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Some data for efficient cross country

Carl Froehlich

Well Known Member
Some flight data for you guys that like such stuff.

Just ~1 hour to go for the 40 hour test period. Yesterday was the high altitude data run (up to 17.5K?). As part of this run I wanted some raw data to develop cross country cruise setting. At first blush it looks like ~14.5K? is where, for the same speed, efficiency starts to fall off.

For these typical high speed cruise settings I got 27.3MPG at 14.5K?, then slowly dropping off to 26.5MPG at 17.5K?.

I?m sure I could enhance the MPG numbers at slower speeds, but shoot fire, we fly RVs!

14.5K? screen shot attached.

Carl

68392008-B0-A2-4-A1-A-9-DD7-6-F02329-A82-AC.png
 
For these typical high speed cruise settings I got 27.3MPG at 14.5K?...

27.3 looks like knots converted to statute, or 23.7 NMPG.

Temperature probe appears to be running warm. Dunno about yesterday, but temperature at 14500 is forecast to be 32F all weekend.

Good performance. What engine/prop is that?
 
I am with Dan on the temp question. You should have been at or almost at the freezing level.

If your OAT probe is on the fuse anywhere, I bet it is getting heat from the exhaust. That will mean your TAS is reading a few knots faster.

Also were you slightly ROP and the EMS has been confused by leaning process you did? That flow at that DA looks more than 10dF LOP as a guess, based on scaling the RV10 engine.

And last of all have you done a GPS box to verify the TAS vs GPS, and hence any static error?

Otherwise that is one impressive speed for an RV7 on a 180HP engine.
 
27.3 looks like knots converted to statute, or 23.7 NMPG.

Temperature probe appears to be running warm. Dunno about yesterday, but temperature at 14500 is forecast to be 32F all weekend.

Good performance. What engine/prop is that?

We were flying at 14K yesterday over nebraska and colorado. Oat was 33 - 35.

Larry
 
Plugging in those indicated figures on a TAS calculator from the Net shows 177 TAS so pretty much spot on. It sure is warm outside at that height though. Plugging in ISA Std temp at that Alt only changed the TAS lower by 4 kts.
 
Otherwise that is one impressive speed for an RV7 on a 180HP engine.

It sure is, esp at 14.5k! At max power, 2600, 8k (DA), my 180 hp 7 is the same but higher FF. The BA prop is faster than my composite.

But - - I think Carl is has an 8, it's faster yet!
 
What am I missing..........

Isn't the Ground Speed (GS) under the TAS? and why is it only 137?
 
Guess

Probably because ground speed takes into consideration winds.

Carl, did you happen to try different RPMs at that altitude? Was the ~2400rpm the best all around efficiency wise? The CAFE reports ( who remembers those) would show different rpms and for the -9, if I recall, they were able to get similar MPG at high rpm and also at lower rpm at altitude. I recall an old twin Comanche racer would run her engines at 2700rpm everywhere all the time. Just leaned them at altitude and 75% power. Just asking for your perspective.
Good stuff. Thanks for sharing.

What am I missing..........

Isn't the Ground Speed (GS) under the TAS? and why is it only 137?
 
Probably because ground speed takes into consideration winds.

Carl, did you happen to try different RPMs at that altitude? Was the ~2400rpm the best all around efficiency wise? The CAFE reports ( who remembers those) would show different rpms and for the -9, if I recall, they were able to get similar MPG at high rpm and also at lower rpm at altitude. I recall an old twin Comanche racer would run her engines at 2700rpm everywhere all the time. Just leaned them at altitude and 75% power. Just asking for your perspective.
Good stuff. Thanks for sharing.

The short answer is no. This simple post about MPG was to share a snapshot of same day, same plane, same WOT/~2500RPM/LOP results at altitudes up to 17.5K’.

But, almost 20 years of flying RVs tells me that:
- For most you will get better fuel economy at ~2400 RPM without sacrificing too much speed. I gravitated to 2450 RPM as a compromise on the other two RVs.
- Above 14K’ the engine power drop off is significant, so I tended to compensate with bumping RPM up to ~2500. Now I have some data that says above ~14K there is also a decline in MPG.

Some other thoughts:
- Modern EFIS and EMS systems will display (what it thinks) is the engine ROP or LOP. On the RV-10 it alway showed LOP before the engine got there, and now on the RV-8 it shows ROP when the engine is 10 to 20 degrees LOP (as you can see from the screen shot where it says LOP under the EGTs but ROP next to 52% power). In other words, if you are only relying on the EMS telling you if you are ROP/LOP, you may be mislead. I guess those smart 29 year old software programmers need to sharpen their pencils.
- I agree the OAT is reading a little high. Perhaps I should have waited to let things settle out. But not the point of my post.

Carl
 
Today we flew the RV-10 from western Minnesota to Pennsylvania at 14,500 ft. Running 2100 RPM and leaned to 8.0 gph, we consistently saw 160 kts. and thereby reached our 20 NMPG talisman.



My testing shows that for my stock setup efficiency is maximized at low RPM. 2000 is even better than 2100. Testing also shows nothing but improved efficiency with altitude, though I haven't tested above 16.5.

We have spent the past two weeks hiking the mountains of Montana and Idaho, and took the RV-10 into mountain strips including Big Creek, Moose Creek, Schafer, and Ryan.



Even after doing this for seven years now, I continue to be impressed and feel so fortunate. There are very few four place airplanes that can hit the 20 NMPG mark, and I doubt any that can haul a load into and out of Schafer or Ryan and then do it. While cruising at 160 kts.



 
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Plugging in those indicated figures on a TAS calculator from the Net shows 177 TAS so pretty much spot on. It sure is warm outside at that height though. Plugging in ISA Std temp at that Alt only changed the TAS lower by 4 kts.

Yeah......You dont think I ran the numbers either? But that is making a big brave and foolish assumption that all the input data is correct.

Take 4 knots for temperature and say 10 knots for static error, and running slightly ROP not LOP (by fuel flow because you can trick the algorithm in the EMS) and then you have a 160-165 KTAS result.

This is not my first rodeo, in fact I learned the hard way 11 years ago and I have corrected dozens since, so it is not an uncommon thing. Just most folk never test the raw data.

As a very wise man once said........ "Trust but Verify"

:)
 
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