bad engine performance today

[buttman]

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?

[Bob Moore]

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

[Maxwell]

"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.

[john smith[_2_]]

Do they have the same prop?

[Snowbird]

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

[[email protected]]

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

[ktbr]

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

Did you try to apply carb heat?

[ManhattanMan]

ktbr wrote:
> carb ice... 150's can be very susceptable.
>
> Did you try to apply carb heat?

@ 35º C ?

[Brian[_1_]]

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

[BDS[_2_]]

"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

[ManhattanMan]

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

Now that makes perfect sense. Couldn't be seen from the cockpit, and it
probably bounced off on landing so nobody noticed what the actual problem
was..

[Jose]

> And the error is
> always on the low side: the tach will under-read because the magnets
> in it weaken with age.

How does a tach work? Were I to design one, I'd count the revs
individually and do the math.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

[Gig 601XL Builder]

Jose wrote:
>> And the error is
>> always on the low side: the tach will under-read because the magnets
>> in it weaken with age.
>
> How does a tach work? Were I to design one, I'd count the revs
> individually and do the math.
>
> Jose

There are several ways they can work and most of them do in fact do the math
in one way or another. But something has to sense the revs.

[Jose]

> There are several ways they can work and most of them do in fact do the math
> in one way or another. But something has to sense the revs.

How would weakening magnets affect a tach? If they get so weak they
start missing revs (that are counted individually), I would expect the
tach to be very erratic at that point.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

[john smith[_2_]]

In article >,
"BDS" > wrote:

> Anything that would cause a decrease in
> power should also cause a decrease in rpm with a fixed-pitch prop.

Are you certain they are the same pitch?
Just because it is fixed pitch does not mean they are the same pitch.
There are "cruise" props, there are "climb" props and there are props
with their pitch somewhere inbetween.

The club I am in has to C172N's.
One has an intermediate pitched prop, the other has a cruise prop.
Both read the same RPM, but the cruise prop equipped aircraft does not
climb as well as the intermediated pitch equipped aircraft.
In level flight cruise, it is 10 kts faster, however, at 2300 RPM.

[buttman]

On Apr 30, 9:55 am, Brian > wrote:
> 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

I pulled the mixture back a little bit on the second circuit on
upwind, but it didn't help performance one bit.

[Gig 601XL Builder]

Jose wrote:
>> There are several ways they can work and most of them do in fact do
>> the math in one way or another. But something has to sense the revs.
>
> How would weakening magnets affect a tach? If they get so weak they
> start missing revs (that are counted individually), I would expect the
> tach to be very erratic at that point.
>
> Jose

Remember I'm not saying that that is the problem the OP was having but if
you are counting something that happens 2800 every minute (46/sec) and you
start missing say 10% you'd see a 280 RPM drop. Even with that 10%
randomized somewhat the tach could keep up with the change enough for it to
show up as wild swings and would probably not be any worse than that caused
by vibration.

[Jose]

> The magnet causes eddy currents in a bit of aluminum which
> causes the aluminum to have a magnetic field...

Thanks. :)

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

[Peter Dohm]

------snipped for brevity-------
>
> Maybe someone forgot to untie the tail and you were dragging a large piece
> of the airport along with you :>)
>
> BDS
>
>
I've heard of that being done, although not in a C152, and really wish I
could have been a "mouse in the woodwork" :-)

OTOH, presuming that the OP does routinely use both of the aircraft in
question, and that the weight was normal, the only thing that occurs to me
is a notch of flaps. They seem easy to see, and difficult to miss; but I
have seen it done with a C172--and I have done it myself in a PA38, which is
a mildly amusing story: I had forgotten to retract the flaps following the
preflight, for a dual flight, and then taxied out and completed the run up;
and was still blissfully unaware. This was c1980, before headsets were in
common use; and just as we took the runway, I noticed my instructor
confiding something into the microphone--so I presumed that we were about to
have some sort or "emergency." I later learned that he was sure that the
Tomahawk would not take off with full flaps on an 85 degree day, and had
told the tower to expect an aborted take off. Actually, it would and did,
and quickly climbed to the top of ground effect (at an abnormally low
airspeed) and then I discovered that it is possible to press the release
button in a little too far (and get it stuck inside the flap handle... But
I promised to be brief; so I will only add that we did safely resume the
climb, I learned to be more thorough, and I also learned not to get the
button stuck inside that damned handle!

Peter

[Blueskies]

"buttman" > wrote in message oups.com...
> 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'm surprised no one mentioned gross weight...

[flypaper]

Blueskies wrote:
> "buttman" > wrote in message
> oups.com...
>> 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'm surprised no one mentioned gross weight...

he didnt say mx was in it. ;~/

[Capt. Geoffrey Thorpe]

"buttman" > wrote in message
oups.com...
> 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).
<...>


How did it fly later in the day after the pastic bag fell off the landing
gear?

:-)

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

[[email protected]]

On Apr 30, 3:54 pm, Jose > wrote:
> > The magnet causes eddy currents in a bit of aluminum which
> > causes the aluminum to have a magnetic field...
>
> Thanks. :)
>
> Jose
> --
> Get high on gasoline: fly an airplane.
> for Email, make the obvious change in the address.

The magnet spins inside an aluminum cup, and the cup drives the
neele. No pulses generated, just a rotating current in the cup that
generates an opposing field that drags the cup around against a small
spring.

Dan

[[email protected]]

On Apr 30, 10:47 am, "ManhattanMan" > wrote:
> ktbr wrote:
> > carb ice... 150's can be very susceptable.
>
> > Did you try to apply carb heat?
>
> @ 35º C ?

Yes, at 35°C, though it won't be heavy icing. See this
chart:
http://www.carneyaviation.com/images/carbice.gif

It all depends on the relative humidty (dewpoint for
pilots). We've seen carb icing here on the dry prairies on summer
mornings. And we often hear of accidents caused by carb ice because
students aren't getting taught much about it and don't recognize it,
nor do they think it's possible at high ambient temps. Ignorance kills
way too many PPLs.

Dan

[Margy Natalie]

ManhattanMan wrote:
> ktbr wrote:
>
>>carb ice... 150's can be very susceptable.
>>
>>Did you try to apply carb heat?
>
>
> @ 35º C ?
>
>
The only time my old Lycoming ever iced up was on a beautiful summer day.

Margy