Horton Flying Wing

[Koopernic]

Argentina is also where Reimer Horton in 1950 made a speech before the Argentinian Aeronautics Society talking about the "radar camaflauge" effect of wood.

Wood only reflects 20% of microwave radiation. Perhaps, if lucky, 20% is absorbed and the remaining 60% is transmitted. The problem is the 60%-80% transmitted will reach underlying metal structures in the aircraft such as engines, undercarriage and scatter perhaps more.

Here is something interesting. The Ho 229 wasn't intended to use wood as a filler. The Mosquito skin consisted of balsa wood sandwiched between two layers of plywood. The Ho 229 used a plastic-wood called formholz which consisted of sawdust, glue and graphite. Graphite is a reinforcing nano material used for the same reason it is used in tyres. It also makes the material semiconducting and radar absorbing. It's a poor radar absorber in that is still reflects the 20% but it now absorbs more of the 80%.

The first prototypes of the Ho 229 were actually not real Ho 229 prototypes but Ho IX test beds that didn't use firmholz. The Ho 229 from the V5 onwards was slightly enlarged to increase wing fineness and raise Mach limit. (Jumo 004 had an accessories gear box)

This fact wasnt noted in the national geographic channel investigation on the stealth characteristics of the Ho 229 because they based their investigation on one of the test beds.

Im not sure how much better graphite loaded formholz would be but I'm guessing half the backscattered radiation would be absorbed by the graphite.

********
The Germans did have a stealth coating. It was called Schornsteinfeger (chimney sweep) but was properly known as a "Jaumann Absorber" that was used on u-boat masts sucesfully absorbing 96% of radar. It is still used today on aircraft such as the F-117, F-22 etc.

Here is how it worked. It is 1/4 wavelength thick so 2.5cm for a 10cm wave. This means the wave reflected from the back of the material is anti phase with that reflected from the front thereby cancelling out. (This is technically a Salisbury absorber). This isn't the primary mechanism. The material is made semiconducting and given exponentially increasing conductivity towards the back. The Germans did this with about 9 layers of graphite loaded cardboard of increasing graphite density that was impregnated with PVC to make it water proof.

So to make the Ho 229 very stealthy you just need to laminated the formholz in layers to give it exponentially increasing semi conductivity.

Schornsteinfeger absorbed 96% at 9cm and about 80% at 3cm and 20cm.

 

[Acheron]

Question was the Horten even viable with WWII-technology? My understanding is that flying wings have certain issues regarding stability I believe?

 

[SaparotRob]

With the Soviets and Allies (UK and US) studying everything that they could get their hands on, I would think stealthy flying wings would have invisibly flooded the skies of the 1950's had it been viable. The U.S. certainly had a flying wing or two post 1945. Heck, we had a "flying saucer". I'm sure the U.K. had one in the works. The former allies certainly had far larger budgets and far more resources to bring to bear than the RLM. As both sides had access to German research and were busily trying to stay ahead of the other, had it been doable it would have been done.

 

[swampyankee]

Flying wings had quite a long history in aviation; the Burgess-Dunne D.8 flew before WW1 (see Burgess Dunne | Historical Aircraft | Royal Canadian Air Force, for example) and its only stability problem was a tendency to weathercock, making crosswind landings difficult. It was well-known how to make a flying wing stable, although it was difficult to provide enough damping.

Studying the technology of a defeated enemy who had demonstrated some areas of technological superiority is standard practice. German technical superiority existed in some areas, including diesel (but not spark ignition) engines, many areas of ordnance[1] engineering, and many areas of chemical technology, and some areas of aerodynamics, but far from all[2].

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1: Ordnance vs ordinance. Using ordinance where ordnance is meant is one of the usage bêtes noire that most consistently annoys me.

2: I think it's difficult to argue that German airfoils were "better"; they clearly weren't as many German manufacturers consistently used NACA airfoils. Their engine cooling design wasn't particularly advanced; nobody did it better than had North American with the P-51. Their combat aircraft didn't have remarkably better (and largely worse) flight characteristics than those of the Allies nor did they have better aerodynamic efficiency.

 

[Acheron]

Flying wings had quite a long history in aviation; the Burgess-Dunne D.8 flew before WW1 (see Burgess Dunne | Historical Aircraft | Royal Canadian Air Force, for example) and its only stability problem was a tendency to weathercock, making crosswind landings difficult. It was well-known how to make a flying wing stable, although it was difficult to provide enough damping.

Given the supposed benefits of the flying wing, I would expect some considerable drawback, otherwise, as SaparotRob pointed out, we would have seen a lot of them by the Cold War, no?

 

[MIflyer]

First, the Horton HO-9 design was more of a flying mockup than anything else. Gotha had to redesign the thing.

Second, the YB-49 was cancelled because (take your pick):

1. It needed a stability augmentation system to prevent it spraying bombs all over the place after it made its turn onto the final bomb run.
2. The SECAF directed that Northrop should merge with Convair but they did not.
3. Recently revealed information shows that the FBI had found the the Soviets had penetrated the YB-49 program, and with the combination of their duplication of the B-29 and their development of an atomic bomb, along with the discovery that the YB-49 was very difficult to pick up on radar, represented a huge threat.

 

[swampyankee]

Given the supposed benefits of the flying wing, I would expect some considerable drawback, otherwise, as SaparotRob pointed out, we would have seen a lot of them by the Cold War, no?

There are numerous disadvantages to flying wings. One is they tend to have poor yaw stability and poor damping in both yaw and pitch (both factors in the failure of the B-35 and B-49). Less obvious ones include the need for long runways compared to conventional aircraft with the same payload capacity (putting the nose up for takeoff requires decambering the wing and reducing its lift coefficient at any given angle of attack; using trailing edge flaps is challenging as they produce a nose-down pitching moment and the short moment arm for the elevators limits pitch authority) and that center of gravity travel and payload volume is also constrained. This isn't to say that successful flying wings haven't been built (Fauvel, Gotha's, Lippisch's, and Horten's gliders, the Burgess-Dunne aircraft, and, most expensively, the B-2), but it does mean that they are very much niche aircraft.

 

[Acheron]

My understanding was that the B-2 uses computers to make the flight stable (fly-by-wire), something not available of course to earlier aircraft?

 

[swampyankee]

My understanding was that the B-2 uses computers to make the flight stable (fly-by-wire), something not available of course to earlier aircraft?

I believe that is the case. Both the B-35 and B-49 were stable, but the poor pitch damping of both was problematic for bombing accuracy. The B-49 also may have had inadequate yaw stability and authority; the pusher propellers on the B-35 added quite a lot of damping in both pitch and yaw.
See The Ride of My Life—on a Flying Wing | Page 2 | History | Air & Space Magazine for an anecdote about flying (and stalling) the B-49. See NASA Technical Reports Server (NTRS) and ntrs.nasa.gov, in general (if you can get past the poor search interface) for more information.

 

[Juha3]

The NatGeo special was/is titled " Hitlers Stealth fighter " It is a nonflying full scale replica that is hanging in the San Diego Museum of Space and Flight - One of the main points in the show was to test its radar signature with the same frequencies as English Chain Home Radar that was utilized in WW 2. They also did their best to replicate the paint/coating to test its actual radar absorption properties.If i remember they came up with if it was on the deck at 50' above the water crossing the Channel at 1,000km hr the English had it was either 6 or 8 minutes to scramble and intercept before it was on them.Even if they did manage to get anything into the air it would've run away from anything then flying very quickly - The show states the Germans test flew a prototype in a mock dogfight with an ME 262 and it outperformed the 262 across the board. Its definately worth seeing -

When I saw the film several years ago those points were IMHO most odd.
1) Already in 1940 British had the Chain Home Low Radar to detect low altitude objects. It was a very different animal to CH radar, 200 MHz frequency vs CH's 23 to 30 MHz. And they began operate CHEL radars, operating 3 GHz frequency, in 1943, just against very low flying a/c (EL = extra low). So why to test the a/c against a radar known to have problems in detecting low flying a/c already in late 30s and not against radars designed to detect low flying a/c and in service in 1945?
2) Horten proto crashed IIRC on its 3rd flight. Who believes that a revolutionary a/c design of rather unknown flight characteristics is put on its paces in a mock combat during its 2nd or 3rd flight? Fairly many, it seems.

 

[Acheron]

The Horten was cool in the intro mission of Wolfenstein, but as an actual aircraft, sounds like another pretty concept that would have run afoul of real world physics.

 

[tyrodtom]

When I saw the film several years ago those points were IMHO most odd.
1) Already in 1940 British had the Chain Home Low Radar to detect low altitude objects. It was a very different animal to CH radar, 200 MHz frequency vs CH's 23 to 30 MHz. And they began operate CHEL radars, operating 3 GHz frequency, in 1943, just against very low flying a/c (EL = extra low). So why to test the a/c against a radar known to have problems in detecting low flying a/c already in late 30s and not against radars designed to detect low flying a/c and in service in 1945?
2) Horten proto crashed IIRC on its 3rd flight. Who believes that a revolutionary a/c design of rather unknown flight characteristics is put on its paces in a mock combat during its 2nd or 3rd flight? Fairly many, it seems.

I've always had doubts about that mock combat on it's second or third flight.
A lot of newly developed aircraft don't even retract their landing gear on early flights, let alone explore the upper limits of their flight envelope.

 

[MIflyer]

Anybody notice the B-35 in that Transformers movie?

 

[SaparotRob]

I hadn't been interested at all in the Transformers movies but a B-35, you say? Which movie?

 

[MIflyer]

It is The Last Knight. Cade Yeager (Mark Whalberg) meets a robot from England that is virtually a dead ringer for C3P0, and who tells him they need to go to see Sir Edmond Burton in England. The next thing you see is a B-35 flying away. I have not watched the whole thing, but it has been on TV quite a bit.

With those props, the B-35 would not have been very stealthy. I think when they built the YB-49 the stealth abilities came as something of a surprise. On the other hand, I guess that the props added to the stability.

 

[Koopernic]

Question was the Horten even viable with WWII-technology? My understanding is that flying wings have certain issues regarding stability I believe?

Flying wings were viable in WW2 and so was the Horton flying wing. They did have issues but they were ready. They'd been proven in 20 years of flying. The Hortons made many propeller driven aircraft.
I'll deal with the issues
1 Static Stability. Conventional aircraft achieve stability by having their centre of gravity so they are nose heavy. The tail plane is given a negative incidence (called decalage) so that it forces the nose up. This balance of forces is what holds the aircraft stable in pitch. Flying wings like the Northrops used wing sweep with about a 4 degree downward twist at the wing tips to get the same effect as a tail. Northrop also put slats on the wing tips to make sure they didn't stall and pitch up the nose.

There is another way to to achieve stability that doesn't require wing sweep or a tail and that is with an auto-stable aerofoil. These have a slight upward reflex at the tail to provide the negative incidence (called decalage) that is provided by negative incidence in the tail of conventional aircraft or by the negative incidence of the tips of swept wing flying wings. Aircraft made this way are often referred to as flying planks. They do tend to required a vertical tail since the sweep also provides yaw stability. In a swept wing the advancing airfoild expereinces more drag which restores yaw.

The Horton flying wings worked differently to the Northrops. The Horton flying wings used an auto stable air foil on the inner wing which transitioned to a conventional air foil at the tip. This only required 2 instead of 4 degrees decalage. It also didn't need slats. On the Ho IX and Ho 229 a bat like tail extension was added to increase this effect.

The Me 163, while not a flying wing because it had a rudder, used a combination of wing sweep, auto-stable air foil and a type of slat called a slot. Slots and Slats when they operate don't increase lift but allow about 50% greater angle of attack which is what ultimately supplies the extra lift but they also tend to pitch the aerofoil down which increases stability. (Which helped the Northrop's)

2 Dynamic Stability. The tail moment arm lT is defined as the longitudinal distance between the centre of gravity and the aerodynamic centre of the tail plane. In a flying wing this is very short and it creates problems that must be carefully dealt with. The result is if there is a change in flight direction or a disturbance there may be a slight oscillation. This oscillation does die of but it may be several seconds which causes problems with gun and bomb aiming. On the Ho 229 the solution was to apply some air brakes (both dragerlons simultaneously) when firing guns. This apparently worked and was reasonably practical. I imagine it would have worked on the Northrop.

There was something called a yaw damper. "A new use of automatic control in aircraft occurred in the early 1940's with the invention of the yaw damper. Dr. Karl Doetsch developed a gyro operated bang-bang servomechanism that drove an aerodynamic tab on the rudder to damp out yawing oscillations(U. It was first applied to a German Henschel He 129 attack bomber (Figure 4). The Hs 129 had a small vertical fin and high yaw inertia due to extensive armour plating which resulted in low aerodynamic damping. Dr. Doetsch's yaw damper provided artificial damping to improve its flying qualities."

See the pdf file I attached.

Basically a rate gyro would detect excessive yaw and switch in a electrical contact that drove an electric motor to apply rudder trim that damped out the yaw. As soon as the yaw stopped the motor would wind the rudder trim back to neutral. This is the kind of thing the YB-49 had developed for it. A pickup on the rudder peddles ensured actual inputs weren't trimed out. Dr Karl Doetsch was emplyed by the RAE Farnbrough after WW2 and a British developed yaw damper was applied to the Meteor jet ( F.8 and some F.4 I think)

The thing to remember jets, in particular swept wing jets needed yaw dampers. It was normal. The problem was simply harsher in flying wings and they needed pitch dampers as well but aircraft such as the B707 had to have yaw dampers.

3 Issue of development. In one of my books on Reich era aviation there is an congenial discussion between Kurt Tank (designer of the Fw 190) and another German aeronautical engineer about the value of flying wings and tailless aircraft. Kurt Tank, ever practical, makes the point that a flying wing bomber to carry the same load as a conventional bomber would not fly any faster nor further but it would only be able to fly higher. He said that the penalty for this would be a lot of extra risk and time in designing, testing and developing the aircraft and that in that time a conventional design would advance. (I think the range would be better but only at higher altitude.)

I think Tank was on the money here. Flying wings, though potentially better just weren't worth the extra hassle in development. Today with CAD and simulation the issues are less.

They also tend to have rather high landing speeds.

Note however tailless aircraft, in many ways flying wings with a vertical tail like the Vulcan Bomber were extremely successful and effective.

 

[SaparotRob]

Thanks Koopernic, your post explained a lot about flying wings that I just didn't get, like why they don't spin around the various axis. Still kind of fuzzy about how they actually turned without a rudder/tail. Great post!

 

[Koopernic]

My understanding was that the B-2 uses computers to make the flight stable (fly-by-wire), something not available of course to earlier aircraft?

Fly by wire came in to use in the 1950s: the AVRO Arrow, the German VTOL program (VAK 191,m VJ101) all used it because of the transition from Vertical to Horrizontal couldnt be handled any other way. I cant remember which US aircraft might have been a NASA Crusader.

Prior to this there were yaw and pitch dampers. See my earlier post about the one the Germans used on the Hs 129 in WW2.

The term flight control 'augmentation' came in. This is where you have mechanical linkages into which automated adjustment and corrections are applied. For instance if you applied rudder on an aircraft with a tall tail like the Bristol Britannia it could roll the aircraft and so a bit of aileron was automatically applied as a correction. If you applied aileron to roll the aircraft the lowered aileron produces more drag than the raised and you get provers yaw and the rudder can be applied to provide a correction or he lowering ailerons can be lowered less than the raised.

The most 'augmented' aircraft in history was probably the SR-71/Y-12 where there was cross coupled augmentation of pitch, yaw and roll controls.

The SR-71 needed it because of the vast differences in behaviour between subsonic and supersonic flight at different altitudes and fuel trims. Full fly by wire probably would have been better but Johnson didn't trust it.

If an engine 'unstarted' at Mach 3 the pilots helmeted head would hit on the cockpit window sides and I suspect the yaw damper is what saved the aircraft from worse. Any way ironically the engine unstart probably occurred because they used slow hydraulic computation for engine control (nose cone position etc) rather than electronic because Johnston didn't trust dang electronics.

The most notorious 'augmentation' system is of course the Boeing B737 MAX MCAS.

Fly by wire aircraft are not unstable (its not allowed EASA or FAA) but have relaxed stability.

Conventional controls let the pilot feel the back force on his controls as the aircraft speeds up so that he doesn't over stress the aircraft. It is linear with air density and deflection but goes up with the square of speed. (IAS)

Fly by wire usually reduces the control surface deflection with air speed air density (IAS indicated air speed) but the pilots side stick or yoke actually demands say a pitch rate and the fly by wire system compares this to internal gyros and produces a pitch rate proportional to control input. There is no need for artificial feel as the FBW limits deflection

 

[MIflyer]

One noted aviation writer said he did the calculations and there is no way the B-2 could get off the ground with the published thrust. So he inferred that there was "something else" at work.

 

[Koopernic]

Thanks Koopernic, your post explained a lot about flying wings that I just didn't get, like why they don't spin around the various axis. Still kind of fuzzy about how they actually turned without a rudder/tail. Great post!

They turned by banking (like a conventional aircraft) but also had 'rudders' in the form of air brakes at the wing tips (some times called draglerons) that could be applied differentially by the rudder peddles. On the Ho 229 this was just a piece of wood the extended out vertically from a groove. On other aircraft it was sort of split ailerons. They would have elevons for combined pitch and roll control and differential air brakes at each wing tip for yaw. Lots of ways people do it.

From Wikipedia

Yaw control
In some flying wing designs, any stabilizing fins and associated control rudders would be too far forward to have much effect, thus alternative means for yaw control are sometimes provided.

One solution to the control problem is differential drag: the drag near one wing tip is artificially increased, causing the aircraft to yaw in the direction of that wing. Typical methods include:

• Split ailerons. The top surface moves up while the lower surface moves down. Splitting the aileron on one side induces yaw by creating a differential air brake effect.
• Spoilers. A spoiler surface in the upper wing skin is raised, to disrupt the airflow and increase drag. This effect is generally accompanied by a loss of lift, which must be compensated for either by the pilot or by complex design features.
• Spoilerons. An upper surface spoiler that also acts to reduce lift (equivalent to deflecting an aileron upwards), so causing the aircraft to bank in the direction of the turn—the angle of roll causes the wing lift to act in the direction of turn, reducing the amount of drag required to turn the aircraft's longitudinal axis.

For pitch and roll control you could have separate elevons or use the above controls in unison.