I'm trying to figure out what L/D max gives you in jets, because I know it's different in some ways than pistons...

Does L/D max speed give you max endurance regardless of how many engines are operating? Also, it gives you best range with one engine, right?

Thank you!

Matt

 

Matt:

Not necessarily. This took me a while to understand in USAF pilot training; maybe my explanation will help.

L/D max is most 'bang for the buck.' It does give the most speed for an amount of fuel flow. Analogy: You're in your car, in 5th gear, driving 45 mph. Car is burning about 1.5 gallons an hour, or 30 mpg. L/D max is best mpg.

Max endurance is the lowest possible power setting where you can stay airborne. It's not costing you much gas, but you aren't making much distance, either. Analogy: you start your car, put it in 1st gear, and leave it there, idling your way down the street at 5 mph. You can't go any slower (ie, reduce fuel-flow) without stalling the engine. In idle, you're burning 0.5 gallon an hour, which is only 33% of what you were burning before. Sounds good...car will run 3 times longer before flaming-out.

But, at 5 mph, you are only getting 10 mpg.

There is usually a sizable difference between L/D max and Max Endure in swept-wing jets as induced drag becomes huge (due to tip vortices and the resultant losses/drag) at high angles of attack.

In a maneuvering fighter, this could be because you put "g" on the jet (once took an F-4 from 500 kts, in Afterburner, to 230 kts, in about 9 seconds, by pulling 8.3 g). In a civil jet, the angle of attack would equal getting slow.

It happens in straight-wing jets and props, but I don't recall it being as a significant difference.

I should add that in jets, where fuel is a significant portion of your total weight, the speed varies with weight. As you burn it off, it usually decreases the best speed. I haven't touched on best altitude, wind, etc.

As to the engine-out questions: L/D varies a little in engine-out situations, as the remaining engine(s) may not be capable of achieving the original "best fuel economy" speed. That is, they may not have enough thrust to hit that speed. Example: the T-38's best range speed is pretty close to 320 kts (no-wind). With one engine, it will be closer to 275 kts, depending on altitude, because one engine can't make it go 320. To use the car analogy again: if one of your spark plugs was bad, you'd probably still get the best mileage in 5th gear, but it might not be able to go faster than 40 mph.

In 4-engine jets, the question would be 1) how many engines are out; 2) do I have symmetrical engines remaining, as asymmetric yaw and resultant rudder trim would affect the total drag on the airplane. (In the 747 [sim-only; thankfully!] two engines-out on one side was a handful, requiring full rudder AND full rudder trim...and I still got a 'Charlie Horse' from holding the pedal to the floor!

 

Thanks for the info, dude. Between this, the books, and the internet, I'm starting to piece it all together.. One thing you said, though

Wouldn't that mean that best range occurs at L/D max? In the ATP written prep, it says that best range occurs at a speed above L/D max... Thanks again, man.

 

Yep, I screwed up. I should follow my own examples.

L/D in the T-38 varies with weight, but is between 250-265 kts. Best rasnge is about 320. Max endure is L/D max, or just slightly below that.

Right examples, but associated the L/D-term with the wrong example.

 

The absolute best source for this information is "AERODYNAMICS FOR NAVAL AVIATORS". I think that every serious pilot should read this book. It is available at Amazon, and many other places online.

You can look at it in PDF form at askacfi.com (look in the library section) or download the whole thing for free.

Joe

 

My immediate answer is L/D gives you max endurance and you need a faster speed to get max range.

Did I pass the interview question?

 

Jets:

L/D max = best endurance
CL^(1/2))/CD = best range

Props:

L/D max= best range
CL ^(3/2)/CD = best endurance

~~~~~~~~~~~~~~~~~~~~~~~~~~~
Books:
Aerodynamics for Naval Aviators by US Navy (less derivations, less math)
Aircraft Performance and Design by John D. Anderson (full derivations and math)

 

Cub Driver,
Is there a simple explanation you can give me that explains why L/D Max is max endure in jets, but not in props?

 

I think it has to do with the effect of the prop wash over the wing producing lift

Of course I could be wrong.

 

In a jet fuel is converted directly into thrust. The lowest fuel burn is at the bottom of the L/D graph since that is where the least thrust is required to overcome the drag.

In a prop plane you are converting fuel into the work of turning the props. You can maximize the results of the work done, in this case moving the airplane through the sky, by minimizing the resistance the work has to overcome, the drag.