What causes engines to develop less-than-rated power?

QUESTION: I just read your article on the history of the 0-320-H2AD engine (What engines should I avoid, Dec. 24, 2004, issue). I have had three Skyhawks with the later engine (two 1977s and one 1979) and have had no problems with TBO — all went well past 2,000 hours.

My question is about power. Do they really put out 160 hp? The manual says the plane should true out at 122 kts at 8,000 to 10,000 feet (fuel flow of 8.5 gph) and a service ceiling of 14,000 feet. All three of my Skyhawks fell well short of that with typical cruise at 105-110 kts, fuel flow of 9.5 gph and service ceiling of 10,000 or so (my son’s Cherokee 140 does better). All three were at or past TBO, but the compressions were all in the mid-70s. What else could cause the engine to develop less than rated power?

Kris Widdison
Yorba Linda, Calif.

ANSWER: Thanks for your letter and I hope all of those naysayers out there regarding the O-320-H2AD read it closely. It’s obvious that you know how to operate and maintain an aircraft and have gotten good service life out of the engines.

Regarding the power output of an engine, I must remind everyone that an engine is TC rated on a dynamometer and not as installed in any given aircraft.

Once the engine is installed in an aircraft, we experience what is commonly referred to as “”installation losses.”” This comes from attaching accessories, such as the vacuum pump and alternator, and also from the airframe induction system itself.

Just for conversation, let’s say you end up with a completed installation loss of 6 to 8 hp. The airframe manufacturers calculate this and all performance criteria are established after this has been put in the formula. The point still remains that the engine is certified for a given rpm and hp and that is the figure used as an industry standard. It’s done in a similar way in the automobile industry. They will tell you a certain car has 350 hp, but if you look close enough — probably somewhere in the fine print — it may say the car develops 278 hp to the rear wheels.

Let’s get this back on track and see what I can come up with that may figure into the points you’ve made. I may hear about this, but from my personal experience, the tachs used in GA aircraft are, shall I say, less than lab quality instruments. Having had lots of experience with the Cessna models you’ve owned, I can tell you that most likely the tachs were reading low in rpm. Of course, this means the engine is turning faster than you think it is and this probably explains the higher fuel consumption, but it sure doesn’t explain the lower performance as far as slow cruise speed, so let’s take a different approach. Did you ever have the pitch checked on the prop or swap props from a like model aircraft? Trying that may answer your questions and solve the problem. The pitch could be off, causing the prop to take a bigger bite. Also, it would act like the old climb props vs. a cruise prop, which was common back in the days of the early Cubs.

One easy way to check this entire thing out is to locate a like model aircraft and ask what the performance numbers are and compare those to what you’ve got. This also might be a great time to have both aircraft fly side by side at given rpm settings and check the air speed. You could also check fuel consumption if you both started off with full tanks and timed your event.

Kris, thanks again for your interesting letter. I’m certain it’ll have a lot of people putting on their thinking caps.

Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: AskPaul@GeneralAviationNews.com.

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