What is the best alt to fly at?

[kentb]

My friend (RV10) and I (RV9A) were discussing our trip to osh this year and the subject came up about what alt to fly at.

So the points came down to this:
If there were not wind factors from alt (I know that that will never happen) what is the best alt (lets assume that nothing was below, but sea level) to fly at to cover the ground the fastest.

The testing that I have done with my plane leads me to believe that somewhere around 9000 ft would be the best speed. I know up higher I can save some fuel.

He believes that the higher he flies the faster he will go.

Does anyone have real data to support what is the fastest alt to fly at?

Kent

[hecilopter]

I think for the normally aspirated engines, 7500 - 8500 DA is going to give the best speed/economy because that is where the engine is running at 75% or so. Go higher and save gas but lose speed (running out of HP) assuming no wind. On the flip side, go lower and fly faster but burn the gas (assuming wide open throttle/max RPM).

If you want absolute fastest speed, wide open throttle, 2700 RPM and fly as low as you dare, but get ready to buy some gas!

I think the reason is that TAS goes UP about 1% per 1000 ft over indicated airspeed, BUT engine HP goes DOWN (normally aspirated) about 3% per 1000 ft. And since speed is an exponential function of HP.... It is a losing battle going higher.

I put out a similar post asking these questions before, and I think George had the answers (he always does!).

[frankh]

You have to have the throttle WIDE open to avoid the pumping losses across the throttle plate...A partially closed throttle is a very inefficient thing to have.

I was surprised how much difference this made on the 7a

Frank

[Paul Thomas]

Fastest speed are achieved flying as low as possible.

[Kevin Horton]

Assuming you have a constant speed prop - decide what percent power and rpm you want to use, then go at the altitude where full throttle gives the desired power at the target rpm. The fuel flow for a given power will be pretty much the same no matter what altitude you are at, but the TAS for a given power goes up as the altitude increases. So, for a given power, you get better mileage the higher you go.

The situation is more complicated if you have a fixed pitch prop, as full throttle may give too high an rpm at some altitudes, depending on how your prop is pitched. The engine is most efficient at full throttle, but the airframe is most efficient at lower CAS (i.e. less than full throttle). Flight testing is required to see where the sweet spots are.

And, as you probably suspect from your automotive experience, you'll get better mileage if you fly slower than typical RV cruise speeds. But most people aren't prepared to fly around at such slow speeds.

[kentb]

Please correct me anywhere I go off...

As you go higher (air thinner) you loose engine power.
As you go higher, you decrease drag.
As you go higher, less engine power requires less fuel.
As you go higher, prop is less effective.

So...
If there are no winds aloft (not likely) when I fly from the west coast to OSH, will I get there sooner if I fly just high enough to clear the rocks (Rockies).

Or would we be better off flying at 17500 all the way.

Kent

[keen9a]

Fly high! If you are skimming the surface at high power, you'll be making at least one extra fuel stop.

[f1rocket]

Do you want to cover the ground the fastest (as you say) without regard to fuel burn? If so, the fastest is the lowest.

Now, I assume you have some concern over fuel burn, then it kind of depends on a lot of different factors. Generally for my fuel injected engine, the higher I go the better time I make with the least fuel burn, but that depends on other factors as well like weather, winds, etc.

It's like juggling Jello. Once day you squeeze it one way and the next, another way.

[kevinsky18]

Remember there is speed to be gained by not landing for fuel. Sometimes the fastest way there is to go slower and not have to stop so often.

[kentb]

I had hoped to see a couple of statements like "best alt for plane Y is at alt X".

Let me ask the question another way.

Lets assume that I am willing to spend between 10 gph and 7 gph.

If I get 10 gph at 5000 ft and 7 at 17500 ft. How much tall wind do I need to make it worth going higher?

Lets further assume (because it is true), that I can't stay airborne more then 3 hours before I explode.

I realise that the number will be different depending on the plane, but has anyone tested their plane enough to know the answer. I haven't done this much testing on mine.

Kent