Percent power altitude

Possibly Cessna and Piper have not seen your chart. I have several POHs he

An Arrow PA28R-201 POH shows 75% power, full throttle, at 6,000 feet/2400 RPM
and 7,500/2700 RPM. 65% power at about 9,800/2400 and 12,000/2700. (IO-360/200
Lyc.)

A Cessna 182T (T-model, not T for turbo) POH has tables that are a little harder
to read than the Arrow's graphs, but the highest % power they show at 8,000 feet
is 74% and at 12,000 feet 64%. (IO-540/230 Lyc.)

A carburated Cherokee Six/260 shows 75% power, full throttle, at 8,300 feet and
65% power at 11,200 feet. Engine RPM is not stated. (O-540/260 Lyc.)

I also have a tiny and very complicated power chart from Lycoming for the O-540
but I am too lazy to figure it out.

I am no fluid dynamicist (and may be about to prove it), but my dim
understanding is that the Reynolds number has a major effect on fluid flow and
in its calculation there is a density term. Certainly the mixture velocities at
various points in each different intake system would be different. Velocity is
also a term in the Reynolds number calculation. So (leaping) it does not
surprise me that intake systems with different geometries would perform at least
slightly differently at different altitudes. Exhaust systems, too, I'd guess.

Possibly I should have been more diplomatic in how I suggested that the OP
needed a POH but that still seems to me to be the case.

On 10/21/2006 12:46 PM, Bob Moore wrote the following:
Mitty wrote
You don't have a POH?

You know, he might not even have an airplane. He just asked
a question and expected an answer, not a wise crack.

No POH required, answer is not aircraft specific, see:

http://www.kellyaerospace.com/articl...bocharging.pdf

Bob Moore

Yes, after age 60, my recall gets muddy.


"Bob Moore" wrote in message
. 122...
| Jim Macklin wrote
| 5 and 7 thousand
|
| Running on (faulty) memory again, Jim?
|
| How about 7,500' and about 12,500', ISA of course.
|
| See my previous replies to this thread for a real chart
| answer.
|
| The formula answer is:
|
| bhp at altitude equals bhp at sea level times the quantity
| (density ratio minus the quantity(1 minus density ratio
divided by 7.55))
|
| Works out to 76% at 7,500' and 64% at 12,500', or
thereabouts.
|
|
| Bad information is worse than no information at all.
|
| Bob Moore

Leaning and density altitude (performance altitude).



"Bob Noel" wrote in
message
...
| In article
,
| Bob Moore wrote:
|
| The formula answer is:
|
| bhp at altitude equals bhp at sea level times the
quantity
| (density ratio minus the quantity(1 minus density ratio
divided by 7.55))
|
| Works out to 76% at 7,500' and 64% at 12,500', or
thereabouts.
|
| OK, I'm confused. Full throttle, 64%, is 12,500' for any
engine?
| How come my cherokee 140 couldn't even get to 12,500'?
| What am I overlooking?
|
| --
| Bob Noel
| Looking for a sig the
| lawyers will hate
|

As a first approximation, it should be the same for any normally
aspirated engine (and yes it would be based on DENSITY altitude). The
engine develops 100% HP only at sealevel. As the altitude goes up what
changes? The density of the air decreases and in response the fuel
delivered decreases (from both the carb delivering less fuel and the
pilot leaning the mixture). This is the same for all the normally
aspirated engines.

Now at somewhere around 65-70% power it becomes impossible to get
excess EGTs and CHTs due to overleaning the engine. So run the engine
at that altitude and lean for max rpm and note the EGT. This is
guaranteed to be a safe EGT! Now use this EGT for leaning at ALL
altitudes. Thus you have found 100 or so degrees rich of peak without
ever having to run the engine at peak (which is too hot and hazardous
in itself).

Doug wrote:
Anyone tell me at what altitude I get 75% and 65% power respctively
(with full throttle, normally aspirated).

Jim Macklin wrote
Yes, after age 60, my recall gets muddy.

Hmmmm....at 71, I must have missed that stage. :-)

Bob

Bob Noel wrote:

OK, I'm confused. Full throttle, 64%, is 12,500' for any engine?
How come my cherokee 140 couldn't even get to 12,500'?
What am I overlooking?


Because 64% of whatever HP you have isn't enough to get you there/
hold you there.

I've got 300HP in an airframe designed for 185. At 65% HP I'm
doing better than they can do with WOT at sea level.

Note, I did say MY recall. I sleep like a baby now. Go to
bed, wake up every hour or two, cry and go to the bathroom.

I my defense, I'm not perfect.



"Bob Moore" wrote in message
. 122...
| Jim Macklin wrote
| Yes, after age 60, my recall gets muddy.
|
| Hmmmm....at 71, I must have missed that stage. :-)
|
| Bob
|

It depends predominantly on your mixture.

Karl
"Curator" N185KG



"Doug" wrote in message
oups.com...
Anyone tell me at what altitude I get 75% and 65% power respctively
(with full throttle, normally aspirated).

"Doug" wrote in message
oups.com...
altitudes. Thus you have found 100 or so degrees rich of peak without
ever having to run the engine at peak (which is too hot and hazardous
in itself).

What makes you think that?

Karl
"Curator" N185KG
Gami ser# 19

Doug,

Thus you have found 100 or so degrees rich of peak without
ever having to run the engine at peak (which is too hot and hazardous
in itself).


Come again? That part in parenthesis is completely wrong.

--
Thomas Borchert (EDDH)