I made my first flight in my CH750 STOL on Sunday and all went well.  However, while returning to home base, I decided to see what airspeed I could attain with my Continental O200A at 2500 rpm.  At 2500 feet PA, I attained 95 MPH and shortly thereafter the airspeed indicator began to irraticlly fluctuate plus and minus 10 MPH.  Vertical velocity was fluctuating plus or minus 50 feet per minute and altimeter was noticeable fluctuating a very small amount.  I saw that the pitot tube  (small diameter kit provided) on left wing was not moving, solid as a rock.  As I slowed down for the traffic pattern and final approach, it reduced fluctuation but still fluctuating slightly on final.  Landing was normal.

In level flight the pitot tube appears to be slightly inclined to the relative wind which may be causing a burble at 95 MPH, so for the next flight, I plan to reposition the pitot tube down a hair and more into the relative wind.

It has been dry here for two weeks and temps in 60's and 70's so I do not suspect moisture or icing.  Possibly an insect got picked up during flight.

Any other ideas or suggestions?

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It sounds like a static port/static line issue... are you using the static port in the side of the pitot tube? If so, maybe check to make sure the pitot tube is pointed straight forward in the yaw direction. 

Maybe do a leak test on the static line too? 

Thanks, Matt for the suggestions.  My static port is just the instrument static ports open to the rear of instrument panel like the earlier certified light aircraft.  This pitot tube does not have a static port on its side.....just an L shaped tube pointing forward to sample ram pressure (relative wind).  

This afternoon I bent pitot tube slightly down and it looks more like a direct alignment into the relative wind now.  I'm flying again in the AM and will see if that helps.  I'll post the results.

Ah, that makes sense. The only thing those three instruments have in common is they all rely on static pressure, which in your case is cabin pressure. Cabin pressure changes as the airflow around/through the door gaps and cabin vents changes, so you'll probably notice variations as you open/close the vents in flight, transition between coordinated and uncoordinated flight, fly through turbulence - basically anything that changes the speed or direction of the airflow around the cabin. The faster you go, the more turbulent air is flowing over the cabin, so that's probably why you noticed the variations more at high speed. 

The pitot tube is only connected to the airspeed indicator, so I don't think that's the issue, but aligning it with the airflow will probably give a more accurate airspeed indication.

Connecting those instruments to a static line that is routed to a single small hole in the side of the fuselage will likely help. Maybe someone else has already figured out the best spot for a static port on the 750 STOL. 

Thanks, Matt, for the additional analysis.  I hadn't thought much about the possibility that increased airspeed might cause more turbulence over the fuselage/doors and affect the cabin pressure, but that does make sense. Yes, most builders located a static port on both sides of the fuselage just aft of the baggage compartment, joined the two lines and ran them to AS, ALT, and VVI..  I may have to do the same.   I'll pay particular attention as I change airspeeds tomorrow and post results here. I may have created a small problem when attempting to simplify the pitot-static system.

Bill

Haha yeah, simpler is usually better - except when it's not. I hope the flights go well!

Hi Matt,

I flew again this morning after bending the pitot tube down and more in line with the relative wind.  That did help smooth out the airspeed indicator, however anytime I make significant changes in attitude, power, uncoordinated flight, there is slight fluctuations in static instruments.  I also remembered that I had the circular plastic vents on the bubble door (all plexiglas except frame) were turned to position the slotted opening into the relative wind.  There is no doubt in my mind that these conditions are the cause of these static instrument fluctuations ( varying cabin pressure).  For now I'm going to let it be since I understand the cause.  I may add standard static ports later.

Thanks, again, Matt, for the analysis.

Bill

Hi Bill,

I'm glad I was able to help. And yeah, as long as you know what to expect then it shouldn't be a safety issue... my only concern would be if you're getting close to stall speed on final and you have the vents oriented so they're sucking air out of the cabin - then the airspeed would read higher than usual, which could lead to trouble. It might not be an issue in a 750 STOL, but maybe test that scenario at a safe altitude first. 

Cheers!

Matt

Excellent suggestion and I will do next stall series with those bubble vents oriented in different positions to determine affects on cabin/static pressures.

Bill

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