XB-70 Valkyrie

[Terence Lynock]

How would you calculate the wing area of the XB-70 Valkyrie? do you use the upper surface area or the lower surface where there is a large engine pod cum weapons bay taking up a lot of space, need to work out just what the wing loading is on the Depron model i am just working on,

                     regards,         Terry

[Simon Chaddock]

Hi

The wing area of the full size was given as 6,296 sq ft (585 sq m) so multiply this by the square of the scale of your model and you have the wing area. i.e for a 1/10 scale model the wing area would be 6,295*1/10*1/10 = 62.95 sq ft.   Conventionally wing area is taken as the "projected" surface so engine ducts are included.

On any airframe the wing loading itself is not the only criteria as the wing section plays an important part. Remeber however deltas do operate a bit differently from a conventional wing in that they continue to generate lift to very high angles of attack but create a lot of drag doing it. Ok when you have the power to go supersonic but care is needed on a model unless you have sufficient of spare thrust.

How is your Valkyrie to be powered?

[Pete Rieden]

Look at the whole horizontal area (including the engine pod) - the area of the shadow cast by the aeroplane. It all contributes to the lift.

Interesting aeroplane. Did you know that the full-size let the foreplane "weathercock" in a zero-lift condition when flying subsonically and only used it to generate lift when supersonic. This offsets the effect of the aerodynamic centre shifting aft. Concorde pumped fuel ballast around to achieve a similar effect, and modern supersonic fighters just fly with a rearward CG so that they are in-trim when supersonic (for lower drag). This in turn is the reason why these fighters have fly-by-wire control systems and artificial stability - to allow lower supersonic drag. There is a common myth that the unstable layout makes them more manoeuverable, but this isn't actually the case.

PDR

[Bev Lawton]

Mmmm....  can't agree here.

Taking the Typhoon as a an example it has been mad DEBLIBERATELY unstable just so that it is more manouverable.

The amount of time a fighter would be at supersonic speeds is very, very low - probably less than 1%

You would NOT design an aircraft so that 99% of the time it is IMPOSSIBLE for a human to fly it without the help of multiple computers sensing 3-D axis movement and adjusting control surfaces accordingly just to reduce supersonic drag.

Modern aircraft are JUST a carrier for weapons (Usually missiles) and you have to bear in mind that the design of the Typhoon is 20+ years old and the desirability of a plane being able to basically turn inside it's opponent (and therefore allow the then very narrow missile lockon angle to aquire the target).

The Russians have achieved this in the Mig 29  --- 35 series by allowing a "Snakebite" manouevre.

However modern missile aquisition systems have altered the game and have almost rendered this sort of design obsolete. They have achieved this in two ways (A) The aquisition angle of the missile is wider

 and therefore can "see" more sky and thus the target aircraft without having to turn faster inside the attacked aircraft. (B) More importantly modern helmet missile cueing systems allow the pilot to "look" at the target  within more than a 180 degree area and then "tell" the missile where the target is. The missile is launched  without actually having "locked onto" the target.

This ability to launch without lockon means even the most basic airframe when upgraded with an appropriate helmet and missile system is more than a match for the most modern aircraft.

Ask the Americans how they got on against the Indians three times in a row against the most basic type of aircraft.

Technically modern aircraft are equipped with missiles and radar to allow BVR Beyond Visual Range engagements - they very rarely employ this tactic as they nearly always have to visually identify the target first.

Modern aircraft have "Fly by Wire" systems because the control systemhas multiple redundancy  - wires being routed many and different ways to avoid damage - something that was difficult in the older hydraulic systems. It matters not what system you have in the end you move a control surface irespective the method of translating a pilots input on the stick into a movement on the control surfaces.

In addition fly by wire is less complicated and very much lighter - less weight on the aircraft = more payload and/or range (ie fuel). Compare a modern "fighter" payload to that of say a Victor or Vulcan bomber!

How can a Fly by Wire system lower supersonic drag?

[Bev Lawton]

The foreplane of a canard design is NOT generally there to generate lift - it is there to generate (via lift I agree) control - in the example of the XB70 considering the vast wing and huge lift generated why would they want/need the tiny amount of lift generated by the canard wing? They are actually trying to REDUCE lift at these high speeds as the higher  the lift the greater the drag - you only need enough lift to fly it at the lowest speed with a full allup weight plus a bit as a safety margin.

The canard wing is there for pitch control as the control surfaces on the trailing edge and aft of the trailing edge become LESS effective at higher speeds due to lots of reasons but the shockwave moving aft along the body at increasing speed is the main reason - bye this time the canard wing is in "clear" air and far more effective.

Concorde (and lots of others) pumps fuel forward as a result of both CofG changes due to fuel usage AND the Aerodynamic changes due to increased speed.

[Dusty]

might help

[Terence Lynock]

One thing that has always confused me is why aircraft such as the F117 need computers to keep them stable in flight, to adjust control surfaces automatically to get the aircraft to perform and do its job yet we can knock up a model at 6 foot span with no computers and the thing flies pefectly.

A friend has a 4 foot span F16 that is as tame as your pet budgie, properly trimmed it is a hands off job and will cruise around quite happily at 2/3rd power and when it is out of go it just floats in, I just didnt believe a model with such a small wing area could be so benign,

                       regards,         Terry

[Bev Lawton]

I'm not sure about the F-117 - initially it was knickname "The Wobbling Goblin" but that was in it's "Have Blue" design phase. Speaking to pilots of the real thing they say it is a very stable aircraft - wether this is down to software improvements in the Flight Control/Fly-by-Wire systems or just the natural aerodynamics of the airframe being sorted out I have no idea.

Pilots say that the F-117 doesn't justify it's reputation for being unstable.

One other thought - just how true to life are the wing sections etc on the models compared to the real thing? The facetted fuselage shapes I would suspect will have very little to do with the aerodynamics.

[Terence Lynock]

True, the actual wing of the XB-70 had few curved surfaces with a strange <======> wing section and the wing was thinner at root and tip than in the centre, the section of the Canard was a thin symetrical section with sharp leading and trailing edges.but with eleators rathar than a full Canard all moving plane.

When the XB-70A -2 was hit by the Starfighter and lost its vertical surfaces I wonder what would have happened if the pilots could have drooped the wing tips fast enough to give it some stability, -

At full droop maybe she would have stayed airborne,

regards, Terry

[Pete Rieden]

OK, I'll try to keep this simple.Firstly manoeuvrability in a fast-jet sense is solely a function of the instantaneous pitch rate (how quickly the aircraft responds to an "up elevator" demand). Simple Newtonian mechanics tells you that this is determined by two factors: the amount of pitch "force" available (the "elevator power") and the moment of inertia about the pitch axis. "Stability" doesn't come into this at all, no matter how much we intuitively "feel" it should. The common belief that unstable aircraft are more manoeuvrable is simply a long-perpetuated myth which has aerodynamicists tearing their hair out..So why make aircraft unstable? Well it's to do with something called "trim drag". To put it simply: We all know that a wing is basically unstable in pitch, so we bias it in one direction with the CG and add a tailplane or foreplane to develop some lift to oppose it and hold the aircraft steady. Now developing lift also develops drag - it's unavoidable because lift and drag are the static and dynamic forms of the same pressure changes. The extra drag due to the lift developed to hold the aircraft steady is called the “trim drag”.

Obviously, as we would like to keep this drag to a minimum, we only want to have the tail/fore-plane developing the minimum amount of lift required to assure stability. The amount required is determined by the amount of weight bias we put on when we choose where to put the CG, and the general idea is to simply ensure that the CG is in front of what we might call the "aerodynamic centre" (horrible term, but let's keep it simple) throughout the flight envelope. The further the CG is from the “aerodynamic centre” the greater the trim drag. Hold onto this, I’ll be coming back to it in a minute.

[continued in part 2]

[Pete Rieden]

[Part 2]

Now in subsonic flight the “aerodynamic centre” can be considered to be at the 25% chord position (a simplification, but I’m trying to keep it simple). But in SUPERsonic flight it shifts back to the 50% chord position for reasons too involved to go into here. So an aeroplane trimmed for subsonic flight will need a bucketload of up elevator to hold level at supersonic speeds – this is why early attempts at supersonic flight got into ever-steepening dives. So we hold the aircraft level with very powerful pitch controls, but these produce vast amounts of trim drag, requiring a lot of power to overcome and dramatically increasing fuel consumption. Hold onto this as well, because we’ll put it all together in a minute.

Now the final element. The Typhoon design requirement was set up in the late 70s/early 80s, and at that time the holy grail of fighter design was “supercruise” – the ability to cruise at around Mach 1.2 without using afterburner. Lightnings could do it, but not for long, and Concorde could do it for hours at a time, but no American fioghters could do it until the late model (post block 60 IIRC) F-16. It was deemed essential for all future fighters to be able to supercruise to increase their radius of operation and thereby allow fewer fighters to cover a larger area. The whole canard layout of Typhoon is dedicated to this requirement.

Now to do supercruise you need the lowest supersonic drag you can get, and high trim drag was as welcome as a rattlesnake in a lucky dip. There are a number of ways of avoiding it – you can use variable geometry, but that is heavy and complex – usually done with swing-wing layouts, although the Vlakyrie is a variation on this theme. You can use variable ballast (which is what concorde does), but this requires a minimum amount of fuel to remain in the aircraft on landing and fighters couldn’t do that (concorde did it by using the IFR reserve fuel, and if they were “stacked” for too long the subsequent landings could be “interesting”).  

But another way of doing it was to set the CG so that the aircraft has the minimum required stability when supersonic, and then use a full-authority autostabiliser to make it flyable at subsonic speed. This was tried with analogue autostabilisers, but they were never reliable or capable enough to be safe. The whole prospect only became practicable with the advent of more modern digital systems and some more powerful/reliable software developments. And that gave rise to the Typhoon, and its “unstable” configuration. It’s all about fuel burn, and nothing to do with agility at all.

PDR

[Pete Rieden]

[message duplicated due to server problems]

[Bev Lawton]

Mmmm interesting Pete - why is it then whenever you see an article etc about the Typhoon in particular they always bang on about this instability and how it makes the aircraft agile?

Funnily enough the XB-70a achieved a similar effect using it's drooping wing tips.

Part of the benefits of the Compression Lift effect it employed.

In flight the XB-70a could lower the outer wing sections either :

25 Degress for speeds 300 knots - approx mach 1.4

65 Degrees for speeds Mach 1.4 - Mach 3+

Lowering the wingtips had 3 differing effects on the XB-70a :

The total vertical area of "fin" was increased allowing them to have smaller vertical stabilisers than were otherwise required.

The reduction in rearward wing area countered the delta wings inherent rearward shift of the center of lift as speed increased, this kept trim corrections and thus Trim Drag to a minimum.

The effect of the Compression Lift was 30 percent more effective because the pressure under the wing was better managed.

Mind you you are going to have to put one hell of an engine on this model to achieve the Compression Lift of the real thing !!!

[Terence Lynock]

Hi Bev,

I have a spare 5.6 litre Dodge Hemi but suspect it may push the wing loading up a little, actually the model started out as a 12'' span chuckie to test the plan but sort of grew a little until eventually it was big enough to fit R/C into so why not?.

The idea was to make sure the plans I have are viable and build a big version later on, this one is about a quarter the size of the large version I have in mind, when this one is retired (if it survives) it will end up hanging from my grandsons bedroom ceiling probably with its innards donated to some other project,

regards Terry

[Pete Rieden]

Bev Lawton wrote (see)

Mmmm interesting Pete - why is it then whenever you see an article etc about the Typhoon in particular they always bang on about this instability and how it makes the aircraft agile?

We've often wondered about this. As far as we can establish the origin of the myth goes back to an off the cuff remark made to a journalist at a conference in the late 60s. But you won't find it being discussed by aerodynamicists.

Bev Lawton wrote (see)

Mmmm interesting Pete - why is it then whenever you see an article etc about the Typhoon in particular they always bang on about this instability and how it makes the aircraft agile?

Funnily enough the XB-70a achieved a similar effect using it's drooping wing tips.

Part of the benefits of the Compression Lift effect it employed.

In flight the XB-70a could lower the outer wing sections either :

25 Degress for speeds 300 knots - approx mach 1.4

65 Degrees for speeds Mach 1.4 - Mach 3+

Lowering the wingtips had 3 differing effects on the XB-70a :

The total vertical area of "fin" was increased allowing them to have smaller vertical stabilisers than were otherwise required.

The reduction in rearward wing area countered the delta wings inherent rearward shift of the center of lift as speed increased, this kept trim corrections and thus Trim Drag to a minimum.

The effect of the Compression Lift was 30 percent more effective because the pressure under the wing was better managed.

Mind you you are going to have to put one hell of an engine on this model to achieve the Compression Lift of the real thing !!!

Well yes and no. In actual fact this is a solution to the same transonic AC shift as I described fopr the pitch axis - it holds true for yaw as well. By and large this is addressed by having huge (and multiple) fins and accepting that there is excess directional stability at subsonic speeds. Again, it's interesting to note the much bigger fins on "normal" supersonic aircraft (Tornado, F14 & 15) compared to "unstable" fly-by-wire ones (Typhoon, F-22). As you say, the original reason for dreaming up the droop-tips was to create the tunnel required for the shock-lift system needed for speeds above mach 2.5. The elegant part of the solution was that at the same time it reduced the rearward wing area (partially compensating for the rearward AC shift) and increased the rearward fin area (for the same reason). It was sufficient to retain supersonic directional stability, but not quite enough to offset the pitch trim change - hence the need for weathercocking the foreplane at subsonic speeds (another extremely elegant design solution).

Note that the "rearward shift of the centre of lift" isn't a delta characteristic - it's a characteristic of all bodies when they exceed their critical mach number.

PDR

[Bev Lawton]

Terry I've copied this :

The flight plan for today is simple: they will make several passes over recording instruments at a speed of Mach 1.4 at 32,000 feet, then, at the request of General Electric, they will fly in formation with 4 other GE-powered aircraft so that GE photographers can take some publicity pictures from an accompanying Learjet. The boom-testing went smoothly, then, dropping subsonic speed and raising the wingtips back to 25 degrees, the Valkyrie joined up in formation with the other aircraft, including, just off her right wingtip, an F-104 Starfighter piloted by Joe Walker.

Different views of the formation (not in a specific time sequence). Walker's Starfighter is directly off the Valkyrie's starboard wingtip.

<a href="http://www2.interceptor.com/%7Ethumper/xb2/s-in-flight2.jpg">

</a> <a href="http://www2.interceptor.com/%7Ethumper/xb2/close-closeformation.jpg">

</a> <a href="http://www2.interceptor.com/%7Ethumper/xb2/s-closeformation.jpg">

</a>

As the photo shoot progressed, the photographers asked several times for the formation to close up, until all five planes were in close proximity, and had been for over 45 minutes. Finally, at 9:26am, the photographers were done, and everyone prepared to break formation and return to Edwards.

Continued in next post.....

[Bev Lawton]

Disaster struck at this moment as somehow, Walker's F-104 collided

with the Valkyrie. The complex airflow surrounding the XB-70 lifted the F-104 over her back, spun the Starfighter around 180 degrees, causing it to smash down along the center of the Valkyrie's wing, tearing off both vertical stabilizers and damaging the left wingtip before falling away in flames. Already, Joe Walker, one of America's great test pilots, was dead.

Al White and Carl Cross heard the impact, but felt nothing. Flying in the T-38 off the left wingtip, Joe Cotton called out "207 (identifying AV/2) you've been hit! You've been hit!" But in those first moments, neither White nor Cross heard the call. Even as Cotton continued "...okay, you're doing fine, he got the verticals, but you're still doing fine," White turned to Cross and asked, "I wonder who got hit?"

16 seconds after the impact, the XB-70 started a slight roll. Al White corrected the roll -- and instantly recognized the Valkyrie's peril as she began a snap roll to the right. Ramming the number six engine's throttle to maximum afterburner, he tried to save AV/2 -- but after 2 slow rolls, the plane broke into a sickening spin, taking any hopes of recovery with it. White pushed his seat back into the eject position, but caught his arm in

the ejection pod's clamshell doors (see inset photo at left) as they closed. Unable to communicate with the struggling Carl Cross, and unable to eject until getting his arm clear, White could only watch his co-pilot fail to get into his pod for ejection. Finally, with the realization that he needed to get out now, Al White worked his arm clear and ejected just moments before AV/2 slammed into the ground a few miles north of Barstow, California.

Continued in next post.....

[Bev Lawton]

Although the drogue chutes deployed from White's pod, he realized the airbag underneath the pod -- designed to absorb much of the impact -- had failed to inflate. Striking the ground, White took a 44G impact -- lessened to 33Gs as his chair broke free of its mountings. Amazingly, although banged, battered, and bruised, he suffered no broken bones. Although White returned to flight status just three months later, he never flew the XB-70 again.

Carl Cross was not so lucky. Unable to escape the Valkyrie, and still strapped into his seat, he was killed instantly when the XB-70 struck the ground in an upright and level configuration.

Something I didn't know :

Overall, the XB-70 has the best lift-to-drag ratio of any manned airplane ever built, being bettered only on the unmanned D-21 drone, an airframe designed to be air-launched, fly at one speed and altitude, and then self-destruct (thereby not needing to land). 

The official NASA enquiry didn't just blame vortices etc it actually blamed the fact that  Joe Walker in the F-104 was flying in front of and below the starbord wing of the XB-70 and as such could only get a good visual reference to the wingtip (to establish HIS relative position to the XB-70) by craning over his left shoulder and looking back - and thats not easy or comfortable in a F-104. Because of this they surmised he was using the nose as a reference point which wasn't suitable and he slowly drifted into the wing tip.

I guess the reasons why they didn't lower the wing tips was (A) It all took place in 16 secs. (B) The F-104 took out the left wingtip anyway.

Not sure if the drooping was effective (as a fin) at low speed - all I do know is that it INCREASED  the effective fin control at SUPERSONIC speeds.