Performance Question

[Lee]

Question: I flew yesterday ... two of us, near full fuel. Temperature was 96 deg on AWOS and the airport elevation is 320 ft (Millington, TN) ... close to 3,000ft density altitude. The prop is a McCauley DM 7653. On takeoff, the highest indicated RPM was 2000 ... once we accelerated, cleaned up and in cruise at 2,000 MSL, RPM would come up to 2400+ with an IAS of around 100-105 KTS as I recall ... I was flyng from the right seat and paralax made it hard for me to read the IAS. I do not know what RPM I should be getting on T/O and the owners manual is well ... "weak" (that would be an apt description) in the area of performance. Comments? Thanks ... Lee

[170C]

I am sure George or others can give you the data you need. With that same prop I get 2250 static rpm & at cruise I can get 2700 rpm & could exceed it, but I don't unless I decend without pulling back on the throttle.

[GAHorn]

Lee, the static rpm for your prop/engine is found in the Aircraft Type Certificate Data Sheet (TCDS). The Member's Page of this website has a copy of the TCDS. 2230 minimum -2230 maximum rpm for the DM prop, 2250-2350 for the MDM, if I recall correctly. There's a strong possibility that your tach is inaccurate. You can easily test it with a digital testor, which many mechanics may have in their toolbox. (Or you can come to the convention where I'll have one available for anyone wanting to check their tachs.)

[futr_alaskaflyer]

Lee-

Another member had a similar question not too long ago, and after checking a few things (harness, timing, compression) he found that though his prop was stamped 7653, it had actually been repitched in its past to something higher. Looking at the few performance charts contained in our owners manual, at almost 5000 feet DA, at 2400 rpm, 100-105 kts isn't out of the realm of possibilities with the standard prop, but it is on the high end of possibilities. Have you pored over your 337 file?

[Lee]

I will check the RPM as suggested ... I think I can come up with a digital tach around here as I have a couple of friends at work who are also mechanics ... interesting! I'll run down the data mentioned and save. R/Lee

[Lee]

I think I have pretty well been over all of the paperwork that came with the airplane ... well organized. I will double check the prop ... good point about a potential re-pitch but, I have my doubts, knowing the previous owner. I do think George's comment about a potential tach error is worth running down ... I'll do that next. Interesting and I appreciate the help! R/Lee

[six.zero.charley]

Did you ground lean for max rpm because of the density altitude? I've taken off from higher elevation areas like Reno and Truckee, which definitely require leaning for max rpm even on standard atmosphere days.

[Lee]

I just got back from flying the bird ... DA today was 2,100', 30deg C ... 2,000-2,100 and as soon as we got the speed up, the rpm was rising. I did lean once we leveled at 2,000' MSL. I am a little hesitant to lean with our field elevation of 300+ feet. The question is whether to lean for takeoff with a DA over 2,000'. I am beginning to suspect that T/O rpm was pretty much what I will see around here on hot days. I have a friend who has a digital tach and I am still going to do some more checking ... I am still getting a feel for the -170 and there is a learning curve. What a delightful airplane though! Thanks for the comment/query. Regards, Lee

[six.zero.charley]

Even though you're at 300 MSL elevation, on a hot day your airplane will perform as if you're taking off from a 3000 MSL strip if the temp 96 degrees out and the density altitude is 3,000 ft. That means your effective altitude is 3,000 ft, and the volume of air available to your engine is as such.
I believe you're supposed to run full rich if making more than 75% power, so look in the POH to see how much power is being made at ~3000 ft or whatever your effective altitude is.

There is the consideration of overheating because of the hot atmospheric conditions, but given the choice between running hot for a little while, and running off the end of the strip because I can't make takeoff power, I'll take the first option.

I, too, just began flying the 170 and am still learning a lot, but this is a lesson I learned all too well in a 152 and 172.

[GAHorn]

When the throttle is wide open, the enrichment circuit of the carb is energized to provide a slightly over-rich condition to provide additional cooling for takeoff and climb.
As you open the throttle and begin to roll, glance over at the tach (presuming it to be accurate and taking into account any parallax) to confirm that take off power is reached (2250-2350) just like you do at work, Lee.

If static rpm isn't reached (when historically it did) then leaning might be in order ...just to reach rated rpm. Do not overlean or you'll risk overheating. (It's unlikely that you'll need to lean for density altitude at MEM tho'. I've only run into the need to lean for TO at/above 4-5K'.)