Take off RPM expectations at high DA

[Hawkeyenfo]

What RPM should I expect to see at higher DAs? Flying across highway 40 yesterday I noticed that on initial power up for take off, 39A was turning only about 2150. Is that to be expected? At cruise (10,500) it would turned 2500 and seemed "normal." My brother has had the plane back in Ne so this was the first time I've flown it consistently (14 hrs) in a few years. Thanks!

[blueldr]

Keep in mind that as the density altitude goes up, the manifold pressure goes down with the resulting decrease in power. However, the "thinner"air puts a reduced load on the propeller. A static engine run up to ambient field barometric pressure, normally full throttle, should remain relatively close to a standard.

[blueldr]

On airplanes such as the Convair 240 or 340 or DC-6 with P&W R-2800-CB16/17 engines, the manifold pressure gauge was equipped with a bleed valve that was pressed to determine the ambient barometric pressure. The engine was then run up to this pressure and the RPM was noted. I seem to recollect that this rpm was something like 2100. This was an rpm below the governing range of the constant speed propeller, so we were in effect testing with a fixed pitch prop. As I remember, the static RPM on this check was a constant and did not change with DA.
The propeller and governor were set to deliver 2800 RPM at max power for take off, using 56 to 61 inches of manifold pressure depending on the fuel used.

[Hawkeyenfo]

I used my rpm gauge and a couple heads with fair hearing to lean it out at 2000 rpm prior to takeoff. Worst was at Prescott, with ramp temp around 76-78 and reported Just over 7000. As for climb out, about 2350-2400 and 200-300 fpm at gross ( two guys, wheel chair, junk and gas). At 10,500, indicated 2500 rpm and burnin 6.6 average.

[cessna170bdriver]

What RPM should I expect to see at higher DAs? Flying across highway 40 yesterday I noticed that on initial power up for take off, 39A was turning only about 2150. Is that to be expected? At cruise (10,500) it would turned 2500 and seemed "normal." My brother has had the plane back in Ne so this was the first time I've flown it consistently (14 hrs) in a few years. Thanks!

That sounds pretty close to what I get up here in Tehachapi, 4k field elevation, 5k-7k DA most of the time.

[Hawkeyenfo]

Miles, what static rpm are u seeing at tehachapi? Now that I have my 170 on the left coast, I'll have to fly up and meet you. Isn't there a great train watching spot up there?

[GAHorn]

What RPM should I expect to see at higher DAs? Flying across highway 40 yesterday I noticed that on initial power up for take off, 39A was turning only about 2150. Is that to be expected? At cruise (10,500) it would turned 2500 and seemed "normal." My brother has had the plane back in Ne so this was the first time I've flown it consistently (14 hrs) in a few years. Thanks!

Tach accuracy is a factor, of course. So is the prop. (Which exact prop do you run?)

If you consult the engine "Operators Manual" (found in the Member's Section) ...I suspect you'll be better able to confirm your engine/prop performance relative to your tach accuracy. The altitude performance pages can indicate the horsepower developed, but the critical information lacking is the prop you are running and that accuracy matter of your tach.

[Hawkeyenfo]

Agree, the tach accuracy needs to be determined. Reviewed the logs, it's turning the McCauley Met-L prop, pitched for cruise.

[GAHorn]

Agree, the tach accuracy needs to be determined. Reviewed the logs, it's turning the McCauley Met-L prop, pitched for cruise.

That still leaves a lot to be considered. Is it a DM prop? ..or a MDM?...or an EM"...and is it pitched for "cruise" at 54", 55", or 56" ..or something different? Is it full length 76" diameter?

[Hawkeyenfo]

Yes it does, I have the log books at my office and will check in the morning!

[Hawkeyenfo]

Checked the log book, my prop is the McCauley DM 7653. It's still at full 76" length. Heading out to the aiport tomorrow to run it up again. Talking to my mechaic about getting an alt electronic tach to confirm the aircraft tach accuracy.

[GAHorn]

If your tach (or a digital/optical unit) confirms the low RPM then the next thing to do is run a compression check and an ignition system check, followed by a detailed induction and exhaust system integrity inspection. (blockage of induction and/or exhaust)

[Hawkeyenfo]

Thanks, definitely good to know what path to follow if needed! I know exactly what I'll be doing the weekend