Pete Hecht:
Member since 10/03 when I purchased a 1976 Sierra N24PD-IO360 200hp. First post-Question/Problem- The POH states that I should get 155 mph at 75%- I can not even get close- up until last week my max was about 110 mph- mechanic then worked on the aircraft and cleaned filters, screens, fuel injectors (two were clogged), adjusted throttle linkage, adjusted timing and did a compression check (75, 64, 76, 76) -I now can get 128 mph- This is still a long way from 155 mph- only remaining issue I know could be a problem is that I cannot get above 2600 RPM in any configuration- oil consumption is about 1 qt/8 hours- fuel consumption is right on the numbers- about 10 gph- Any thoughts- is 128 mph all that I can expect? any advice is appreciated.
Editor:
Pete, you should be able to get 2,700 RPM almost regardless of conditions. That’s why you have a constant-speed prop, to enable you to do exactly that. I’m a bit skeptical that it is a power shortage that is preventing it, if you can’t get it at low altitude. I would suspect that the prop control cable adjustment or the prop governor adjustment is incorrect. It could be something in the prop itself (I won’t get into that nitty-gritty right now), but I suspect it to be one of the easier things I have noted. You have to get that fixed first. I think that the reduced RPM is your major performance constraint.
You have indicated that the magneto timing has been checked; it should be at 20 degrees, and both mags have to be there. If one is set to less than 20, you’ll lose a little power. I don’t think that this is your problem, partly because it has been checked, and almost any A&P can do this properly.
Your compressions are all fine. Even at 64 on that one cylinder, that would have no measurable effect on full power output. At worst it could cause a slightly uneven idle, but I think you’d have to be under 40 to even cause that.
Look up into your exhaust pipe with a small but very bright flashlight. You should either see nothing but a big empty chamber (meaning all the baffles have been burned out and ejected), or you should see an orderly arrangement of baffle plates or tubes. If you see what looks like a jumbled mess, and anything that appears to be blocking the exhaust outlet, you must have the muffler overhauled. Any exhaust obstruction can get very serious very quickly, with enough power loss to bring you down in a partial-power glide.
Make sure that the induction air filter is clean, that none of it is separating, and that the correct side is facing forward (depends on type). Remove the filter and check behind it, to make sure that the air box gasket is not extruding into the air stream. Make certain that the alternate air door is secure (intact hinge), and that the spring is keeping the door properly closed.
While you have the filter off, double-check that when the throttle is full forward, the throttle blade in the fuel injection servo is wide open (parallel to the air stream).
Make sure that all the external access panels are secure, and that the doors are reasonably snug against the door jambs. They won’t usually be perfectly tight, but an edge that sticks out a half-inch isn’t right (and creates a ton of drag).
Once you have the prop RPM issue corrected, and have verified that there are no induction or exhaust obstructions, take the plane up to 7,500 feet. Set it to full throttle and full RPM (it should/must easily reach 2,700 RPM). Note that at 7,500 feet, full throttle, and 2,700 RPM, and the specified fuel flow range, you will be getting 75% power. You cannot get more than 75% at this altitude, with everything set for full power. At full throttle, and any RPM below 2,700, you are not getting as high as 75%, if you are at 7.500 feet or higher.
If you have an EGT gauge, lean to 125 degrees rich of peak. If you don’t have an EGT gauge, set the indicated fuel flow to about eleven or twelve gallons per hour. At these settings, you should easily indicate 125 knots or more. My C24R will indicate 142 knots plus, depending on loading, but I have done some minor drag clean-up on it, and it has some fairings, different prop, etc. that the A24 does not have.
I usually cruise around this altitude range (6,000 to 8,000 feet), as it is the most efficient range for the unsupercharged plane. Best efficiency is gained with full throttle, controlling power with RPM and fuel flow. You usually can’t run full throttle cruise much below 6,000 feet, as manifold pressure will be above 24″ if you are much lower. I usually use full throttle and 2,500 RPM, with a leaned fuel flow of 9.5 gallons. The resulting power is probably between 65% and 70%. At these settings I usually indicate 120 knots, with a true airspeed of 135 knots (155 MPH).
These indicated/calculated numbers have been confirmed many times via multiple GPS groundspeeds/wind correction calculations. Take special notice that it is TRUE airspeed that matters, not INDICATED airspeed. IAS will vary with altitude, temperature, and air density. TAS takes all these factors into account, to provide a valid comparison number.