F-101 Voodoo, McDonnell


F-101 Voodoo

The F-101 was a larger and more powerful development of the F-88. It was heavy and large, but also powerful and fast, with an impressive range. Due to continuously changing requirements, the F-101 had a checkered development. Its original role was to be that of a 'deep penetration' long-range escort fighter. Instead, it served as nuclear attack fighter, all-weather interceptor, and reconaissance aircraft. Although it once equipped the majority of USAF interceptor units, it never was a major type in the US inventory, because of some control problems which limited its effectiveness. The F-101 was also flown for a long time by Canada. The reconaissance version was much used in Vietnam. 807 built.

Type: F-101B Voodoo
Function: fighter
Year: 1957
Crew: 2
Engines: 2 * 6750kg P&W J57-P-55 afterburning turbojets
Internal fuel capacity: 7771 L (2,053 US gal)
Wing Span: 12.09 m
Length: 20.54 m
Height: 5.49 m
Wing Area: 34.19 m2
Wing loading: 607 kg/m²
Empty Weight: 13141 kg
Max.Weight: 23768 kg
Max. Speed: 1965 km/h
Ceiling: 16705 m
Rate of climb: 250 m/s
Max. Range: 2500 km
Armament: 6*msl
Unit cost: 1,819,000 USD


Background

Initial design work on what would eventually become the Voodoo began just after World War II, in response to a contract by the newly formed USAF for a long-range high performance fighter to escort bombers, much as the P-51 Mustang had done in its time. Designated the XF-88, a prototype first flew from Muroc on October 20, 1948. The military initially rejected the need for bomber escort, but experience over Korea indicated that the current bombers were vulnerable to fighter interception. In 1951 the USAF issued a requirement for an escort and designs from all the major manufacturers were submitted. The McDonnell design was a larger and higher powered version of the XF-88, and won the bid in May 1951. The F-88 was redesignated the F-101 Voodoo in November 1951.

Design work was relatively simple, as the only major change from the initial specification was to change the Westinghouse turbojets for a much more powerful Pratt & Whitney design. The new engines required certain changes to the intakes and a lengthened fuselage to triple the fuel load. The design was approved, and an order for 39 F-101As was placed in May 1953 without any prototypes built. Performance was considerably greater than the barely supersonic F-100 Super Sabre, but would fall short of later Mach 2 class fighters. However the Mach 1.7 performance is very comparable to the current McDonell Douglas, now Boeing F/A-18E/F Super Hornet which trades off maximum speed for turning performance.

The first flight of the F-101A was on September 29, 1954. The end of the war in Korea and the development of the B-52 negated the need for fighter escort and Strategic Air Command withdrew from the program. The aircraft would be employed primarily as a 2-seat air defence interceptor, nuclear fighter bomber, and a reconaissance platform which saw service over Cuba and Vietnam.

The Voodoo's replacement as a bomber would be the McDonnell Douglas F-4 Phantom II. While the Voodoo was a moderate success, it may have been more important as an evolutionary step towards the Phantom, the most versatile and successful western fighter design of the 1960s if not all time. The Phantom would retain the twin engines, twin crew for interception duties, and a tail mounted well above and behind the jet exhaust.

Problems

The Voodoo had numerous problems during its lifetime that seemed downright scary; among these were:

Nose pitch up
At low speed or high speed/high 'G' and certain angles of attack it was thought that airflow separation occurred on the wing surfaces outboard of the wing fences. This down wash would envelop the horizontal and vertical tail surfaces and the aircraft would pitch up, airspeed would bleed off to about 110 kts.- and the stall would grow and progress inward and forward along the wing until only the inboard leading edge was producing lift. This would lift the nose of the aircraft and it would fall into an spin, oscillating between 20 and 70 degrees of pitch up.

In this oscillating stall condition the engines also stalled from lack of air induction. (In case you think this is an unusual action for a jet engine, just go out to your local jetport and watch the older 'commercials' idling on the taxi-ways in high wind conditions. Watch for the times that the wind blows across the intakes and you will see and hear the engines choking and burping and being not very happy at all.) There was not enough air across the controls to make a difference at this point so you just had to ride it out, or eject.

If you had lots of altitude and you hated parachutes the procedure was to wait for the aircraft to unload to the lowest angle of attack then deploy the drogue chute to try to keep the nose down. If your head stopped banging on the canopy you tried an engine restart and -if the engines lit- you firewalled the throttles and powered out of it until the aircraft was flying again at 230 to 350 kts., then you released the drogue. Below 15,000 feet standing orders were to eject since the Voodoo lost altitude quickly without lift. To remedy this problem the Air Force installed a hydraulic system, attached to angle of attack sensors on either side of the nose, that would not allow the stick to be pulled past specific points at specific speeds.

Nose pitch down
Similar to nose pitch up, this happened if the stick was pushed too far forward too fast at certain speeds. The flow of air over the upper wing seemed to separate in these conditions and cause the wing to loose lift. The aircraft would first fall nose first, then tumble as the tail (which still had lift) overflew the nose. The engines might stall and there might not be enough air over the control surfaces for control, but if you were quick and the engines remained lit you could catch it with the throttles it as you went through the horizontal again and fly out of it in sort of a non-spinning snap roll. The same hydraulic stick limiter designed for pitch up seemed to work as the remedy.

Inertial roll coupling
If you did a fast, large, roll at the same time as pulling G. the lower wing tip would stall outboard of the wing fence - causing an increase in the roll rate and dropping the stalled wing back - so that the roll became an asymmetrical bobble. Sort of like a snap roll again, but with no rudder fed in. If you tried reversing the controls to roll back, the ailerons would only cause drag on the stalled wing and the stall would worsen. The wing would drop until it really caught the slipstream then bounce up again... then stall again, etc. Unless you unloaded back pressure the condition would not stop, and in an oscillation or two you would find yourself in a nose pitch up condition... see above.

Missile Problems
The heat from the electronics bay cooling system exhaust ports just under the cockpits was found to be bathing the underslung port side AIM 4-D infrared missile, causing damage to the missile electronics and causing it to (I am only guessing at this point) get confused and home on the aforementioned heat vents when it was fired. I assume this because the missile would detonate two seconds after it was fired. Turbulence inducers and belly strakes to divert the heat around the missile solved this nasty problem. Luckily the nuclear rockets seemed unaffected by the heat wash during testing.

Instrument Panel Attack
It was not unknown in the early Voodoos for the instrument panel to fall into the pilots lap. This could be distracting, especially on take off, and could make it difficult to read some of the instruments unless you managed to hold the panel in just right way.

I can just see it, you ram the throttles forward and into the afterburner slot and the aircraft leaps ahead, comes to speed and you rotate and pull the wheels up in a 'jock'-like manner - the panel falls off and hits the control stick forcing it back into your lap. The AOA limiter, sensing too steep an angle of attack, hydraulically shoves the stick forward into the panel... which shoves it back... which causes the limiter to shove it forward into the panel... which shoves it back... which ....etc.

And all this time the guy in the back is asking "Excuse me pilot, just exactly what are you doing up there?" or words to that effect; and you notice for the very first time that the farmer swathing the field at the end of the runway has a small tattoo of a rose on his left shoulder; and you are trying to recall just what is rated as an acceptable impact - as seen from the point of view of the two Genies you have slung down below. They used stronger bolts, or glue, or something and fixed this cutie.

From the late 1950's the Voodoo suffered from a spare part and support equipment shortage, fire control system maintenance headaches, and a complete loss of purpose when the USAF - realizing that a pure interceptor was doomed to failure - that any interceptor would have to be able to dogfight as well - started phasing the Voodoo out of the interceptor squadrons and converted some for work in photo-recon units. Canada kept them flying as front line aircraft until 1987, and Taiwan flew theirs (also a lend-lease deal) until about 1985.


Ejection Seats

Seats for the F-101 Voodoo were manufactured by either MacDonnell or Weber. Neither were zero-zero capable and neither were good at low altitudes (even though they were rated for 50 feet) and they had a minimum speed limit of 120 knots (to add a forward vector and increase your distance (not time) in the air). Maximum speed limit was 525 knots, so you can see that the rocket had a big initial kick to clear the tail, but it didn't last very long.

The seats were designed and normally configured to accommodate a back-pack parachute and were equipped with an over land survival kit, but a parachute support bulkhead could be removed to provide additional equipment (life raft, hibatchi, etc.) if desired. I seem to remember that anything additional fell out of the seat and hung from a rope attached to your harness; the extra weight would cause the 23 foot navy conical chute to reach a disconcerting velocity until the raft hit the water. At that point the extra weight was removed and the parachute resumed its normal descent, letting you down just slightly after your seat hit. There was also a seat/man separator mechanism to get rid of the heavy bits at the apogee.


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