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Best Speed for a given Fuel Flow

petehowell

Well Known Member
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
 
Lowest rpm possible

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? ;-)
 
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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
 
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...
 
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??
 
MPG...

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/Tiger/performance chart.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.
 
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Pretty good!

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.
 
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.
 
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.
 
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
 
Pete,

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.

Bob Kelly

I don't understand why you would want to run your engine with hot air. I always thought that the colder you could get the air into the manifold the better. I think that would be the same as running your engine at a higher density altitude .
 
I don't understand why you would want to run your engine with hot air. I always thought that the colder you could get the air into the manifold the better. I think that would be the same as running your engine at a higher density altitude .

It would be, but there are other variables in play. This thread should help explain why. The basic problem that heat can help solve is notoriously uneven mixture distribution.
 
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.

Pete,
You are most likely are familiar with Curve # 12883 in the Lycoming 0-320 Operators Manual. This is for an injected 0-320 but it clearly shows that FF decreases considerably with reduced rpm for the same hp using the same method of leaning. For example, leaned to Best Economy, FF for 65% at 2,700 rpm is 8 g/h and at 2,000 rpm is about 7.1 g/h. So 2,700 rpm uses nearly 13% more fuel for the same hp as does 2,000 rpm. The only remaining variable is the relative efficiency of the C/S prop in turning hp into thrust at various rpm? This would obviously vary with different makes and models of C/S prop??

Fin
9A Flying
 
'Hate to brag...no, strike that! I like to brag, that at 14,500 dalt, 2760 rpm, TAS is 197 mph at 5.6, 5.7gph. I can slow it down to 150 mph TAS at 2100 rpm for 3gph, but why?
 
Good stuff Fin

Pete,
You are most likely are familiar with Curve # 12883 in the Lycoming 0-320 Operators Manual. This is for an injected 0-320 but it clearly shows that FF decreases considerably with reduced rpm for the same hp using the same method of leaning. For example, leaned to Best Economy, FF for 65% at 2,700 rpm is 8 g/h and at 2,000 rpm is about 7.1 g/h. So 2,700 rpm uses nearly 13% more fuel for the same hp as does 2,000 rpm. The only remaining variable is the relative efficiency of the C/S prop in turning hp into thrust at various rpm? This would obviously vary with different makes and models of C/S prop??

Fin
9A Flying

It looks like slower is better - supporting Chuck Lindberg's undersqare ops - The question is in the last part of your message, is there an RPM where the prop is most effcient, and how important is that?
 
Theoretically the most efficient rpm will be some composite value between torque peak where VE is highest and a somewhat lower rpm where frictional losses are lowest. Pumping losses are more a function of throttle angle than rpm. Since Lycoming engines have a very flat torque curve, it is probably safe to assume that VE does not vary much over the range of say 1800 to 2400 rpm. Logically this confirms the theory that very low rpm at WOT will give the best SFC on these engines (assuming even mixture distribution).

Lindberg was the one teaching low rpm/ high MAP for best range in WW2 on P38s. We have to remember that these were turbocharged engines where we have more control of MAP than with atmo engines. It has been shown that energy is recovered from the exhaust in the form of boost which reduces pumping losses on the intake stroke. This is where a fair proportion of extra hp comes from in turbo/supercharged engines. In effect, the compressor's work is forcing the piston down rather than having the crank pull it down as in an atmo engine. By using high MAP and low rpm we get the best composite of high VE and low frictional losses.

We do have to figure prop efficiency in here though. Your prop may not be well optimized for this flight condition so that adds another variable.
 
Hair on fire

Slower than most RV'ers are willing to fly. I'll take the philosophical approach and say there is NO one speed that is "efficient"; it depends on your point of reference.

There is min speed which is not efficient*
There is max speed which is not efficient
There is best cruise, trade a little efficiency (range) for time**
There is best endurance which is max time aloft or close to L/D max.
There is best long range, which is slow but gives max 'real' air mile range.

* efficient = going some where with min or reasonable fuel.
**'Best cruise' is like a carson's speed. Most of us throttle back a little, lower RPM if we have c/s prop, lean the heck out of it (which lowers HP and also speed) and try to pick a good altitude with wind and terrain in mind. That is going to get you about 75%-90% there, ie efficient on a given day.​

In a vacumn, no other factors of wind. weather or terrain, the speed is going to be down around best L/D, but not many RV'ers like flying at 100-125 mph? The old wise man said, "Life is too short to fly slow". I know a few pilots who fly C-140's and Cubs and love flying low and slow. Yes it takes them 3 or 4 days to get to Oshkosh from the West Coast, but they don't care.

Truths, the more HP your engine makes the more it burns, Doha! No kidding right. However the way to get that HP or power down can be done with the throttle (and mixture) OR it can be done with altitude.

The problem is we fly in the sky with winds and WX and people. So picking a good altitude takes airmanship. It might be any where from on the deck to 18,000 feet? General rule fly faster into head wind and slower with tailwind. Also not many people LOVE sucking O2 from a mask. I flew a turbo twin for a company, un-pressurized and sucked O2 all day, sometimes. It plays havoc with the nose and nozzle passages. Also there's hassle and cost. So if you limit your day time max altitude to 12,500 or 9,500 night (#) you will limit your choices. If going for long range trip, minimizing burn, with any tail wind of any kind, light solo weight, usually flying into the "teens" is a good thing. It does mean flying slow however. You really have no choice because the engine is making less power and you will go slower even WOT. Lower powered RV's, at gross, are not going to like the teens, especially the shorter wing RV's (RV4/6) on a hot day. 8,500' to 10,500' is a good ball park, compromise altitude to start with for almost any RV.

(#) -Consider staying lower at night or using O2 due to loss of night vision, especially if you smoke. I don't smoke but night vision suffers with lack of O2.

Engine and propeller affect this as well. Of course c/s props really shine in cruise efficiency since you can dial your RPM as desired. In general lower RPM in cruise does increase prop efficiency. Fixed pitch props, you get what you get, which can be OK, depending on the pitch you choose. Of course you have to trade-off or balance t/o and climb performance with cruise performance with a fixed pitch prop. In general the more HP you have the higher you can fly efficiency. Or flip it around, the more HP you have, the higher you might need to fly to get max efficiency.

Yes efficiency considers both the wing/airframe, engine and pilot/crew. In general the airframe has a min drag speed, which is close to the min HP required. Again less HP less fuel burn. The engine tends to be efficient at WOT but well under 75% power, so that means the engine likes to fly at 8,500' or higher. This affects the first one, min drag TAS. Mix in winds/WX you have best altitude. The last factor is the pilot and crew, "we can't fly one twenty five". If flying 50 miles at 125 mph, not 190 mph, the difference is about 8 min. Now if flying coast to coast, it is a 7 hour difference, basically a whole extra day. Another day in a hotel efficient? What is your definition of efficiency? One is two 7-8 hours days (or so) and the other is three 7 hour days.

I hope that is as clear as mud. The fact is there is NO one speed. :D

PS: Go back into the RVator and look for some flight test Van did in a RV-9 with a O-235. For fun he tried to fly slow, max endurance/range speed and see what the fuel burn was. It was very low. I don't recall, but he did not optimize it but it was down in the 3gal/hr! Have you ever flown at 3.x gal/hr? It just shows how efficient the RV9 is and you can lower fuel burn with that black and red knob-ee thing-ees. Go up and flight test it and see how little you can burn. I know one of my cars has a instant MPG readout and a cumulative all time MPG and trip MPG. It was very accurate. It did make me aware of how to drive more efficiently. The best way to get more efficiency is be a better pilot, more aware of economy of flight.

The subtle efficiencies have nothing to do with speed, like better navigation. Some say a autopilot saves time because it tracks more precisely, since there is no s-turning your way to destination. I agree. No wind or traffic takeoff? Than land in direction of flight. Start let-down in a timely fashion to arrive and pattern with out adding power and driving around......Pick time of flight so there is no delay for departure or arrive. I do this at air shows. I never leave with the gaggle after the show. Too much fun watching the departures anyway. You can save lots of fuel with better planning.
 
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Finally, I get to correct George

Slower than most RV'ers are willing to fly. I'll take the philosophical approach and say there is one speed, it depends on your point of reference.

There is min speed which is not efficient*
There is max speed which is not efficient
There is best cruise, trade a little efficiency (range) for time*
There is best range, which is slow and gives max range
There is best endurance which is max time aloft or close to L/D max.
...
(emphasis added)

The speed for best endurance /time aloft is approximately 76% of the speed for L/D max and the Carson's speed is 1.316 times the L/D max speed. L/D max is approximately the best range speed, too, with corrections for wind. But that's what you meant, I know. Carson's speed in an RV is way below 75%. Even in my old C-150, 75% was beyond Carson's.

Of course, the original question wasn't about this but about how to best use the engine for a given speed. Or at least that's what I think he was asking. CAFE says Carson's speed is the optimum speed for efficiency.
 
About time

(emphasis added)

The speed for best endurance /time aloft is approximately 76% of the speed for L/D max and the Carson's speed is 1.316 times the L/D max speed. L/D max is approximately the best range speed, too, with corrections for wind. But that's what you meant, I know. Carson's speed in an RV is way below 75%. Even in my old C-150, 75% was beyond Carson's.

Of course, the original question wasn't about this but about how to best use the engine for a given speed. Or at least that's what I think he was asking. CAFE says Carson's speed is the optimum speed for efficiency.
I was seeing if you where paying attention....ha ha :D Yep great points. Back to my real point in too many words, short, efficiency is an elusive thing and varies based on many factors, not the least is the human that wants to get there today. Some pilots define efficiency as getting there faster.
 
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a question

Hey!, any of you guys knows what exactly is the Carson's Speed?, I need a clear concept. Thank you in advance.
 
Anyway, the method was WOT, keep on adding prop to min RPM, and lean, lean, lean.

This is consistent with what John Deakin preaches in his articles, although I was never able to discern exactly what "minimum RPM" meant. Kinda freaks me out when the engine gets quieter and quieter...

erich
 
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