By: Rocket J. Squirrel (a.k.a. Bob Waldmiller)
Originally published August 1993
Remember back in May when I performed the One Hour Flight Test Program on N554FL to gather some realistic climb data? Well, as I mentioned last month, I discovered the source of my dismal climb performance was due to a failed baffle inside the muffler. It took a while for the new muffler to arrive and once I got the plane back together, I knew it was going to perform a good deal better. On the initial test flight, a week before Oshkosh, I did all my usual preflight checks and was immediately impressed by the 100 rpm gain in static rpm. On climbout, it became blatantly apparent that this was not the same airplane I was used to. I was getting a good 1000 ft/min rate of climb and sustained it until reaching 5000' MSL...WOW, I was impressed! Since I was low on fuel and flying solo I figured it would be interesting to quantify the impact of a failed muffler baffle on the airplane's rate of climb at gross weight. So the sequel to the story goes like this:
I basically duplicated the One Hour Flight Test Program as described in the May 93 issue of this newsletter. I used all the same assumptions and equations to generate my new data. The only real difference was that my test weight was about 100 lbs heavier than before due to all the extra fuel I had onboard this time. Essentially I had excellent consistency with my previously computed data. For instance, the amount of horsepower required to overcome the aircraft's drag during the climb should be nearly the same for both tests. This is due to the fact that only change in the aircraft's configuration was the 100 lbs of extra fuel I was carrying the second time. My number crunching indicated that the horspower required to overcome drag was less than 1 percent different that during the previous test--that's pretty good consistency! With confidence in my data, I then continued my data reduction to develop the following chart:
Immediately evident are three things. First the rate of climb is better than before which shows that the new muffler has a positive effect on the aircraft's performance. Secondly, even though the rate of climb at sea level is almost exactly what Piper published in their operating manual, at altitude the difference is still large. Thirdly, as I mentioned back in May, any change in climb power will change both the overall rate of climb and the slope of the line on the climb chart. Since Piper shows the same slope for the 150, 160, and 180 horsepower versions of the Cherokee, I again claim that Piper's methods for determining the Cherokee 140's climb performance are bogus and their data should not be used.
So how much horsepower did the failed muffler baffle consume? Well, if you look at the 7500' standard altitude values of the rate of climb, you'll see a 114 ft/min increase with the new muffler. To climb a 2150 pound airplane this much faster requires 7.4 more horsepower worth of useful work. Since I assumed that the propeller is only 75% efficient, this means the engine is now producing 9.9 more horsepower at 7500' than before. Imagine what I'd get if I installed those high compression pistons in my Lycoming O-320-E2A!!!
Previous Article "One Hour Flight Test Program"