fuel efficiency. Seems that there was a pretty large fuel savings.

Anybody else remember this, or an I just having a senior moment here??

az_gila · #6 03-09-2008, 05:48 PM

I know this isn't an RV...

....but here are my Tigers fuel/speed numbers charted and turned into miles per gallon.

http://home.earthlink.net/~gilalex/T...37;20chart.xls

There is not a lot of difference... higher is a little better, but not that much.

Note the chart has speeds in knots, but miles per gallon in regular miles for auto comparisons...

I would expect a fixed pitch RV would not show much difference either....

... just much faster speeds and better MPG...

gil A

PS The chart numbers are from the handbook, but do seem to be what I actually achieve.

hevansrv7a · #7 03-09-2008, 05:52 PM

Quote:

Originally Posted by steveKs.

A couple days ago after discussing fuel efficiencies on this forum so..while on a short cross country, I pulled back to 2100 and 19 inches at 5500' and was seeing 6.2 at 150 knots.

It looked to be a good tradeoff at approx 4 gal/hr flow for 20 knots, if I am not in any particular hurry...

About 28 sm/gallon at a very high speed.

6.2 gph is about 37.2 pounds/hr. If the BSFC is .43 then the HP is 86. Using 85% of 200 as a guess for max available thrust HP, then to go from 86 to 170 HP would be an increase of 1.977. The cube root of that is 1.255. Apply that to 150 kts and you get 188kts or 216 mph.

That's pretty fast, but within Van's published numbers. I'm ignoring that you can't use LOP safely at max power. All these calculations are rough, but just for a context check. Induced drag is ignored, for example and .43 BSFC is just a guess.

AlexPeterson · #8 03-09-2008, 06:24 PM

Quote:

Originally Posted by Mike S

I hope this will help---------maybe giggle someones memory. Finish out the details I cant quite pull out of my poor tired brain.

I remember reading in a book by one of the WW2 greats-----Hoover, Yeager, Boyenton, Anderson----about ferrying fighters IIRC.

Anyway, the method was WOT, keep on adding prop to min RPM, and lean, lean, lean.

As this was a ferrying activity, speed wasnt as critical as fuel efficiency. Seems that there was a pretty large fuel savings.

Anybody else remember this, or an I just having a senior moment here??

You might be thinking of Charles Lindbergh - he went to the Pacific as a civilian and taught the P38 guys how to get range. I'm sure that there were others, but that is one that I'm aware of.

Mike S · #9 03-09-2008, 06:35 PM

Quote:

Originally Posted by AlexPeterson

You might be thinking of Charles Lindbergh - he went to the Pacific as a civilian and taught the P38 guys how to get range. I'm sure that there were others, but that is one that I'm aware of.

Alex-----you are probably correct-------sure fits into the bits of what I could remember.

One of those tidbits I filed away for "someday".

Thanks.

videobobk · #10 03-09-2008, 06:46 PM

Gil has some very interesting numbers here. It seems that mpg figures change with power setting much more at lower altitude. At 12-13,000', power setting doesn't make much difference. I plan on watching this when I get around to seriously testing at the higher altitudes. This will probably be a factor even with the faster RV's. As always, plan all your flights to be downwind. BIG savings there!

Bob