Max Range

[uwochris]

Hey guys,

I got a Q in regards to flying at max range IAS.

I realize that at this IAS, we are essentially trying to fly at the AOA that provides the max L/D ratio; however, since we do not have an AOA indicator, we must rely on the next best thing, which is IAS.

Now, after doing some reading, I understand that this AOA is constant, regardless of altitude or weight. However, I have read that max range is NOT affected by altitude, which puzzles me. I can see that as we change altitude, we must maintain that same IAS (all else equal) in order to maintain the same AOA, and that TAS will be different. However, I figured that as we climb, we can lean our engines more, saving us some fuel, and increasing our range.

What are your thoughts on this?

Thanks in advance.

 

[TurboS7]

In a Lear 24 you fly 210 at 3000 for fuel economy at 450 you fly about 210 indicated and you are flying at mach .78.

 

[Hugh Jorgan]

However, I have read that max range is NOT affected by altitude, which puzzles me.

Max range AOA is not affected by altitude, but the corresponding airspeed associated with that AOA will be. Also, Max range is more correctly defined by TAS and then converted to an IAS. Some aircraft have good rules of thumb that can get you a fairly accurate rough estimate based on weight and altitude, but to really nail it, you should calculate the max range TAS based on weight, temp, alt and then make a wind correction and convert it to IAS. That oughta keep your fingers busy on a long transit.

 

[uwochris]

Thanks for the responses guys.

Although the AOA will not be affected with altitude, will the actual range be different?

ie) an a/c flying at the max range AOA at 5000 ft vs. another a/c (exact same make and model and weight) is flying at the max range AOA at 10,000ft.

I'm assuming that the one at 10,000 will go further (higher range) because he will be able to lean his mixture out more. I don't know if there is a consideration here that this pilot will also have to fly at a higher power setting to be able to fly at the specific AOA... if there is, then perhaps my assumption is wrong.

 

[Hugh Jorgan]

Although the AOA will not be affected with altitude, will the actual range be different?

The AOA at which max range can be found will not change, but the TAS and fuel flow will vary with altitude. So will the wind. Usually, it's better to fly higher because one will obtain a faster TAS and lower fuel flow at the higher altitudes. In order for one to correct for headwind, the AOA actually needs to be decreased to compensate. There becomes a point in which an increase in altitude can sometimes create enough of a headwind component that the ground speed vs. fuel flow is less efficient than if a lower altitude were chosen. Each aircraft should have performance charts that will show the best altitude for a given weight and temperature. Then some number crunching must be done do see if the fuel flow versus groundspeed will be worth the climb.