Most Efficient Cruise Altitude and Speed

I know this is a loaded question, but I'm on a long distance cross-country mission right now and wanted to quickly ask the community that the ball-park cruise altitude and speed would be to obtain the maximum flight distance. this is with no weapons and 3 external fuel tanks. (about 85% max takeoff weight).

thanks!

Im not 100% sure at all.

But 280kn is the most fuel efficient cruise speed.. But i cant remember the height.. I would asume it is 20.000ft but it ofcourse depend on your weight and weapons as well. But 280kn maybe try to fly at different height and activate cruise speed and then write down how much fuel you use in 1 minute.

I've had a lot of luck around 280 kts @ Angels 45. Yeah, it takes a LOT to get up there, but at that altitude you barely use fuel.

Cheers!

I know the CF-18 Demo team cruised at 220 knots at high altitude for their trips, maybe that helps?

thanks everyone! sounds like i need to get way the f up there and 220-280 knots, i'll give it a try

4.2Alpha for Range and 5.6 Alpha for Endurance

4.2Alpha for Range and 5.6 Alpha for Endurance

thanks!

FYI, When FPAS page is implemented you'll get all that automatically.

FYI, When FPAS page is implemented you'll get all that automatically.

^This, the plane will suggest which Mach number and altitude are most efficient/longest lasting (there's a difference!) on that page, It's on their to-do list, be it a little way down. After the radar fixes, JHMCS, HARM, and LTWS i think.

FYI, When FPAS page is implemented you'll get all that automatically.

awesome can't wait

I've had a lot of luck around 280 kts @ Angels 45. Yeah, it takes a LOT to get up there, but at that altitude you barely use fuel.

 

 

Cheers!

280 KIAS at FL450 would equate to around Mach One, not exactly fuel efficient.

Climb is flown from IAS to Mach, so something like 300-350 to Mach .78-.82 would be more appropriate. Best would to apply the alphas listed and see how that converts to Mach at whatever altitude you are flying. The higher the better for range depending upon winds.

Optimum cruise for 42klb is about 37,000' pressure altitude. Optimum Mach for DI about 50 is M0.83. At 37,000' such Mach is 270KCAS/476KTAS on a standard day.

You should expect between 0.11 and 0.12 nm per pound fuel at height. To give an idea of sensitivity of specific range vs. Mach, 5% of efficiency is by varying the Mach from optimum -0.06 or +0.05.

As fuel burns optimum changes. By 30klb GW optimum is 43kft but similar mach and 0.14 nm per pound range.

I think I understand the climb schedule as 350 KCAS until M0.83 and then constant Mach climb the changeover happening at 25kft. The climb takes about 1250 lb. fuel, 6 minutes, 50nm.

Optimum cruise for 42klb is about 37,000' pressure altitude. Optimum Mach for DI about 50 is M0.83. At 37,000' such Mach is 270KCAS/476KTAS on a standard day.

 

You should expect between 0.11 and 0.12 nm per pound fuel at height. To give an idea of sensitivity of specific range vs. Mach, 5% of efficiency is by varying the Mach from optimum -0.06 or +0.05.

 

As fuel burns optimum changes. By 30klb GW optimum is 43kft but similar mach and 0.14 nm per pound range.

 

I think I understand the climb schedule as 350 KCAS until M0.83 and then constant Mach climb the changeover happening at 25kft. The climb takes about 1250 lb. fuel, 6 minutes, 50nm.

really great info, thanks!

How much of the extra fuel burned to get to altitude can be got back when you come back down? What's the most efficient way to come down?

Descend at idle.

Generally it's worth the fuel spent at high power to climb all the way to the most efficient cruise alt. The exception is very short trips where you don't have the time to get there before you have to descend again. In this case it's better to level off early and cruise for a bit at a lower alt.

Gotcha.

Generally it's worth the fuel spent at high power to climb all the way to the most efficient cruise alt.

But does that include using afterburner? I assume no?

Correct no burner. Apologies I should have been clearer.

On the hornet is it possible to switch the airspeed displayed on the HUD between True/Indicated/Ground speed? I know on the F16 that is possible. I assume the DCS hornet is showing IAS? On the DCS hornet at High altitude I always ignore the displayed airspeed and use the mach display.

On the topic of max range, good rule of thumb to use for is climb to 1/3 of the range to where you're going. For example: if a field is 90nm away, climb to 30k ft, then slow to max range AoA (4.2.) Another rule of thumb is start your descent at 1/2 of your altitude, so for the 30k ft example you would start at 15nm from the base and it is idle power until getting to max endurance AoA (5.6, it's the same AoA as best glide ratio) then starting the descent.

These aren't as precise as actually using the checklist to find what altitudes and speeds to fly but it works well for the most part and it's easy math to do in your head.

When "On the bones" start your Idle descent when the airfield gets to 4degrees on the HUD Pitch ladder :)

When "On the bones" start your Idle descent when the airfield gets to 4degrees on the HUD Pitch ladder :)

"On the bones" ? I haven't heard that expression . Can anyone define ? Thanks .

The absolute best range descent profile is cruise Mach until some calculated KCAS. The standard one size fits all profile speed is 250KCAS after an assumed M0.85 cruise Mach. The condition in which the calculated speed matches the standard one for DI 50 is a 42klb GW. You'll find that squeezing every last drop of fuel efficiency is impractically protracted to do regularly. With higher drags and lesser GW the optimization speed is less than 250, as low as 170KCAS in extreme large DI and low GW. Usually it probably won't be much less than 210 though. I doubt the savings between 250 and the lower calculated speed are significant.

More or less it'll be a constant AOA relationship near the L/Dmax angle (it would be exactly that with no engine thrust at all).

Initial Conditions:

37.5klb GW

50 DI

38.5 kft

Standard Profile:

M0.85-250KCAS

12.4min

340 lb fuel

71nm

Slope 5.1°

Max Range Profile:

M0.84-236KCAS

13.3min

330 lb fuel

71nm

Slope 5.1°

Woo, save 10 lb fuel takes 54 seconds more.

We can always call the tanker. :)

"On the bones" ? I haven't heard that expression . Can anyone define ? Thanks .

Australian fighter slang for out of gas .... "on the bones of your arse"