Attempting to figure out Hypersonically stable aircraft [FAR involved]

[TheGripenSaidWhat?]

Hello there, with my first Mothership-based Interplanetary mission under way I figured a good way to kill some of the time between correction burns would be to develop my SSTO program. I have a college-level understanding of physics, so I figured that for a supersonic flight one will need longitudinally longer wings to maintain control. Hence my first SSTO Craft, the Hyperion pictured below.

During initial tests I've found that the yaw stability was much too low, so I increased the height and length of the two tails, as seen above. However, as I approach 20k the stability of the aircraft is even worse now. In my latest disaster of a flight I could't even pull out of a manic spin where I both rolled and yawed uncontrollably, despite fighting it with rcs the whole way down. After the sad demise of Edwise Kerman. I checked the FAR Static graph to see if I had messed up the design somehow. Lo and behold, the Dark Magic that has apparently possessed my aircraft:

If anyone could offer any insight or help as to what is going on or what I'm doing wrong, it'd be greatly appreciated. Only Mods involved were Deadly Re-entry, FAR, and Ioncross. Thanks again!

[ferram4]

Your design looks like it's going to be a little too stable in pitch for you to control it properly at supersonic speeds. You really want the Cm (moment coefficient - yellow line) to have a lower-magnitude slope for easy pitch-control; which means that you want to push the CoL further forward. Make sure that you do your static analysis at a variety of Mach numbers because the physics change a lot with Mach number.

Another problem is that the sudden increase in Cm as stall begins indicates that your vehicle can't recover from a stall. The solution to this is to make the canards have a larger span or to reduce the span of the main wing; this will cause the canard to stall first, forcing the vehicle to pitch down.

You've also got a lot more wing there than you need, you can get rid of quite a bit of it and still fly fine.

To help with yaw stability I'd say to try shifting the fuel tanks forward so that the vertical stabilizers you have there can be more effective.

Keep in mind that as you go higher and faster the aerodynamic damping on your aircraft will decrease, meaning that you will probably need to use some active control inputs to keep in under control.

[Van Disaster]

Can you do a shot with your CoL + CoM balls? and just to check, what are the jobs of both pairs of horizontal control surfaces? eyeballing it suggests that you have too much wing aft of the CoL.

I went digging around my designs - this one seems reasonably similar to yours ( perhaps less fuel ) and is obviously hypersonic given it's a spaceplane. Not awfully stable in pitch ( mostly because of giant canards ) but it'll fly along happily at 21km full or empty. Spin recovery generally involves flying backwards for a while(!) but that seems to be a characteristic of FAR, or at least my use of FAR.

It doesn't *look* like any hypersonic plane I've seen, mind - I can't remember one with that sort of span, my shaky rememberance of aeronautics suggests that the ends would be outside the shock cone but I can't remember what the effect of that would be

Caveat - not tried in 0.20 with the latest FAR ( I'm rebuilding just about everything with B9 SABRE engines anyway ) but the general layout should still be fine, it's not a terribly draggy shape.

As a final suggestion, I'd say use a nerva for the rocket stage & ditch most of the fuel. You don't appear to have a payload, so less is more in this case. Tiny is beautiful!

Edited July 10, 2013 by Van Disaster

[TheGripenSaidWhat?]

Thank you both to ferram and Van Disaster. I've edited my design and here is the new Hyperion:

As you can see I've ditched the extended wings at the end and sides while keeping the twin tails due to stylistic choice. I've also extended the reach of the advanced canards by smacking them on a tail fin which I then pitched one notch downward. Both of these things helped reign in my CoL towards the CoM and hopefully will aid in it's flying (Ignore the fact that I'm now missing yaw control surfaces as I realized that fact as I was writing this post xD).

In addition the Cm now has a delightfully smaller negative slope as shown below. It still splits as my angle of attack gets around 20 at subsonic speeds but both the deviancy of the split is smaller and the split disappears much quicker as I approach Mach 2-3.

As for before Van Disaster, the CoM was around the center of that second Mk 2 Fuel tank with the CoL being a little bit in front of the center of the third tank. A great distance more than what it is now. Unfortunately removing fuel is out of the question as I aim to use SSTO's to refuel my space/gas station that's currently in orbit, although if it comes down to it I'll cut the extra. I'm going to try some test flights now and report back my findings.

And Van, I believe that having the wing jutting out past the shock cone results in the outer parts generating much less lift than the inner, moving the center of lift forward and destabilizing the plane a bit, at least with swept or delta wing designs. Quite a problem, but thankfully not simulated in KSP (yet...)

EDIT: Sooooo adding more lift to the front was a horrible idea, as it brought the Center of Lift pretty much centerline with the CoM. So I removed the tailfins while keeping the advanced canard, and now it flies much better. Still possible to get into an unrecoverable stall but only if I work towards it. On it's own it won't do it. It'll need a little bit of tweaking to get the heavier LV-N on the back, but I'm sure with some further testing I can do it. Thanks again to both of you!

Edited July 11, 2013 by TheGripenSaidWhat?

[Van Disaster]

That looks considerably smoother than most of my designs have you considered using one of the Mk3 fuselage mods with a rocket fuel tank component?

In your case I'd still use NERVAs rather than the aerospike - you'll need at least two - my gut is telling me probably more - but usually I've found one per jet engine is enough ( er, in older FAR versions! although drag changes by the time you'd use them would be minimal ). Fuel consumption is so much less that it's worth the hassle.

I'd wonder if leaving a bit of wing outside the cone would be a way of countering mach tuck, but I suspect the bit of wing in the shockwave itself might not be so happy...

[nabeav]

Another problem is that the sudden increase in Cm as stall begins indicates that your vehicle can't recover from a stall. The solution to this is to make the canards have a larger span or to reduce the span of the main wing; this will cause the canard to stall first, forcing the vehicle to pitch down.

This is not a solution to stall recovery. The inherent flaw in canards is that they will always stall before the wing, but a horizontal tail will not (due to downwash). That is why conventional aircraft use horizontal tails most of the time, even though it provides a negative lift, decreasing the efficiency of the craft. Forcing the vehicle to pitch down will not recover a stall, as you will often just pitch back and forth, unable to regain control using the canard.

[arq]

Also, I find adjusting the pitch of individual lifting surfaces to be risky. Because of all the drag experienced on re-entry (especially from Minmus or other planets), it is very easy to flip or spin the aircraft (though maybe it isn't as bad with FAR, I've only played with it a little). The issue with angled wings is that it may control your CoL when upright, but if you end up upside-down the angling will have the opposite effect, sometimes greatly complicating recovery.

Just think carefully about it, and if you have recovery issues then remember that this may be why.