Best Speed for a given Fuel Flow

[Norman CYYJ]

Originally Posted by videobobk

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 .

[AlexPeterson]

Originally Posted by Norman CYYJ

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.

[Finley Atherton]

Originally Posted by petehowell

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

[elippse]

'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?

[petehowell]

Originally Posted by videobobk

Pete,

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

Bob Kelly

Hi Bob,

I plan to hit Greensburg with my daughter this spring - we'll stop down for a visit to OVO.

[petehowell]

Originally Posted by Finley Atherton

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?

[Norman CYYJ]

Originally Posted by AlexPeterson

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.

Thanks for the link, interesting read.

[rv6ejguy]

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.

[rzbill]

http://www.eaa1000.av.org/technicl/p...s/perfspds.htm

Are we talking about Carsons speed with the addition of efficient engine settings (IE low rpm, WOT)?

[gmcjetpilot]

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.

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.