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For educational purposes I am designing a small single seat twin engine aircraft that will be as efficient as possible at sea level and at a variety of speeds 150-500+ mph. The goal is to be able to travel long distances at high speed while offering great acceleration and loiter characteristics.

All of the information I've found is regarding cruising altitudes where at speeds of 400mph or less the turboprop is more efficient then a turbofan.

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The chart below shows the results at altitude.

My top two choices are a variable pitch unducted fan or a high bypass turbofan. Both would be mounted in the wing with unducted unit having the blades aft of the wing. Both could possibly include afterburner for accelerating to top speed.

None of these charts take into account the drag of the different sized engines and their respective arrangements. The TSFC is usually quoted at design intention or static thrust and not sea level at high airspeeds so it's hard to compare known engines.

The turbofan aircraft could likely be designed quite a bit differently as you could use thrust vectoring and a variable nozzle reducing the number of necessary control surfaces and therefore drag which is extremely important in the thicker atmosphere.

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    $\begingroup$ For unbeatable fuel economy and high speed, you can't beat a turbo-compound piston radial like a Wright R-3350. The specific fuel consumption is down around modern car levels; maybe 2/3rds of a turboprop of the same HP. Unfortunately, you have to live with vibration, oil leaks, maybe reliability issues... but who can't fall in love with the sound. $\endgroup$
    – John K
    Feb 12, 2020 at 23:26
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    $\begingroup$ With regards to @JohnK 's comment, does your concept have to have a turbine engine? If not, would you accept experimental stage engines? Also, your specified speed range is quite broad... $\endgroup$
    – Jpe61
    Feb 13, 2020 at 8:55
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    $\begingroup$ @jpe61 It is only turbine engines I am considering at the moment. It needs to be smooth and reliable, like a gas turbine. $\endgroup$
    – usernamechecksout
    Feb 13, 2020 at 20:15
  • $\begingroup$ Thing with turbines is, that they are not optimal at lower altitudes (warmer air). A wild thought: usa a propfan with afterburner ;) $\endgroup$
    – Jpe61
    Feb 13, 2020 at 20:36
  • $\begingroup$ Jpe61 From my understanding the propulsive efficiency is much higher at lower altitudes due to the thicker air offering better compression and also mass flow for the fan. I believe the temperature differential itself only has few percentage points of efficiency to offer. It's the fact that there is less drag at altitude that offers much better overall efficiency. Also I suggested an after burning propfan in my question. $\endgroup$
    – usernamechecksout
    Feb 14, 2020 at 22:25

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Penguin ancestors took to the sea for food, not to set speed records. There is absolutely no reason to design an aircraft to cruise at sea level. Plus you take away a very important lifesaver in the event of engine failure, the glide. Far better to fly in thinner air, where true airspeed can be pushed all the way to your Mach limit while the indicated airspeed is far lower.

Simply apply your graphs to higher altitudes for greatest efficiency, and, unless you plan to go over 400 knots, the turboprop should be your choice, unless you find a good mechanic for your turbo-compound radial.

The weather is also a bit smoother up there too.

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    $\begingroup$ "There is absolutely no reason to design an aircraft to cruise at sea level". Correct. I never stated it would "cruise" at sea level. It isn't an airliner. By long range I meant maybe 200 miles. $\endgroup$
    – usernamechecksout
    Feb 13, 2020 at 1:01
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    $\begingroup$ @usernamechecksout your graph is excellent. You would do well to study the strategies of airliners, as they are obsessed with fuel economy. It's all about the airspeed, until you go so high Mach number limits your speed (unless you go supersonic). The higher you go, the greater true airspeed (TAS) is. Another path to efficiency is minimizing drag. A third, that even major names have been very slow to espouse, is cheaper fuel. CNG or LNG is 1/3 the cost of aviation fuel. You're on the right track, go with the unducted fan (propeller). $\endgroup$
    – Robert DiGiovanni
    Feb 13, 2020 at 12:35
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    $\begingroup$ That's wrong. It may be more efficient to fly at 35,000 feet, but you don't find a lot of submarines at that altitude. $\endgroup$
    – Harper - Reinstate Monica
    Feb 13, 2020 at 19:28
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    $\begingroup$ @Harper - Reinstate Monica actually you do, except their hulls have lower pressure out side and the bow planes are slightly larger. I once thought a panic stricken Navy put rockets on the Sea View (VTTBOTS) in an attempt to keep up, then realized their minisub wasn't half bad. $\endgroup$
    – Robert DiGiovanni
    Feb 13, 2020 at 22:16
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I was trying to find an article about propfan efficiency I've read, since I recall a propfan fitting your specs quite nicely. I'm sure you have read more on propfans than just the Wikipedia article? Roughly speaking a propfan would combine some of the best aspects of turbofan and turboprop in a way that would be beneficial for your project.

However, during my (so far unsuccessful search) I stumbled upon this: multiple purpose subsonic naval aircraft (mpsna) multiple application propfan ... (pdf), at a fast glance this seemed interesting and some of the mission profile kinda matches yours, right? Comparisons between propfan and turbofan start at page 138.

It is an old paper, but check it out.

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