NT-33A Variable Stability Simulator

[Old Viper Tester]

The NT-33A, USAF s/n 51-4120, was a variable stability in-flight simulator. It was owned by the Air Force Flight Dynamics Laboratory and, later, the Air Force Research Laboratory. The aircraft was maintained and operated by Calspan Corporation of Buffalo New York, under contract to the USAF.

 

This T-33A was modified in the late 1950s for flight controls investigations and simulating the flight characteristics of other aircraft designs. Schedule permitting, the aircraft was sometimes made available for contractor or foreign government evaluations, usually to correct anomalies found in new aircraft designs. The aircraft was also used by both the USAF and US Navy test pilot schools as part of their flight controls evaluation curriculum. This is where I first encountered the jet.

 

The F-94B nose houses computer units interfaced to the aircraft flight controls. The rear cockpit is modified with banks of controls to change the coefficients and variables of the flight control equations, thereby changing how the aircraft responds to control inputs.  The Calspan safety pilot would occupy the rear seat while the research pilot or student would fly the aircraft from the front. The safety pilot could take control of the aircraft at any time using the basic T-33 flight controls. or if specified flight conditions were exceeded, usually yaw/pitch rates or Gs, the system would automatically kick off the variable stability control laws and revert to T-33 flight control operation.

 

The aircraft has changed some over the years. During the 1960s, the aircraft tip tanks were modified to have the aft portion of the tanks act as clam shell speed brakes. This modification was later removed.  In the late 60s/early 70s, the F-94 acquired rows of vents on each side to facilitate cooling as more computing power was added to the variable stability system.

 

My TPS class team project was to investigate changing the roll axis of the aircraft through flight control changes and its effects on maneuverability. Most of the tests involved evaluating the ability to handle various target tracking scenarios.

 

Lots of piccies…

 

Ready for brake release...

 

 

 

 

 

 

 

Returning to Eddie's Air Patch...

 

A long pass down the Edwards Tower Fly-By Line...

 

 

 

 

 

Nose-on view...

 

Computers circa 1978...

 

Some of the variable stability input controls in the rear cockpit

 

Side-stick controller in the front cockpit. There's a center control stick as well.

 

Nose detail in 1988

The stickers on the nose are from left to right, top to bottom: Swedish flight test center, IAI Kfir C2, NASA, USAF Test Pilot School, US Navy Test Pilot School, unidentified, SAAB JAS 39. The first and last are interesting in that the aircraft took part in flight controls development of the Gripen, yet in the following year, the Gripen would have the first of two mishaps attributed to Pilot Induced Oscillation (PIO) caused by flight control software issues. After the 1989 mishap, the NT-33A was again being used to investigate the cause and possible corrections to eliminate the PIO situation.

 

The NT-33A was retired and put on display in the National Museum of the USAF in 1997 after many years of being the oldest aircraft in the USAF active inventory. When it went to the museum, it retained the dummy refueling probe used in one of its last research projects and remains there on display...

 

Thanks for looking,

Sven

Edited November 7, 2017 by Old Viper Tester
Added info

[Corsairfoxfouruncle]

Excellent photo’s thanks for sharing them.

[Phantom726]

Brilliant! Thanks for sharing!

[Giorgio N]

That's another set of fantastic pictures, thanks for sharing ! It's an aircraft of which I knew nothing before reading this thread and that looks very interesting from a modelling perspective. It's also very interesting for an engineer like me

Have to say that I'm very envioius (of course in the positive sense of the word) when I see these pics, you must have had one of the best jobs in the world !

[Old Viper Tester]

51 minutes ago, Giorgio N said:

you must have had one of the best jobs in the world !

Thanks Giorgio.

 

I can truly say that I consider myself very lucky in my USAF career. All of my assignments can be boiled down to three categories: flight test engineer, technical intelligence analyst, or systems acquisition programs flight test manager. All were very interesting and rewarding. I sometimes wish that I could have gone to pilot training or got to do some rigorous engineering design in line with my aero engineering degree. Most of the engineering I did in the USAF was evaluating what others (contractors) had proposed or produced. But then, the USAF paid for my engineering degree, so it was fair that they determined how I used it!

 

Sven

[72modeler]

18 hours ago, Corsairfoxfouruncle said:

Excellent photo’s thanks for sharing them.

Yes! Never knew about this one- incredible detail photos!

Mike

[F-32]

Saw it last year, fascinating jet

 

Lockheed NT-33A by tony_inkster, on Flickr

[CurrantBunbury]

Very interesting, and completely new to me. Thanks for sharing your pictures.

[Smudge]

Thanks for sharing. I always enjoy your pictures.

 

I just bought a 1/72 Heller F-94 Starfire and a Sword T-33. I was wondering how much the T-33 fuselage had changed to accommodate the Starfire 'upgrades'. Now I know, not much, the nose just bolts on!

 

Interesting about the Gripen and PIO, didn't a YF-22 suffer a similar mishap? I think the upshot of it was that the pilot was pushing down, the plane went down but a bit too much, so pilot pulls up, plane goes up, too much, pilot pushes down. This goes on for a bit..........pilot aims for ground.........hits it!.........

 

The early days of fly by wire and computer generated 'feel' on the control stick 

 

Of course you engineers will know more about that than me..........I hope!

 

 

   

Edited November 12, 2017 by Smudge
spelling!

[Old Viper Tester]

On 11/12/2017 at 4:43 PM, Smudge said:

I just bought a 1/72 Heller F-94 Starfire and a Sword T-33. I was wondering how much the T-33 fuselage had changed to accommodate the Starfire 'upgrades'. Now I know, not much, the nose just bolts on!

 

Interesting about the Gripen and PIO, didn't a YF-22 suffer a similar mishap? I think the upshot of it was that the pilot was pushing down, the plane went down but a bit too much, so pilot pulls up, plane goes up, too much, pilot pushes down. This goes on for a bit..........pilot aims for ground.........hits it!.........

Considering the F-94 design actually started with a T-33 fuselage, just swapping the nose just before the windscreen is what I was counting on as well. The farthest I got was making a resin duplicate of the Heller F-94B nose with the intention of putting it on a Hasegawa T-33, but that was the state of play about ten years ago and it is still in the box.

 

I don't remember the cause of the YF-22 problem. In general the PIO gets started because of delays in the flight control response, due to mechanical design (dead band, friction and breakout, etc) and, possibly, pilot reaction time, the pilot control inputs end up being 180 out from what the aircraft is doing. Continued inputs generally make the amplitude of the oscillation worse. An F-4A actually disintegrated attempting a low-level speed record at Holloman AFB (Sageburner?) due to PIO exceeding the structural load limit of the airframe. There is an amazing film tracking the (practice?) flight showing the aircraft breakup, the fuel ignites in a fireball and the two J79 engines proceed through the fireball. 

 

Altitude permitting, and assuming the aircraft is well damped, the best course of action is to stop making inputs and hold the controls at neutral until the aircraft settles down. Then try to maneuver again. Fly-by-wire can often be worse, because in addition to any mechanical delays, the flight control laws may add delays as well, depending on the number of operations/calculations required to enact a control movement and similar considerations for the feedback loop. We're talking milliseconds here, but it can be a real concern. Beyond that, we get into a bunch of feedback and control theory in which I probably remember enough just to be dangerous.

 

For the stick "feel" in fly-by-wire, in most center stick aircraft, the stick does move and the system has resistance mechanisms to give the cockpit controls feel, just as springs and bob-weights provide feel for some mechanical flight control systems. The "feel" often being a function of angular rate of motion and/or Gs.

 

The sidestick controller in the F-16 originally did not move at all, and even though the pilots would feel in their bodies or visually recognize the aircraft motion/response to their control inputs, they would not get the tactile sense in their hand of the stick moving and thus over control the aircraft. The answer was to let the stick move, but only about a quarter inch deflection measured at the top of the stick. That little bit of motion is enough to give the tactile feedback that applying pressure to the stick is actually moving something (which is moving the aircraft) and most pilots readily adapt to it.

 

Sven

[HOUSTON]

Sven,

Firstly THANK YOU FOR YOUR SERVICES . 

 

Secondly  liked reading about the "history"  of the aircraft which I always find fascinating and SO very interesting.

 

Sir your photos are SUPERB and I enjoyed looking at them. What a Beautiful  looking aircraft.

 

Thank you.