What is the best alt to fly at?

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

Altitude and fuel/speed

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!).

Around 8k or plus....

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

Fastest speed are achieved flying as low as possible.

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.

Hummm..

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

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

It Depends

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.

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.

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

Kent,

There's no black & white answer on this, and anybody who gives you a hard & fast answer is just narrowing down the versatility of your airplane. "What altitude to fly" depends on so many different factors...not just wind & fuel burn, but also temperature, turbulence, weather, terrain, length of the leg you're flying, how much O2 you have onboard, emergency landing options (i.e. "cone of safety"), etc.

I can tell you that I will more often pick a "comfortable" altitude than I will pick the most efficient altitude. i.e. if I'm bucking a headwind, I have been known to climb up 2 or 4 thou to get a smoother ride. I'd rather fly 10 minutes longer in comfort. Other times my mood sways the other way around...and I'll fly right off the deck if that's what it takes to cover ground quickly. What mood are you in?

And realistically, in an RV on a 3-hour leg, what's the drop-dead biggest delta in ETE that you're gonna see? In all likelihood about 20 minutes. Honestly.

I use Weathermeister's "Flight Optimizer" feature pretty religiously to get a "sense" of the optimal altitude for speed and/or economy -- it spells it right out for you! But admittedly, that's just one data point of many that go into my decision-making process. For example, let's say Weathermeister tells me that 6500' is the best altitude for both speed & economy...but I gotta climb over Mt. Wilson on my way. So I'm up to 8500' already...do I descend down to 6500' once I pass the ridge? Usually not!

Just my 2 cents.

I fly high

I stay above most turbulence and avoid the altitudes with other GA aircraft. Today, I climbed to 12,500' going from Yuma CO to Meadow Lake (110 nm apart) and 00V at 6840'.

You should go fly at at various altitudes calculate your TAS and fuel burn so you can get MPG. Then you can trade off speed/time/fuel burn.

I just fly high and don't worry about it.

Ain't no Cessnas where I fly

"What mood are you in?"

Dan's answer wins in my book. Too many variables to assign a "hard" altitude. Flying the Midwestern US during the summer, I just climb until the temperature is comfortable.

John Clark
RV8 N18U "Sunshine"
KSBA

John Clark said:

Dan's answer wins in my book. Too many variables to assign a "hard" altitude. Flying the Midwestern US during the summer, I just climb until the temperature is comfortable.

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Does turbulence ever play a factor in your search for "comfortable" flying altitude? Do you factor in an altitude that will get you above the thermals for example? Or do you just fly through them and take them as they come? I ask because I hate the turbulence these thermals create and do as much as I can to avoid them when I fly currently. Do any of you flying your RV's now take this turbulence into consideration when making your decisions?

Try it a different way...

The great thing about RV's is they climb well even at higher altitudes.

If you have GPS with ground speed, time to destination and a fuel flow Gage,

make an altitude change and see if it helps or hurts.


Your best speed, no wind will generally be the altitude that you can run 75% with the throttle wide open. Problem is, is the wind can have a large effect.

Sometimes an altitude change of just 2000 ft can make a significant change in your ground speed.

This is one of the things that takes new RV pilots (at least ones that have just flown low performance aircraft) a while to get proficient at. Many pilots aren't accustomed to moving so fast over the ground (traveling though different airmasses so quickly) or having the ability to reach altitudes in 10 minutes that can take a C-172 five times as long to reach (if it can get there at all).

In a nut shell...there isn't a one altitude answer. The conditions can have a lot to do with what altitude is best.

BTW, don't be afraid to experiment if you see your ground speed change. Traveling at RV speeds it doesn't take very long to cover enough sky for the air conditions to have changed quite a bit. All depends on how stable of an air mass you are flying through, and what the winds are doing that day.

dan said:

I can tell you that I will more often pick a "comfortable" altitude than I will pick the most efficient altitude. i.e. if I'm bucking a headwind, I have been known to climb up 2 or 4 thou to get a smoother ride. I'd rather fly 10 minutes longer in comfort.

Click to expand...

I totally agree with Dan

The majority of the time I will chose comfort over saving a few minutes of time or a couple gallons of fuel.

But if you are in a comfort zone...then you can experiment to get the best performance possible.

RVbySDI said:

Does turbulence ever play a factor in your search for "comfortable" flying altitude? Do you factor in an altitude that will get you above the thermals for example? Or do you just fly through them and take them as they come? I ask because I hate the turbulence these thermals create and do as much as I can to avoid them when I fly currently. Do any of you flying your RV's now take this turbulence into consideration when making your decisions?

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I don't fly an RV, but my evil twin - er, good twin does

Heck, yes, bumps play a role in my decision on a final cruise altitude, particularly here in TX in the summer. Getting above the mixing layer is easy in the spring and fall, since it's rarely above 6000ft. In the summer, it's higher, just above the base of the little (and sometimes larger) puffies that form in the afternoon. Be on the ground by noon or 1p and it's less of an issue.

In the spring we get a lot of turbulence induced by the low level jet coming in from the Gulf. The jet can run 35 kt as low as 3000ft. Often you have the choice of going faster with bumps or slower with smooth air. It really depends on my mood and the distribution of the bumps - If they are all consistent but tolerable, I'll put up with them. If I get strong bumps mixed in, I'll look for other altitudes. Hitting one's head on the airplane is never pleasant.

Bumps

RVbySDI said:

Does turbulence ever play a factor in your search for "comfortable" flying altitude? Do you factor in an altitude that will get you above the thermals for example? Or do you just fly through them and take them as they come? I ask because I hate the turbulence these thermals create and do as much as I can to avoid them when I fly currently. Do any of you flying your RV's now take this turbulence into consideration when making your decisions?

Click to expand...

Absolutely. I try to go for the best "ride." The short wing of an RV is a rough ride in bumpy air. I guess the considerations in planning altitudes for a long cross country leg are 1. terrain clearance (obviously) 2. Wind. 3. Weather (clouds) 4. Temperature. 5. And as Dan said "the mood you are in" or what there is to see. I'm always up for going a little lower than optimal if the view is good.

John Clark
RV8 N18U "Sunshine"
KSBA

Can not be answered

kentb said:

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

Click to expand...

"Whats the best altitude to fly at?" What ever altitude you want, it's the joy of flying. U-da man, the big cheese the captain of the ship.

Kent: Your question is of course un-answerable, at least in a definitive way, as in the sun always sets in the west way you want.

The question about is the wind favorable enough at higher or lower altitudes to justify flying at those altitudes is a BIG QUESTION MARK, mainly because you rarely have perfect known actual wind, save for the FT's (winds aloft forecast). Asking ATC for PIREPS is a good technique, since so many planes now can give you winds aloft at altitude with GPS and a little E6B calc.

The answer as some one said is with flight planning. Also as Dan said there are practical aspects of flying, convenience and comfort. However from what your wrote you sound like you already are going up into the teens to have a look. Just start taking records and calculate the trade offs. Clearly the more wind on the tail the better.

Old wisdom: Fly fast when flying in head winds and slow when you have tail winds. Also tail winds never help as much and headwinds usually are worse, because head winds last longer.


The answer lies not only in the wind but weather, HP, winds, temps, weights, leg distance, terrain. To eliminate some variables assume generically: piston plane non-turbo engine, no wind condition and leg distance not a factor (long legs).



[1] RV's make more power higher up because of the better induction and exhaust and higher cruise speed.

dan said:

Realistically, in an RV on a 3-hour leg, what's the drop-dead biggest delta in ETE that you're gonna see? In all likelihood about 20 minutes. Honestly.

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True but 20 minutes may be 3 gals of gas at $4/gal or $7/gal.

However JUST PULL THE BLACK KNOB BACK AND THE RED ONE AS WELL AND YOU WILL SAVE FUEL. The real saving is playing the winds. If you have O2 you and have the option on the west coast you can sometimes get into the Teens and get 30 kts you might not get in the less rarefied atmosphere. Other wise I am with Dan, what feels good.

Also flying high you can't see as much. I asked an older 747 flight attendant what her favorite plane was. DC-7. Why? She said: "We flew lower and you could see more of the country side".

Another Wrinkle to the question?

While working toward an RV I'm curently flying a Long EZ. Even at 10 to 11 thousand feet I can still redline my tach. Can this possibly mean I am still developing full HP? I asked the question on the Lycoming forum and got a couple strange answers. I asked Catto, the prop mfg. and he said not to worry as he ran a Pitts at 3000 RPM for 2000 hours with no problem. So two questions:
1. Am I still making HP at altitude with an NON injected O-320 and should fly as high as I want to take advantage of the speed or not and
2. Is running at 2700 RPM or above bad for the engine?
Thanks all,
Dave

Well there is Lycomings recommendation

RVadmirer said:

While working toward an RV I'm currently flying a Long EZ. Even at 10 to 11 thousand feet I can still red-line my tach. Can this possibly mean I am still developing full HP? I asked the question on the Lycoming forum and got a couple strange answers. I asked Catto, the prop mfg. and he said not to worry as he ran a Pitts at 3000 RPM for 2000 hours with no problem. So two questions:
1. Am I still making HP at altitude with an NON injected O-320 and should fly as high as I want to take advantage of the speed or not and
2. Is running at 2700 RPM or above bad for the engine?
Thanks all,
Dave

Click to expand...

No you are not making full HP, that is impossible. If you have a MAP gauge you could calculate your % power. There is no way you are pulling 29" MAP at 11K. Yes, you are making MORE hp due to higher RPM (for a given MAP), but I am sure your RPM's are not high enough to overcome the low MAP you no doubt have a 11k. Every 100 RPM gives you about 4-7 hp. For each additional 100 RPM you get diminishing returns due to engine "pumping loss", basically the power curve, rpm v power.

What you can say is your prop is under-pitched for efficient cruise. Key word is efficient. It may be a great takeoff, climb and overall prop.

Is flying above 2700 rpm bad? Well Lyc will say so.

Lycoming says 320's and 360's must be torn down if RPM exceeds 2970 RPM!

Lyc has a service bulletin on the topic of going over red line: http://www.lycoming.com/support/publications/service-bulletins/pdfs/SB369J.pdf

If you plow thru it you will see over 10% they get upset. 10% is 2970 rpm. The note is #6 = remove engine and tear down! ouch!

If you are over 5% (2835 rpm) you have to do all kinds of inspections on page 4 and 5 of the ref SB above, than repeat in 100 hr intervals or something. They also say you will not get max life from the engine.

My opinion is RPM within reason, like 2,800 rpm (which is less than 5% over red line - 2,700 rpm), but there is no doubt you will wear the engine at a faster rate. My personal 2,800 rpm limit jives with Lycs level of terror v rpm. I personally would not make a habit of running near the 10% over-speed range but would not tear my engine down either for a little "my bad".


Experimental guys (as Catto said) have been flying around at 2,900 or even +3,000 rpm routinely for decades. The Reno formula guys turn their little Continental 200's well into the high 3 grand range. I mean its up to you, but I would try to keep it at or less than red line for routine cruise, but than that is me. I would be OK with 2,800 rpm cruise, but I'd watch it like a hawk, do oil analysis on occasion, flying over red line RPM constantly. Of course temps and pressures in range is a must; over heat and high RPM is a recipe for disaster.

Also high RPM lowers prop efficiency. In your case tweaking a little power back would be fine, but if you have a gas bag, try to fly higher; it would be interesting. However I would not climb to rarefied atmosphere to suck gas just because the RPM is a little high, but if the RPM bothers you, than I'd just adjust RPM with throttle. I take it a new Long-EZ prop is not in the budget as you build your RV.

Closing the throttle a little is not really that horrible. As was mentioned you might loose some efficiency with the throttle plate closed slightly, BUT MAY BE NOT? Actually a little butterfly angle can add turbulence to the induction air, which atomizes the fuel better and leaning more efficient. That is the theory any way. It can also act as a blockage and bias the fuel more towards a bank of cylinders causing uneven leaning. Knowing the Lyc a 0.1-0.3 in-hg map change will not make a big diff. You will go slower but you will burn less fuel, probably more efficiently. If you want to GO FAST, leave it wide open.

Do some flight test and fly it different ways and record your data and figure out what is best. However I am uncomfortable with RPM over 2,900.

Bonanza performance

I'm building an RV8, but I have an old Bonanza. The charts for the Bonanza show that the performance increases as altitude increases up to about 9 to 12 thousand feet. Efficiency also increases with altitude up to about that altitude. I generally fly about 9 to 10 thousand feet as the best combination of efficiency and performance. There is little to be gained above that altitude in a normally aspirated plane.

A Fixed Pitch set of data

This chart is the Tiger Pilot's Handbook data massaged to get speed/fuel flow/miles per gallon information for many density altitudes.

I don't know what "best" would be... it depends which of the three variable you want to optimize...

The changes with altitude should be fairly representative of a Fixed Pitch installation, with the speeds scaled slower than an RV...

http://home.earthlink.net/~gilalex/Tiger/performance chart.xls

You will need Office Excel to read the chart.. a reader only sw is available from Microsoft...

gil in Tucson

Says it all...

The Tiger chart indicates at 10,000' you can save 3 gph with only a 5 knot loss - works for me! Will be doing some testing. I've seen some great speeds at altitude. Was not aware of the ineficiency of the carb at partial throttle. Haven't read my Lycoming Engineering manual from Sacramento Sky Ranch yet.....
Sure is fun to fly high. If you're not a Cub or T-Craft flyer high is fun. Not a lot of traffic and good visuals.
Thanks to all for the help!
Dave V.

Fly high...

RVadmirer said:

The Tiger chart indicates at 10,000' you can save 3 gph with only a 5 knot loss - works for me! Will be doing some testing. I've seen some great speeds at altitude. Was not aware of the ineficiency of the carb at partial throttle. Haven't read my Lycoming Engineering manual from Sacramento Sky Ranch yet.....
......
Dave V.

Click to expand...

And if you keep the engine at the same % power, you get more speed for the same fuel flow... or better mpg.... depends how you look at it...

63%/64% on the engine --
2,000 ft = 122 kts TAS
10,000 ft = 130 kts TAS

I made the chart for a MPG discussion on a Tiger mailing list...

gil in Tucson