F-101 maneuverability (Read 2,474 times)

Quite a while ago, there was a question about maneuverability of the Voodoo.  Fairly recently, I ran across a bunch of data for the F-101B, V-N diagrams and so forth, and translated it so that I could plot it on an energy-maneuverability (E-M) diagram to see no only G-loadings at a given speed, but turn rate and turn radius as well.  The data set that I used was for 40,000 feet and figured that the basic aerodynamics were not any different for the F-101A.  Here is the basic plot that I came up with:





From the first flight, the F-101A had severe compressor stall problems that took McDonnell and Pratt & Whitney over two years to completely fix.  While P&W were able to improve the surge margin of the J57-P-13 engines pretty quickly by the end of 1955, the inlet ducts remained a real problem.  The original "Type I" ducts that the first few Voodoos flew with were completely inadequate.  According to Gus Julian, who flew the USAF Phase II evals in the Spring of 1955, they could not even get basic information such as stall speeds because the engines would start stalling before they could get to that point.  That was at the start of the original Phase II evaluation in December 1954 but they had to cancel it as there was no point continuing until MAC figured it out.  MAC began work on an improved inlet design, the Type II or so-called "12-and-up" duct intended for the twelfth production aircraft and later.  The second Voodoo, 53-2419, was held on the production line to be modified with the new duct for renewed Phase II testing, which Gus Julian flew in April and May 1955.  

At that time, MAC had cleared the F-101A to 4Gs during test, but they would still run into compressor surge at between 2 and 3G below Mach 1, and about 3 to 4G at supersonic speeds.  Trace those G values toward the red line showing the pitch-up boundary on the chart, and you can see how, early on, the compressor stalls "masked" the pitch-up boundary.  As MAC continued to improve not only the inlet, but the duct geometry as well to ensure smooth airflow to the J57 engines, they could expand the maneuvering envelope.  By December 1955, MAC had cleared the F-101A to 5Gs and pilots began to maneuver into pitch-up territory without even realizing that it was there.  The first pitch-up fatality was Capt. John Dolan on 16 December 1955 when he was familiarizing himself with an aircraft just off of the production line in order to participate in scheduled Phase VI testing at Edwards.  He was up at about 40,000 feet flying figure-8s in the contrails to get the feel of the airplane when he apparently pulled it in a bit too far, pitched up, and rode it all the way down in an upright flat spin.

The warning horn and stick-kicker helped keep pilots from getting into that territory, but short of moving the control surfaces and making some other configuration adjustments, there was nothing to be done to alleviate the problem.  This is why the following MAC design, the F-4, has the angled tail and short nose.