I was reading Sparky Imeson's book and he was talking about a rule of thumb he uses to determine the most efficient altitude to cruise at during a cross country.

He says...

Do not climb more than 10 minutes for every hour of enroute flight time. So if rate of climb in the plane is 500 ft./min then 5000 ft. AGL would be the most efficient cruise altitude for a 1 hr trip if weather and other constraints allow.

Do any of you use this rule of thumb?

If not, how do you figure out how high to fly and conserve the most fuel?

The classic text "stick and rudder" makes roughly the same arguement in a chapter devoted to that topic. I usually follow that rule, but I also factor in the winds aloft. With a cruise speed of 80mph, I can get hammered by a relatively small headwind. Other factors for determining altitude are where the turbulence is, hostility of the terrain, cloud layers, and sight seeing potential.

tom

No, there are just too many other variables involved in choice of altitudes, such as ceilings, terrain, hemispheric cruising altitudes, etc.

Guys who write books seem to be compelled to give us lots of "rules of thumb" for our book buying bucks.

Most efficient cruising altitude is something you can calculate, based on the above, PLUS a calculation of power settings, mixture settings, etc.

Oh, yeah, and it'll depend on the plane as well.

MTV

I base VFR cruising altitude on how long the flight is, the terrain, direction of flight, cloud bases and wind/turbulence. Coming back from the Mojave a few months ago, we actually did better by descending to an altitude that netted us more range due to the wind.

In a no-wind case (rare for me, it seems), the NA 185 seems to like 6500-8500' for fuel economy vs. TAS. In NV, I often go higher usually due to terrain and in AK, I go lower depending on the items in the first paragraph. I generally like rules of thumb, but this one for cruising doesn't compute for me.

From an efficiency/economy perspective, for normally aspirated engines, around 8,000' MSL seems to offer the best mix of speed (reduced atmospheric drag) available power and terrain clearance...unless you're west of the Rockies! Out here, it doesn't take long at all to get up there. Heck, on hot days, density altitude puts you there before you even get out of bed.

I think the point is that depending on the length of the trip, if you spend any more time climbing, you're wasting time. I also shoot for a density altitude of ~7,500 for overall efficiency on LONG flights. When jumping over to another field, I regularly beat friends with faster aircraft because I only climb as high as the terrain dictates while they spend time at climb speed to get up to altitude. If you are climbing at Vy for 10 minutes before getting the nose down and accelerating to cruise, and I'm at cruise 1 minute after takeoff, the slow citabria might be able to hang with you faster types if we're only going 60 miles.

No rules of thumb I know of, sounds like book selling tips to me. Here are two for you though.

1. Climb as slow as possible, just enough to clear terrain if no favorable winds.
2. Climb as fast as possible if climbing into strong favorable winds.

Like MTV says depends on several factors, what your flying makes a big difference.

If you can get this to run, it's kind of a cool little utility:

http://www.x-plane.com/cruisealt.html

I'm not an "always by the book" kind of guy, but I recommend using the Aircraft Manual to "help" determine fuel consumption. Also Winds aloft forecasts can help with planning, but remember they are only forecasts, and they are based on True North not Magnetic.

As others have said here, terrain, turbulence, and sightseeing are other factors, as well as airspace and NOTAMS. If flying over bodies of water, keeping in mind how high you'd want to fly to glide to shore.

My experience in picking a cruising altitude is let the gps decide.

I set my gps so it always shows my ground speed and vertical speed (much better than a VSI even IVSI).

After I lift off I trim the plane to the book Vy. Then I adjust it so gives the best vertical speed. This is going to change because of weight, Den alt, etc.

Is I climb as fast as I can I keep a mental tally of the ground speed. Looking for trends of ground speed. Using this technique I usually overshoot the best alt by 1-2,000' (Doh! my groundspeed just took a dive.) then settle back into the best altitude.

Now that I have the 396 with XM weather I can get very lazy and just look on the wind aloft page (updates every hour) and pick the best altitude that way.

-Todd Giencke

1SeventyZ wrote:If you can get this to run, it's kind of a cool little utility:

http://www.x-plane.com/cruisealt.html

Must have X-plane flight sim for this to work.

-Todd Giencke

tgiencke wrote:

1SeventyZ wrote:If you can get this to run, it's kind of a cool little utility:

http://www.x-plane.com/cruisealt.html

Must have X-plane flight sim for this to work.

Crap, I can't even get it to work now because I only have version 8.64.

Most normally asperiated piston powered airplanes true out the fastest at around 7500. I think you will find that you are more efficient, the higher you go. My Maule is an odd one, it trues out the fastest between 9 and 11 thousand. It's got too much wing for a good traveling machine, I guess.
You can download the free flight planner from www.flightprep.com and with an internet connection it will download winds aloft and if you put accurate burn rates and climb and cruise airspeeds, it will calculate total fuel burn and time to destination allowing you to pick themost efficient altitude. Just remember GIGO

JC wrote: the most efficient altitude to cruise at during a cross country.

Efficient????

Shit, I just shove the throttle forward and try to keep from hitting the rocks. Hopefully the airplane knows what it's doing.

Gump

GumpAir wrote:

JC wrote: the most efficient altitude to cruise at during a cross country.

Efficient????

Shit, I just shove the throttle forward and try to keep from hitting the rocks. Hopefully the airplane knows what it's doing.

Gump

Gump, You're killing me

See ya, Bub

I make quite a few flights between S49 and 7S9 I usuallly just use 8500 going west and 9500 going east. That is if the cloud cover will let me do that. I can't tell the difference when I go to lower or higher on my gal./hour. I fly a 182 B model.

I seem to recall a "rule of thumb" that most efficient altitude engine-wise was that at which full throttle leaned out resulted in 65-70% power. usually 6500-7500 feet. Not taking winds aloft, etc into account.
Of course, Murphy's law would dictate that the most engine-efficient altitude is that at which headwind component is maxed out. Doh!

Eric

I just eyeballed the miles/gallon column in my POH to find the best mpg for the highest cruise rpm.

Since all the Rules of Thumb I have ever read require arithmatick. I have narrowed my rules of thumb down to one.

NEVER let your Thumb get lost!

So far, Gump is the onliest one that has hit the nail on the head, "rule of thumb" wise.......

I like it.

MTV