If I were you, I wouldn't be complaining too much... Those are better
numbers than I'm able to muster (but I don't have too many speed mods either). The
2700 RPM only at or above 7000' DA is pretty much what I get (my POH doesn't actually
say that, though).

Nathan Young wrote:
: I have a '71 PA28-180, with a fixed 60" pitch Sensenich prop. This is
: the standard cruise prop for a PA28-180.

The TCDS on at -180 require *exactly* 76" prop... cannot be smaller due to
vibration problems. If it's been trimmed at "overhaul," you could be slipping more
and spinning higher because of it.

: The POH indicates that 2700 RPM should only be able to be achieved
: through 7000ft DA.

Like I said... probably about what I get, but above 7000' DA, you can barely
make 75% anyway. Pretty thin operating envelope.

: I am able to hit redline (2700 RPM) in level flight up to 11500 feet
: density altitude. POH indicates that it should achieve about 2550.

Things get kinda wonky up high like that. Just from my observations, DA
doesn't seem to be the whole story. I certainly wouldn't be relying on the maximum
RPM at a high altitude like that to be accurate. Way too many other variables to
count on it.

: It has operated this way as long as I can remember (several years) so
: it is not a new phenomena.

: So what could cause this? Prop pitch, poorly calibrated tach? Any
: others? My plane has all of the K2U speed mods, so the reduced drag
: probably plays a role.

Yeah, it would. As others have mentioned you should check the static runup
and tach calibration.

: I am able to achieve close to 150mph true airspeed at max cruise,
: while the POH indicates 140mph for book. This makes me think the prop
: RPM indications might be accurate.

I would think that all of your speed mods might buy you 5 kts over book, but
it sure sounds like your prop is under-pitched. If you make higher than TCDS static
runup, it would also imply that. The only other good way I can think of to check is
to see your fuel burn. I was able to detect a 50rpm low indication in my tach by
comparing the fuel burn numbers to the book values. After putting a strobe on it to
cal, it was verified.

: Appreciate your thoughts.

Which speed mods do you have?

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************