With a viable engine and airframe that would at least taxi, we knew we were getting closer and closer to actually flying. With the canopy and all the missing panels installed, it was time for more taxi checks, including engine acceleration runs to full power.
The Rolls Royce Pegasus, MK 104 installed, has a hydro-mechanical fuel control. That means no computers, but metering valves, chambers, and all sorts of contraptions to ensure the right amount of fuel makes it to the engine, under all flight circumstances from high altitude to the hover. It’s a marvel of engineering in itself and about the size of a VW engine. There are adjustments all over the fuel control.
With a stopwatch mounted to the right canopy rail, we time every engine throttle slam before every takeoff. We time from 27%, which is a normal idle, to 55%, which is all that the brakes will hold. Beyond 55%, the brakes will skid, scraping off valuable tire rubber as the plane skids. Since we’re paying for the tires, we want to preserve as much of them as possible.
Depending upon the time for this engine acceleration check, we can adjust the fuel control to give the proper ‘slope’ of the time vs. RPMS curve. This is only the bottom half of the accel check. We must also check the time from 55% to 103% or 107% with water injection, too. Those accel checks can be “sporty” at the least. The reason is with one hand on the throttle, and one hand on the stopwatch, there’s nothing on the control stick. The airplane is pointed down the runway and left to it’s own Newtonian, response, which is to accelerate down the runway like a bat out of Hell. Like I said, it can be “sporty.”
Add to this, I had not experienced the full-force acceleration of the Pegasus engine in about 18 years, and we had very little fuel in the airplane. It was every bit of a rocket ship.
I recall airplanes in the fleet that would require quite a bit of adjusting to get the accels right. This was done at a high-power ramp, with the airplane chained to a concrete pad. The holdback chain is massive, designed to ensure that the airplane doesn’t move one inch, even at high power. I recall it’s actually a ship’s anchor chain, with some modifications.
Unfortunately, we didn’t have any of that. Our checks would have to with the airplane pointed down a 4,000 foot runway, which in Harrier terms, is quite small. Once I started moving, stopping before the end of the runway would be a trick. If anything went wrong, there was very little time to react.
But our engine accels were right on the money. Everything was perfect, as far as I could tell, which was just fine because I didn’t want to be doing too many of these engine runs on this short runway.
With all the systems checking good so far, we were ready to jack the airplane and start the first of a near endless cycle of landing gear retractions and extensions, along with cycling the flight controls. The hydraulic system had not been properly exercised in several years and we expected to have leaks, but there were none to speak of. So far, everything was checking out just fine.
Our next big move, was to get the ejection seat either re-activated, or a current, live ejection seat installed.
That was going to prove to be quite a challenge…