FAR, helping me understand supersonic drag

[AeroGav]

I have no training in aerodynamics but have been a plane spotter for many years, so now may be a good time to polish up my understanding of the concepts.

I am aware of four main types of airframe drag

1) parasite drag - same as your car gets. Pretty simple and i think i understand this correctly enough. Mainly an issue when going fast at low level.

2) lift induced drag - related to angle of attack. Again, pretty simple and think i have this one down. Mainly an issue when flying very slow or very high in level flight, or pulling hard g's in other conditions.

3) transonic drag/area rule. Not sure what the correct name is. My understanding is this is most important when travelling close to mach 1 - but fades either side of it. Correct, partially correct, wildly wrong? The F106 delta dart is an early example - first prototypes couldn't break through the sound barrier, till they "area ruled " the fuselage - ie. make it fatter ahead of the wing and then pinch in at the wing root. This "coke bottle" fuselage enabled the airplane to get supersonic.

4) Bow Shock. OK, TBH I thought the proper name for this was "wave drag", but looking at FAR, it appears that "wave drag" is more what i understood 3) to be. This is probably total quackery but this is what my understanding of it was -

At supersonic speeds, a shockwave forms at the nose cone of the aircraft, so supersonic designs try to keep the rest of the aircraft within this "bow wave" to avoid creating additional "bow shocks" from bits sticking out beyond. To avoid this you can

a) use shorter span wings, like F-104 starfighter, and a long long nose
b) larger sweep angles eg. English Electric lightning
c) use a long nose and short heavy tail, mount the wings as far back as possible. eg. most 4th gen fighters (su-27, F-15 etc)

The problem with c) is that it gives the tailplane a very short movement arm, so a large downforce is needed to pitch up. A lot of drag is needed to create that downforce, and that downforce is also subtracted from the lift produced by the wings, which is why a lot of Gen 4.5 aircraft now have canards, since air combat seems to be moving back to higher altitudes again. Canards have their drawbacks ofc, namely wings less efficient since not operating in clean air, generating negative lift to push the nose down when flying level in high dynamic pressure regimes, but again not what these 4.5 aircraft are optimised for.

So, is this "Bow Shock" a thing at all, or did i just make it up to explain why high speed aircraft seem to have swept wings, shorter spans or wings placed quite far back along the fuselage? If it is a thing, is it modelled by FAR, or any other mod?

[wizzlebippi]

Wave drag is due to shockwave formation, transonic or supersonic, anywhere on the aircraft. A bow shock or shock cone is just the shock wave produced by the nose of a supersonic aircraft. A bow shock is a source of drag, but is not the entire source of wave drag. The main picture in this article should help.

[url]http://phys.org/news/2015-08-schlieren-images-reveal-supersonic.html[/url]

Anywhere you see a dark line in the schlieren photo is a compression wave and the lighter lines are expansion waves. Edited November 16, 2015 by wizzlebippi

[AeroGav]

[quote name='wizzlebippi']Wave drag is due to shockwave formation, transonic or supersonic, anywhere on the aircraft. A bow shock or shock cone is just the shock wave produced by the nose of a supersonic aircraft. A bow shock is a source of drag, but is not the entire source of wave drag. The main picture in this article should help.

[URL]http://phys.org/news/2015-08-schlieren-images-reveal-supersonic.html[/URL]

Anywhere you see a dark line in the schlieren photo is a compression wave and the lighter lines are expansion waves.[/QUOTE]

Thanks for the reply, good to talk to someone who knows about these things.

So, are intersecting shock waves bad, and avoided, like i assumed? Let's say that aircraft is travelling faster and higher and the shock cone off the nose was coming back at a steeper angle, the wings were longer, further forward and less swept, and the wing tips were hitting the bow wave. Would that cause much extra drag, even if those wings were razor thin like on an F104?

I've not downloaded FAR yet (giving myself a break from spaceplanes, with a rocket-centric career game), but i get the impression that FAR is all about area ruling your aircraft to minimise the wave drag , shock waves vs component placement doesn't come into it so long as area ruling is good. Of course the two do overlap, much of the time.

I've also noticed that the Skylon appears to have canards that would get caught in the nose shock cone at high mach, so my theory goes out the water there.

[K^2]

[quote name='wizzlebippi']Wave drag is due to shockwave formation, transonic or supersonic, anywhere on the aircraft.[/QUOTE]
Right. But the important feature of wave drag is that it is at its worst near transonic. So slightly bellow to slightly above speed of sound. Hence the ability of some fighters to punch through the transonic with afterburners, then switch off to supercruise and not waste the fuel.

Ignoring variations in density with altitude, changes in flight configurations, etc. Typical C[sub]d[/sub] profile is fairly constant until transonic, rise sharply in the transonic region, then fall back to a value "somewhat" higher than sub-sonic, where again, it remains almost constant until you hit hypersonic speeds and it starts to ramp up.

[wizzlebippi]

@AeroGav: Yes, anything penetrating the shock cone causes a significant increase in drag. This is why the SR-71 has a delta wing positioned very far aft and creates lift from the fuselage with vortices (why the fuselage has flanges). However, in the case of a space plane, you operate over such a large speed and altitude range that dealing with that extra drag is unavoidable. This probably can be mitigated by keeping speed under the critical mach number until the extra drag is manageable. Or just with extra thrust like the Mig 25. If I remember correctly, NACA tr-1135 contains all the information you need to estimate the critical mach number for having a wing tip penetrate the shock cone.

@K^2: Some early fighters actually had to dive as well to achieve supersonic flight.

[Sampa]

from what I have learned, the biggest problem with supersonic flight is the physics change when you break the sound barrier.

[cantab]

The lift tends to shift backwards which makes challenges for trim. IIRC Concorde pumped fuel around to compensate.

[Van Disaster]

[quote name='K^2']Ignoring variations in density with altitude, changes in flight configurations, etc. Typical C[sub]d[/sub] profile is fairly constant until transonic, rise sharply in the transonic region, then fall back to a value "somewhat" higher than sub-sonic, where again, it remains almost constant until you hit hypersonic speeds and it starts to ramp up.[/QUOTE]

This is visible in the FAR graphs if you look at the sweep Mach ones.

@anyone: If you want to work out what parts stick out of the nose shock cone, the mach angle formula is dead simple: sin a = 1/M , where a is half the angle at the top of the cone & M is mach number. For anything up to hypersonic ( Mach 5 is the accepted start ) it's pretty reasonable, after that you start to worry more about heat. It's pretty unreasonable to think you can keep everything inside at Mach 25 anyway...

@Aerogav: the idea behind area ruling is to minimize the rate of change of the cross section area of the craft. It's not just for supersonics, subsonic craft which fly close to supersonic will have parts of the craft in supersonic airflow. Edited November 17, 2015 by Van Disaster