# Does the temperature of the body of an aircraft increase its efficiency?

I am working on a prototype for an airplane. I was wondering, can the temperature of the body of the plane make it more efficient? If the structure is at 25°C, does it need less energy to maintain a steady state flight than a structure at -20°C?

I had this idea about excess energy that could be collected during the day, and was wondering if there is any chance that when the batteries are fully charged and there's still excess power being generated, I could use it in order to increase the performance of the aircraft.

• I'm not completely sure what you're asking here. The first part of your question is about the temperature of the aircraft's fuselage (I think), but the second part seems to be about collecting and using solar energy. It might be helpful if you can add some more details about your design, especially if it's electrically powered or not. If yes, this question might be relevant. Feb 14, 2017 at 14:48
• For the airspeeds/aircraft you would be powering with a solar/electric system, the temperature of the fuselage would not have any appreciable performance benefit. You would expend a LOT more energy trying to cool the fuselage than you have extra available, plus the added weight of (Peltier?) would really degrade performance. Feb 14, 2017 at 15:16
• Typical aircraft structural materials (e.g. aluminum, composites, titanium, etc) don't appreciably change any functional or performance characteristics over the temperature range most aircraft encounter. The only exception I can think of is the SR-71 Blackbird that accounted for the skin heating effect from high-altitude/high-speed flight expanding the fuselage panels. Feb 14, 2017 at 17:52
• Most importantly, "when there's still excess power being generated", efficiency is not a concern! Presumably at that point you're flying 100% electrically powered, with batteries charged 100%. And to make things worse, you're optimizing for a rare situation, while you would need to carry the weight of this contraption around all the time when it would be dead weight. Feb 15, 2017 at 15:29

As your speed increases, your temperature will increase due to fluid friction of the air. When metal gets heated, it becomes weaker and more malleable. Think about when a blacksmith is hammering on a piece of iron over a hot forge. Due to this, your Vne should be reduced so your wings don't fall off.

As far as your initial question, if temperature affects efficiency; not really, unless your body gets so hot and you are going so fast as to alter the shape of the aircraft (which could happen, I suppose.)

• The SR71 does go so fast that the shape of the aircraft is altered in flight. That's why the skin is corrugated and the fuel tanks leak when it's on the ground. I don't know that this heating and expansion has any impact on the plane's efficiency, though. Apr 24, 2017 at 13:01
• @FreeMan talking about the SR71 in the current tense is a bit weird, considering it was last in service almost 20 years ago.
– Jamiec
Apr 24, 2017 at 14:53
• @Jamiec, that's true, but I'm fairly certain that if they were to put one back in the skies, it would still do all the things it used to do. ;) Sad to think that it's 20 years gone... Apr 24, 2017 at 15:18
• @Jamiec Does NASA not use them any-more for science purposes?
– alex
Apr 24, 2017 at 19:34
• @Jamiec That's a shame.
– alex
Apr 25, 2017 at 8:39