Max range experiment for turbines.

[Adam Frisch]

Hi guys.

On piston aircraft, maximum range is reached at best glide speed as we know. Interestingly, it is also irrespective of altitude. Now, unless you have a lot of time on your hands, perhaps glide speed all day long isn't the best. For that purpose the Carson speed was developed. It says that 1.3xVbg will yield a good comprise between long range and speed.

But for turbines I think it gets a little more complicated. Obviously any engine will be most efficient at max power, that's when it produces the most efficient combustion. True for both pistons and turbines. But drag also exponentially goes up at high speeds, so the most efficient solution would be to have the smallest turbine you can run that would sustain best glide.

So, my question is if you turbine fliers could help me out here and do an experiment on next flight: Can you check your fuel flow at best glide speed next time you fly? I'm curious if you pull back a turbine to best glide, will the inefficiencies at that low heat cycle in engine eat up the low drag benefits, or would there still be a benefit like on pistons? If for whatever reason, you had to maximize your range, that could be useful information. Also, it would be interesting to see just how far you could go with a TC, if time is not a factor.

[donv]

The AFM should have a chart for maximum endurance. I'm pretty sure at least the Jetprop AFMs have this.

I will look when I get home, or when I'm at the airport next.

[donv]

Actually, I didn't quite read your question correctly. Max endurance and max range are not necessarily the same thing-- are you looking for max range?

In that case, there are definitely long range cruise charts in the AFM.

Or are you interested in the difference in speed between max endurance and max range?

[Adam Frisch]

I'm looking for max range and agreed, max endurance and max range are different.

The LRC tables in the AFM aren't nearly as low as one can go. On all the planes I've owned, I've never seen numbers below 55% published. Cruising at Vbg (or if no such number exists, Vy) should only require 25-35% power. No numbers for that. That's why it needs to be empirical data.

[JimC]

You'll find that the increase as you go from 55->45->35 is minimal, especially the very last bit. Cessna published max range data for some of their singles (I had it in my 182) but you'd see that it wasn't  worth it unless, just maybe, you were over the ocean far from land. You could add hours to the flight but only end up going 50 miles farther. My 182's endurance was over 12 hours at max but you traveled less than 80nm extra for more than 2 hours additional flight time at the very limits.

[donv]

I think the LRC charts in the AFM are going to give you the best no-wind range, actually. Going slower will give you better endurance, but it won't necessarily help on range.

Of course, if you have a strong tailwind, going slower will help. Going into a headwind, you may want to go faster than LRC to get the best range.

[av8r]

The charts you are looking are the specific range charts. They will give you nautical miles of range for each pound of fuel burn. Interestingly enough is is always recommended that you set up for and use 99% of specific range. A 1% reduction in specific range yields more than a 1% increase in TAS. One book I use for study on this subject is the Aeronautical Knowledge for Naval Aviators.

[av8r]

I misstated the name of the manual in the previous post. It's Aerodynamics for Naval Aviators, and they are available from multiple sources.

[ghancock]

Totally don't have a turbo prop but ordered me a new book anyway :-)

Glenn