Its good to know that you dont even get what is advertised on the box...!
Okay let base the example on a standard day / temp etc etc ...and assuming that you dont have lots of additional extras running off the engine i.e. AC.. are the lower compressions making a significant effect on the HP.
Which then follows onto the performance figures in POH...How much of a margin should you then add to get somewhere near the actual figures for the plane.
To complicate matters, there is not neccessarily a correlation between the compression reading and any loss of horsepower. You would need to know where the leakage was occuring. As 411 mentioned, TCM has loads of data that prove leakage past the dynamic seal, (rings), during a compression check does not represent what is occuring under engine operation, in fact they significantly lowered the compression test limits for leakage past the dynamic seal.
Leakage past the valves during the comp test is a better indicator of what will result in a loss of horsepower during operation.
all true, but again, leakage past rings detected during a compression check does not mean that leakage is occuring during engine operation. So assuming that there will be a reduction in rated horsepower based only on a trend of lower compression readings is faulty logic
I refer you to the orginal poster's question. Compression checks on aviation engines utilising the starter motor are very rare. The TCM data does not support your position. Leakage past rings during a static compression test is usually higher than when the engine is operating.
Ferrydude is quite correct, and TCM has loads of data on the subject for confirmation. Likewise Pratt&Whitney with larger radial engines. UAL was authorized to operate their R2800CB16 engines (installed on DC-6B's) to 3,300 hours until overhaul, and when one of these engines was sent to P&W for analysis, it was put on a dyno and checked for power output, the results surprised no one...yes, specific BHP was reduced, but only by three percent, and this was in the dry takeoff mode, wet takeoff, two percent.
Gentlemen, an interesting discussion. I gather that Keith Williams contends that leakage, if any, by piston rings will cause a decrease in IHP. 411A and Ferrydude seem to contend that TCM data does not support this.
Is there an academic type of explanation of why you may have some blow by of piston rings yet not decrease IHP? I find Keith's explanations quite convincing....
Separately Keith, have you ever written a book or published papers/articles on aerodynamics/performance/engines etc?
Again, the original poster is asking what sort of penalty he should assess to performance charts based on compression testing results indicating in the low 70's. Plain and simple answer is none. A determination of actual ring leakage during engine operation cannot be made from static testing. I refer you to the TCM testing wherein rated horsepower was obtained during engine operation that failed static compression testing!
Erm, fromthe original post;
"Lets say after several years use the compressions decrease to low 70's."
This does not refer to compression checks? What would you suppose he meant by "low 70's"??????
"But I am arguing that if wear-induced leaks cause cylinder pressure to be reduced in a running engine, then this will cause a reduction in power output."
And while you may be technically correct, it is a moot point. In most cases the reduction in power output is insignificant. No reduction in horsepower. The TCM test data support this. Where did your data come from?
No, I have not published any books or papers on this subject
Well, you should, it may be your second calling if you're looking for one. I was once told that for nearly every complex problem, there is a solution that is simple, neat, logical and wrong. Fortunately, your responses seem to fall only into the first 3.
During operation of an engine, blowby passing through the ring gap is under very high pressure and it reaches what is termed "sonic velocity". At this point any increase will not result in any increase in gas velocity (hence the rate of escape of the gases). From this point of view, leak-down tests are of little or no value in determining engine condition, as they measure leakage under static and low pressure conditions.
The gas which does escape the piston rings, and which is returned to the intake charge, is termed blowby. The control of blowby to an acceptable level is important because excessive blowby means loss of engine power, leads to ring sticking and an increase in emissions. That is why manufacturers tightly control piston ring shape and flatness, two factors important for good gas sealing. Blowby is measured under laboratory conditions by sealing off the engine's crankcase and attaching an accurate gas meter by means to the breather on the oil filler cap. A typical 2-litre engine can expected to have a blowby figure of 20-30 litres per minute under full load conditions.
A reasonably good measure of trending any HP degradation is available to the operator of a fixed-pitch prop aircraft, and that is to keep a log of static runup RPM at home base. Record the OAT, wind, and QFE along with RPM and you'll get a good picture of what's going on. Every 50 hours might be a good interval for these checks, at least until you have a good database.
Unfortunately a variable (constant-speed) prop complicates the matter, unless there's a torquemeter like the R-2800 has.
The original question was raised because you see a lot of aircraft for sale with the seller stating that the compression is "x" per cylinder.
This got me thinking abt the actual engine performance and other directly related issues and whether the performance figures in the POH are correct for a new and an engine that is obviously mid life and has lower compressions.