Northrop XP-56J

[Tophe]

Well, in the secret files that I own at the hospital (psychiatric, yes, why?), the XP-51ZJ3 actually broke the 1,000mph barrier (it is written 10,000mph with probably a mistyping), on December 31st 1945, but this was a different aircraft, with one turbojet for cruise and 2 rockets for the final dash (and a second pilot if the first one faint due to the huge acceleration):

[wuzak]

As kit says it's logical and looks good 

You mention Titanium being used in the build of the XP-56 ? I thought it was Magnesium alloys which is why it was such a fire hazard and they had a heck of a job welding the thing ? I think they had to weld it in a sand pit so they could keep putting the fires out. Or am I thinking of another aircraft entirely ?

Yes, it was magnesium alloy.

Northrop had to invent a method for welding the material together.

And the material came from teh supplier with variable thickness, so Northrop had to plane the sheets to get them uniform.

Also, it was truly a stressed skin aircraft. It didn't use the multitude of ribs, at least in the wing, as other types.

[wuzak]

And the XP-56 reached 465 mph, the XP-56J reached 600mph (steelpillow will confirm), so the XP-56ZJ reached 1,200mph, this is mathematics, no?

465mph was one of those performance estimates the manufacturer gave to the USAAF.

Actual performance was somewhat less. Estimates after flight testing put it at around 340mph. Only 40-50mph slower than the similarly powered F6F.

The jet version would avoid some of the XP-56's issues though. One of the theories for lack of performance was that the turbulence from the tubby fuselage and wing interface seriously reduced the efficiency of the prop.

[NARSES2]

ou mention Titanium being used in the build of the XP-56 ? I thought it was Magnesium alloys which is why it was such a fire hazard and they had a heck of a job welding the thing ? I think they had to weld it in a sand pit so they could keep putting the fires out. Or am I thinking of another aircraft entirely ?

That's funny, I always thought Magnesium was spelled with a "T". Thanks for the tipoff. I have now corrected my spelling, ahem. 

Glad I wasn't going mad

[steelpillow]

Good, but even if they made a jet variant of that plane, it would also be to difficult to fly.

You just want me to burble on about how the wingtip extensions fixed that, don't you? The negative incidence is key. J W Dunne explained all ca. 1910. Of course, they fix both prop and jet variants alike, only they were never fitted to the prop variants.

Oh, yes, and they also reduce the nose-heaviness experienced by the second prototype with the gear down. That's easier to understand: because they exert negative lift, one of the things they do is to push the tail down a bit, helping to lift the nose.

[jcf]

Dunne is actually irrelevant to the XP-56 as his biplanes, and relatively unsuccessful monoplanes, were designed
to have 'automatic stability', i.e. to be inherently stable without control inputs, not a desirable feature in a fighter
plane. Truly Dunne-style large exaggerated downturned wingtips as used on his monoplanes would only serve to
increase drag on the XP-56 and exacerbate the control difficulties.

The XP-56 problems were directional stability, partially addressed by the enlarged dorsal fin added to the second
aircraft, poor fore and aft balance (nose heavy) partially corrected by re-ballasting to alter the CG and low speed
lift loss caused by turbulence created by the flaps. The high-speed ground handling issue that caused the first one
to wreck was put down to incorrect landing gear geometry, redesign was required but they decided they had neither
the time nor the money so minor changes were made to the second aircraft and handling limits were implemented.

I have a partially completed jet conversion of the 1/48 XP-56 kit and looked into the issues in detail, while a Goblin
would hypothetically fit into the space occupied by the R-2800-29 engine (similar diameter but ~ 10 inches longer)
the fact that the engine mounting is aft of the main spar and several hundred pounds lighter would increase the
CG issues. Redesigning the structure/systems layout etc. to move the mounting forward would be expensive, although
I suppose one could say it was done to allow for future mounting of the larger Ghost.

BTW any conversion of the aircraft would probably have originally been given a completely new designation number,
which would then most likely have been canceled and the next letter in the alphabet applied as a suffix, see any number
of real projects from the period. Most likely B as A would probably have been used for a limited production piston version,
YP-56A perhaps. Whatever the case it wouldn't have been 'J for Jet' as the USAAF/USAF of the period didn't do things
that way.

[steelpillow]

Forgive my plain thinking but I have made a close study of Dunne's work, both his actual patents and writings, and also those of others. Your description of Dunne's aerodynamics is far from what I have seen. He was wholly relevant and Jack Northrop is well known to have acknowledged his influence.

Nor were Dunne's monoplanes as "relatively unsuccessful" technically as some authors have suggested. They flew extremely well, only the first one was too heavy to fly and had to be lightened. They were only "less successful" in the sense that the military market demanded biplanes, which both both Burgess and Nieuport license-built instead of the monoplanes.

The feature which granted automatic longitudinal stability was in essence a progressive "wash-out", a gentle reduction in incidence from root to tip. If you look at photos of the preserved XP-56, you can see this in the wings. As with the tailplane on any conventional fighter, the idea is not to confer stability as such, but to neutralise the inherent instability of the conventional wing. Nortrop's wash-out is consequently less than Dunne's, but it is still there.

Dunne's approach to directional stability was far more subtle. You probably think he was inspired by the Zanonia seed. Far from it. He observed that the seed had poor directional stability, tending to scatter off in random directions at the slightest gust. This was a good evolutionary strategy but a poor aerodynamic property. So he resoundingly rejected the Zanonia model. Despite this, even before war broke out the myth had been established and knowledgeable speakers at the Royal Aeronautical Society sought to dispel it. Like may others at the time, he found the answer in the wings of soaring seagulls. But these were too complex a shape to manufacture easily so he developed something a little cruder mechanically but equally sophisticated aerodynamically.

The turned-down wingtips actually confer directional stability as well as longitudinal. To understand this, you have to understand the principle of differential drag. The wingtips exert lift, albeit downwards. Thus, they also exert induced drag. Consider what happens if the plane yaws to one side. One tip moves forward. Its angle to the airflow actually increases, increasing both downthrust and drag. This both tips the wing down into a turn towards the wind and also drags the wing back. The rearward wing reduces its angle to the air, reducing its downthrust and induced drag. It lifts up and accelerates forward, helping the plane to turn into the wind. Thus, there is no need for a tail fin. When Northrop reduced the longitudinal stability conferred by these wingtips, he also unwittingly reduced directional stability.

Directional control is, in both Dunne's and Northrop's case, again conferred by differential drag. Dunne's ailerons, being near the tips, had negative incidence. When operated, the down-moving aileron both lifted the wing and reduced drag while the up-moving aileron both lowered its wing and increased drag. This meant that the plane naturally banked into any turn. Correctly balanced, separate controls for pitch and yaw were not necessary and Dunne abandoned them for safety reasons.

It was this control system that most fundamentally stood in the way of a nimble fighter. Northrop abandoned it, instead producing bellows-like split ailerons which could open out like an air brake: the rudder bar or fore-aft movement of the joystick moved both upper and lower parts together, while sideways movement of the joystick operated the appropriate set of bellows.

All in all, the Northrop design is much closer to the Dunne pattern than the contemporary designs of Hill, which follow on from Dunne in a direct lineage.

Northrop needed only to fix the directional stability problem to gain a usable fighter. They first tried to fix it by adding a tail fin, but it was not enough. The problem lay in his mucking with the swept-back tips and that's where the solution had to lie.

So much is fact. I take off from here. The problem of directional stability was the one which the XP-56J wingtips addressed. If you have followed the above you will realise that they also increased longitudinal stability, one reason they had to be kept small.

Oh, and yes the Goblin was under-weight. It was accompanied by lead weights to obtain the correct balance. The more powerful Ghost planned for production was a heavier engine and would have located farther aft with a stretched tailcone, giving a properly balanced design.

The "J" was informally adopted. Officially, this was just the third prototype XP-56. The modifications were little more extensive than the changes made between the first two prototypes.

The production Ghost would of course have been license-built in the US.

Can't wait to see your version though.