bad engine performance today

Today I was doing some touch and go's in a C152 with one of my
students to prepare for his first solo. It was a very hot day (35C),
but density altitude was only 2500 or so.

I noticed as we were climbing upwind, the plane was climbing very
slowly, even right at Vy. The runway is very long (11,800 ft), but
when we reached the opposite threshold, we were only about 500 AGL.
Normally at the end of the threshold we are between 900 and 1000 AGL,
and thats with switching to cruise climb after 500 AGL...

Then when we got to downwind, my student pulled the power back to 2300
RPM (cruise) and we were only able to hold about about 70 knots (90
knots is normal).

Just before this all happened, I was up with another student in a
different plane (still a 152 but a different tail number), and the
performance was about the same as it normally is.

The thing that confuses me about all this, was how the engine RPM was
perfectly normal. Full throttle was about 2500 which is where it
normally is. Wouldn't an engine that is producing less power have less
RPM's as long as it's a constant speed prop?

I know it isn't a airspeed indicator problem because I could clearly
notice a difference in ground speed from comparing the two flights.
After the flight I went around the place to see if there was anything
on the airframe that could have caused a lot of drag, but it all was
normal. What could it have been?

buttman wrote
The thing that confuses me about all this, was how the engine RPM was
perfectly normal. Full throttle was about 2500 which is where it
normally is. Wouldn't an engine that is producing less power have less
RPM's as long as it's a constant speed prop?

I think that you probably meant to write "fixed pitch prop".

Bob Moore

"buttman" wrote in message
oups.com...

I had a simular problem years ago in a 150. I didn't notice an RPM drop
either, but it turned out the carb heat was out of adjustment.

You might double check for a good indication of RPM drop in run up. For me
it was one of the first really hot days of the summer, and although I did
see some RPM drop on run up, the drop was more pronounced after the
adjustment.

Do they have the same prop?

Tachometer malfunction- shows too high rpm? Just a guess.

On Apr 30, 8:43 am, "Snowbird" wrote:
Tachometer malfunction- shows too high rpm? Just a guess.

Tachs often read low. In Canada we have to check them yearly,
and they're allowed to be no more than 4% off in the middle of the
cruise RPM range. Works out to 100 RPM at 2500. And the error is
always on the low side: the tach will under-read because the magnets
in it weaken with age.
In flight training we have an exercise demonstrating reduced-
performance takeoff. We cut the RPM by about 50, and the climb almost
vanishes. It takes very little power loss in a light aircraft to hurt
performance.
But that would mean that the engine would have to be putting
out less as well as the tach being off on the high side, which is
unlikely. A more likely problem involves a pitot or static error that
makes the airspeed indicator under-read; the pilot will keep the nose
lower to get the Vy reading he's after so that climb suffers, and the
ASI will be low in cruise. Groundspeed estimations need to be done
with the stopwatch to prove it. I bet there's a static blockage.

Dan

carb ice... 150's can be very susceptable.

Did you try to apply carb heat?

ktbr wrote:
carb ice... 150's can be very susceptable.

Did you try to apply carb heat?

@ 35º C ?

Were you leaning the engines before take-off. As it gets hotter this
can be more critical for best power. I haven't done the math but I
suspect that warm you are getting close to a 5000 ft density
alititude.

As noted in the post it doesnt take much of an RPM change to affect
performance so you might not notice on the tach. Also even small
differences in the propeller can affect performance greatly

Brian

"ManhattanMan" wrote ...
ktbr wrote:
carb ice... 150's can be very susceptable.

Did you try to apply carb heat?

Carb heat, carb ice, etc. - none of these things make sense to me in this
case. If two airplanes have the same engines and the same fixed-pitch
props, and their tachs are reading accurately, then rpm should be an
indication of power. In other words, it would seem to me that they cannot
both be turning the same rpms and be doing the same amount of work if they
are not putting out the same HP. Anything that would cause a decrease in
power should also cause a decrease in rpm with a fixed-pitch prop.

With the numbers given we're not talking about a small decrease in power
either - 70 kts cruise versus 90 kts cruise I believe were the numbers
quoted.

Maybe someone forgot to untie the tail and you were dragging a large piece
of the airport along with you :)

BDS