PA38 Climb Performance

[fireflybob]

We operate two Mark 2 PA38s.

One of them seems to have a significantly poorer climb performance that the other one. We had thought that the installation of a new engine would improve the climb performance but it has not done so. Static RPM is the same on both a/c. We have both a/c reweighed and there is only a trivial difference in empty weights (in fact the a/c with poorer climb is about 12 lbs lighter).

The propellor pitch on both a/c is exactly the same.

We have asked engineers to check the rigging but nothing seems to be untoward.

I just wondered if anyone had any bright ideas as to why the climb performance was significantly different.

[BizJetJock]

In my days of doing lots of CofA air testing on light aircraft, the majority of climb performance issues were down to pitot/static system errors, so you're not actually flying at the speed you think.
Worth a look anyway!

Hope it helps.

[fireflybob]

BZJ, thanks for that!

[BigEndBob]

Try slamming the doors to see if the vsi jumps around, usually sign of a static leak.
You might find the climb performance of the good climber is the one in error!

[MrAverage]

Since you have two you could fly both in loose formation trying both climb and cruise (with the "good" one setting accurate speed and the other matching pitch and power). You'll soon reveal any ASI trouble. I would first get a laser rev counter on each during a power check to ensure RPM is accurate.

[MrAverage]

Don't know how the @ got in there, I definitely typed an a.

[stevef]

You could ask your engineers to carry out a pitot-static instrument test. It'll check indicator accuracy and show up any leaks. Shouldn't take more than an hour.

[Genghis the Engineer]

The "l word" is banned on PPRune ostensibly to prevent banner adverts of any devices which get pointed at flying machines.

"How is my client supposed to know that pointing it at aeroplanes is dangerous m'lud when he bought it from a well known pilot website " sort of thing.

G

[skyhighfallguy]

when you test the planes, have you weighed the pilots?


hey chubby, you take that one, hey slim, you take that one. hey, chubby's doesn't climb as well.

wonder why?


D'OH!

[fireflybob]

when you test the planes, have you weighed the pilots?


yes, of course we have taken all that into account.

[NutLoose]

Did you replace the Lord mounts?

[fireflybob]

Did you replace the Lord mounts?

Tell me more?

[BigEndBob]

Measure the fuel burn. Might be poor tuning.

[Genghis the Engineer]

Damned if I can see how the Lord mounts are going to change performance.


If t'wer I, I think I'd fly two identical performance climbs - same day, as near as possible same W&CG, use a GPS to cross-check pressure against geopotential altitude, and groundspeed for the same heading and altitude within an hour or so of each other. Record everything readily available (far better and safer, have somebody in the right hand seat do that whilst you concentrate on flying accurately and looking out): so IAS, GPS G/S, GPS Alt, Pressure Alt (make sure QNHs match), VSI, Ts&Ps, RPM, then afterwards total fuel burned. Record that every minute or so, or every 500ft or so through the climb.

Then line up the data and see where the differences are - you should start to get some useful clues. Bung it all in Excel and plot some graphs - and wherever the graphs are significantly different (personally I'd plot everything against altitude, with time another variable - also work out RoC numerically from altitude and time and plot that on the same scale as the VSI reading) you'll find the significant differences.

G

[edsbar]

Changing Lord mounts sure can effect performance, many times I have seen them installed in the wrong positions or with the wrong or missing spacers effecting the thrust line turning the plane into a dog. Get an inclinometer and check the difference between the levelling point (top of tail one behind rear window) and the pushrod tubes.
Mechanical tachos are also problematic, get it checked you may be surprised how far out they can be.

[smarthawke]

As well as checking the pitot/static system for leaks and instrument calibration (which should be done every Annual anyway), have you tried swopping propellers?

Just because the props look the same and have the same part number doesn't necessarily make them equal.

Have the props been strobed to check if the tachos are reading accurately at max static?

[Cows getting bigger]

As others have said, most likely an instrument issue. You may have weighed the aircraft but have you checked the balance?

[fireflybob]

As others have said, most likely an instrument issue. You may have weighed the aircraft but have you checked the balance?

Cgb, the CG position for both a/c is almost identical.

Thanks for all the suggestions, we are having a pitot/static check conducted this week and will see how that goes first.

[cessnapete]

Watching recently a Tomahawk, two up, stagger off our strip, "Tomahawk" and "climb rate" seemed a good oxymoron!!

[stevef]

How many feet per minute are you getting with both aircraft btw?