pitot tube

[Pilotpeter]

Not sure wether this is the exact place for my question but in the past I've been very happy with the awnsers I got for other questions so here goes.

Our company has two nearly identical cessna 172 skyhawks. The difference between them is that one has aux tanks and an Isham conversion which I think is not of any importance for my question.

While I was flying these two regularly I noticed at some stage that in cruise flight with same powersetting and same cruise speed that if I looked outside I could see a big difference between the angle of the pitot tube and the horizon. In SLF in one plane the pitot tube is parrallel with the horizon and in the other plane the angle is about 5 degrees. (When I put the pitot tube of the second plane parrallel I have about a 5 degree climb angle).
This struck me as a bity strange and since one can't adjust the pitot tube I was left wondering where this difference came from? Maybe you guys can enlighten me. Thanks a lot in advance.
Greets
Peter

[NutLoose]

Possible cause is the angle of the wing. On the High wing Cessna aircraft the aircraft is rigged to fly straight and level by adjusted the rear spar mounting point this is a bolt which is fitted through an offset (eccentric)bush, rotating this bush lowers or raised the trailing edge of the wings, so if you have an aircraft that does not trim out level the engineers can raise or lower the trailing edge thus affecting the whole angle of attack on that wing to compensate, It is possible they are both set slightly different. and hence you are seeing the difference in the angle on the pitot tube.

[Pilot DAR]

I would not expect to see too much difference in the airspeed readings of the two planes as a result of the pitot tube position difference. If in doubt fly them in formation (not too tight please) and compare readings, also considering the calibration record of the two airspeed indicators, which is more likely to produce an error.

As for the reason for the different angle, with due respect to the other poster, the cam adjustment at the rear spar produces only a small fraction of a degree of angle of incidence change over its full travel. Very important for rigging, but no affect on visible wing angle of incidence. Check instead that the pitot tube mounting tube is securly attached in the suspect aircraft. It is an airfoil shaped tube, which projects down from the wing. It is held in place by screws and riveted tabs. Any of these pieces could be adrift. Also, in 1974, Cessna changed the wing airfoil of the 172. If the two planes span these year, that could be a part of what you are seeing.

Pilot DAR