[danieldyer]

Roughly speaking, the drag is proportional to cross-sectional area, whereas engine thrust required is proportional to mass, which in turn is proportional to volume. So, you end up running into a square-cube law situation: http://en.wikipedia.org/wiki/Square-cube_law

Also, the Reynolds number will be completely different for a model aircraft compared to a full-sized aircraft, so they will behave quite differently aerodynamically.

It's also worth pointing out that even the fastest full-size jet aircraft can only just break the sound barrier at sea level (the F-111 did Mach 1.2 at sea level) – going supersonic typically requires flying at a high altitude, which obviously isn't practical or legal for a radio-controlled model aircraft.

[termain]

>Also, the Reynolds number will be completely different for a model aircraft compared to a full-sized aircraft, so they will behave quite differently aerodynamically.

For low speeds, maybe. For high speeds, you're well into the high Reynolds number turbulent regime. I've worked on R/C aircraft designs and I'm reasonably sure we got decent results with inviscid aerodynamics. At sea level, you're going to see Re in the millions.

Regarding the Square-Cube law, I'm not sure it's that relevant here. It's pretty clear you could put two of those engines in a AMA sized model, so not exceeding 55 lbf. That'd give you a thrust-to-weight ratio of greater than 1.0.

[danieldyer]

> Regarding the Square-Cube law, I'm not sure it's that relevant here. It's pretty clear you could put two of those engines in a AMA sized model, so not exceeding 55 lbf. That'd give you a thrust-to-weight ratio of greater than 1.0.

Yes, a small turbine-powered RC aircraft can easily have a higher thrust-to-weight ratio than a full-sized aircraft, but it's not thrust-to-weight ratio that determines top speed; it's thrust-to-drag ratio.

This is where the square-cube law comes into it. If we take a full-sized delta-winged high performance aircraft like the Eurofighter Typhoon, we could very roughly approximate the difference in drag between that and the model to be proportional to the difference between the square of their wingspans.

I'm guessing the aircraft in the video has a wingspan of 1 m, and the Typhoon has a wingspan of 11 m, so the drag should be greater by something like a factor of 121.

However, the Typhoon's engines provide a combined 180 kN of thrust, which is greater than the 160 N thrust of the Jetcat P160 by a factor of 1125.

So, you can see that the thrust-to-drag ratio of a full-sized jet fighter is something like an order of magnitude larger than for a model aircraft like this, which is the main reason why model aircraft are unable to attain supersonic speeds.

Of course, supersonic flight for model aircraft would pose all the same problems it poses for full-sized aircraft; onset of compressibility affecting control surface response, engine inlet geometry and so on.

[termain]

>Yes, a small turbine-powered RC aircraft can easily have a higher thrust-to-weight ratio than a full-sized aircraft, but it's not thrust-to-weight ratio that determines top speed; it's thrust-to-drag ratio.

And parasitic drag dominates, right? Ok, so yeah, I was thinking in terms of induced drag. Thanks for the discussion. I enjoyed it and it was a good refresher.