My take on the Northrop/Horten flying wing concept

[Daniel Prates]

So I decided to try out the 1940s Northrop/Horten flying wing concept. I am not posting this on spacecraft exchange because it seems more like a generic airplane-design topic.

Jack Northrop felt that the concept of a flying wing was sound because it eliminated parasitic drag from tail booms, tail empennage, engine nacelles etc. It would however create problems of its own regarding placement of CoL in regard to CoM, and control surfaces in regard of the CoM (its much more complicated than that, but  summing up...). Already knowing how the Gotha/Horten and the YB-49 looked like, after some fiddling around I came up with this:

 

 

The inner control surfaces are inactive. The middle ones are the ailerons (as with such long wings I do not need a lot to get good roll torque) and the outermost controls are the elevators. Being so close to the CoM, I needed the elevators to be as far back as possible, and the picture shows I could only get them so far back, which is not a lot:

 

 

You may ask by looking at the picture above: why not sweep the wings farther to the back, so that the elevators actuate a little farther from the CoM, thus being more effective? Well, it turns out that it would result the CoM dislocating back a lot while the CoL dislocated only a little, making the plane an impossible design when near empty of fuel (notice the red center there, courtesy of RCS BUILD AID). When low on fuel, as it is, the CoM almost goes behind the CoL. So the sweep of the wing can only be that much. As a consequence, the main performance issue I found is that pitch is somewhat ineffective. 

For that matter: fuel tanks are behind the intakes, and in the center section - as far ahead as possible. Wings are not wet. If I place fuel tanks inside the wings, the CoM again dislocates way far to the rear and again the design is rendered impractical.

A few more design features you can see in the picture above: a) TWR is around 0.87, good enough for a fast plane; b) I chose the engines I did because I wanted to couple speed with range and time aloft. Hence they couldn't be joyride engines but also couldn't be a punch through the wall like an X-15. In the end it was a medium/fast design choice, allowing for decent endurance - an interceptor design would have to aim at some other setup of course; c) you can see it has some 40 minutes of endurance, which of course depends on altitude (more on that later). With stronger engines that would be much, much lower, so I am happy with my choice; d) full weight is a bit over 10 tons, empty weight is closer to 4.4 tons, allowing for easy landings and low stall speeds; e) being not-that-heavy-really, but at the same time with such large wing area, you just know it floats like a balloon with just a little speed, so high cruise altitudes are to be expected. 

The main design feature arising from the lack of a rudder is that yaw is controlled by aerobrakes on both sides (I placed on top and below the wing to avoid it influencing the pitch). They double as speed-breaks:

 

It turns out that yaw control is decent with that system. Roll control is excellent and pitch control is, well, good enough, kinda. But it allows a perfectly controllable plane in most situations, so that's that. Lets take it for a spin! 

Rotate is above 40m/s, which is not surprising given its large wing surface. Climb rate is also excellent:

 

Speeds were quite bad at low altitudes. This design allows for eliminating useless drag, but it is not exactly low-drag either. I don't know if KSP is perfect when simulating this, but it was to be imagined that with such a thick wing my drag characteristics would result a poor performance at low altitudes - in this case, just below supersonic at 1000 meters at full throttle:

 

HOWEVER, all that lift carries you very high where the air is much thinner, and at 11.000 meters I achieved no less than mach 3! It shows what a combination of 'lowish'-drag with high lift can achieve, specially with decent jet engines:

 

 

Sorry, forgot to take a pic of that, but I actually took it as high as 18.000 meters. But by then it was nearly uncontrollable, specially the elevators, which were nearly useless. Endurance also fell a lot above 10.000 meters, due to the much thinner air affecting engine performance. So I wouldn't say the safe envelope is that high.  Flying around 10.000 is where I got the most of it.

Just some more vanity pics of the return flight and I'll wrap things up:

 

 

My conclusions:

1 - It was hard to deviate from what Northrop and Horten already concluded on their own. All changes I did eventually lead to a worse of even impossible design. And it is no surprise my own thing came out looking so much like their own;

2 - Speed is great in higher altitudes, and terrible in lower altitudes. In my understanding this is because drag remains a bit high due to the large wings, so it is meant to be optimal as a high-flyer. A plane meant to do what a Panavia Tornado does, for instance, could never be designed like this. A high-altitude bomber, however, could. 

3 - Lift is in ample supply, which opens a lot of opportunities for several different design purposes.

4 - What I really didn't like are the poor characteristics regarding pitch control. I will consider a future 'B' version with canards on the tip of the nose to correct that.

5 - The thing is a test plane only, so it carries only the bare minimums. But all the characteristics indicate that it could mature into an effective, fast long-range bomber or a medium/high passenger plane very easily. I could easily double or triple its payload or fuel capacity, so the concept has a lot of potential. 

 

 

Edited July 13, 2019 by Daniel Prates

[Snark]

Really nice writeup!

Moving to Spacecraft Exchange.