Airspeed static problems

[Dave Roberts]

We recently purchased a 1956 Super Cub. We are very happy with it but have some issues with the airspeed indicator. Mostly reading to low according to the GPS. Probably around 13 mph slow at all speeds. If I pull the static line off in the cabin it will read right along with the gps. Open the left window and lose the airspeed again(almost the same as when the static is hooked up).

I am assuming when I am opening the window with the static line off I am pressurizing the static system. That leads me to believe when I have the static system hooked up it is somehow pressurizing. It is the standard cub static with the four holes under the pitot tube between the jury struts.

What am I missing.

Thanks,
Dave

 

[Gordon Misch]

Just a shot in the dark, but it might be worthwhile to broaden your suspicions to "something's wrong with the pressure differential". It may be that the pitot line is leaking, and the cabin is at less than outside standard pressure, thus compensating for low pitot pressure. Just a guess - - - Maybe you can come up with an easy way to test both lines for leaks?

Edit: Oh - maybe a line has some moisture and is freezing?

 

[skywagon8a]

To add to 12 Geezer's thoughts, you might check in the wing root area and inside the wing where the connections are made. It is likely that they are just soft rubber tubing which could be cracked from age. Thus a small leak.

 

[Dave Roberts]

Thanks for the info. I took the fairing off the wing root and took a look at the pitot and static lines. Looks like pretty new lines and clamped(safety wire). Put the soft rubber line on the pitot, rolled it up until the airspeed was about 81 then clamped. In 15 minutes it had dropped to 70. Didn't think that was much of a leak. Is that enough to cause my problem?

When I was checking this before I did blow the lines out to make sure they were clear. Actually a couple of times. Kept thinking that was where my problem was. On the static line leaking. The line going to the tube on the back of the airspeed didn't seem to tight. Is that a possibility? I had it in my head that if I pull it off it works, so probably wasn't a problem. But then I've had ideas that were bass ackwards before.

Thanks
Dave

 

[Gordon Misch]

Are all of the static holes in the static tube (out on the jury strut) open? Possibly some imbalance that could give a false static pressure?

 

[mike mcs repair]

can you disconnect static line at gauge, then at gauge end of line check/suck/blow bubbles easily out on jury strut end?? sounds like maybe sand/ice/water down in jury strut static tube?? (kinda impossible to clean on plane..)

also disconnect EACH connection on static line and make sure they didn't leave a piece of masking tape on the alum tube... but i guess you say you blew through them.... hmmmm

 

[Dave Roberts]

Gordon, Yes, all four holes are open.

Mike, I had the pitot static off the airplane and made sure air would pass through all four holes and blew through the line too. I am kinda running out of ideas. I will keep checking and try what was suggested.

What angle related to the pitot should the static be? It appears to be pretty close to the same angle.

Thanks,
Dave

 

[mike mcs repair]

...
What angle related to the pitot should the static be? It appears to be pretty close to the same angle.

Thanks,
Dave

pitot, in level flight, horizontal or a little pointed down if you care it reads better in slow flight.....
static about same... horizontal...

 

[cruiser]

A squeeze bulb like is used on blood pressure cuffs is a safe way to pressurize the pitot system. Block the static side, parallel a calibrated gauge to compare and search for leaks if the speed falls off. Jim

 

[Dave Roberts]

pitot, in level flight, horizontal or a little pointed down if you care it reads better in slow flight.....
static about same... horizontal...

Checked that Mike. I may have been a little off laterally. Checked all the hose connections again. Static may have been leaking a little in the cabin. If it gets above 0 tomorrow, may take it out for a flight and see what happens.

Dave

 

[jwflys]

I'm having the same problem with my airspeed indicator. It indicates about 10 MHP slow.

I checked the pitot and static lines myself. Had a good mechanic look at it also.

Is it possible that the instrument is just old and tired? I know they are pretty simple. Possible to overhaul? I've heard that some guys put helicopter airspeed indicators in their planes. Anyone have a good source?

 

[bob turner]

The way I look at it - airspeed slow, look for a leak or blockage in pitot system. Airspeed high, look for problems in static. A leak in static in a Cub should not be catastrophic.

If your airspeed indicator has always been off, it is probably a calibration problem.

We have a Super Cub with static source problems - disconnected the static, and with either door or window open all is well. Close everything and the airspeed goes up ten mph.

We fly by attitude anyway. My attitude is "thank God I am airborne".

 

[Steve Pierce]

I have never seen an airspeed in a SC that was very accurate except for the Cub Crafters Super Cubs under their own type certificate, they have an accurate static port. Also, when you deploy flaps that disturbed air is rolling back over the static ports on the pitot tube.

 

[Dan Gervae]

Old thread....but, I have a static issue I'm sure. I'm 7-10 mph fast indicated....I disconnected the static at the AS and same....when I open the window it goes down to where I believe it should be....I just have the static line run into my wing leading edge.....Any recommendations here? brand new gauge and new lines....cause its a new plane

 

[BC12D-4-85]

Read and maybe try this per Post #13?

Gary

 

[wireweinie]

Use a mast that has pitot and static ports. This gets the static signal away from artificially high/low pressure areas of the airframe.

Web

 

[skywagon8a]

Old thread....but, I have a static issue I'm sure. I'm 7-10 mph fast indicated....I disconnected the static at the AS and same....when I open the window it goes down to where I believe it should be....I just have the static line run into my wing leading edge.....Any recommendations here? brand new gauge and new lines....cause its a new plane

What do you have for a pitot tube? Generally if you have a combination pitot/static tube this is not an issue. Your static just being a tube open to the inside of the wing is sensing ambient wing internal pressures. Air is sucked out of a wing due to the shape of the airfoil. This creates a lower than ambient pressure which when coupled with an airspeed indicator create a higher then normal indicated speed. Think blow into the pitot and suck out of the static port. If there is excess suck the indicated speed will be higher.

This type mounts on the jury strut.

https://www.aircraftspruce.com/catalog/inpages/unheatedptubes3.php?clickkey=3046038

This type can mount on a plate further back under the wing.

https://www.aircraftspruce.com/catalog/inpages/pitot15144.php?clickkey=3046038

Both have the static ports adjacent to the pitot. The alignment of the tubes should be within +/- 10 degrees of the relative wind. Thus a slightly nose down angle gives a more accurate low speed/high angle of attack reading.

If you can find one of these, mount it midway between the jury struts and wing struts attach points. This is what I have on my Cub, it is very accurate except when below about 38 mph.

 

[BC12D-4-85]

I had the 2nd one in Pete's pics on a PA-18. Seemed to work well. Factory unit caught my parka hood and got ripped off the wing one winter.

Gary

 

[Dan Gervae]

I have the stock jury strut pitot…I actually have a pitot static tube somewhere in my junk…maybe I’ll break it out and use it??? I took it off the back of the instrument with no change….open the window and instantly loose 8 mph. I laughed as I opened and closed the widow and watched the as indicator bounce. I will mess with it this weekend

 

[frequent_flyer]

Old thread....but, I have a static issue I'm sure. I'm 7-10 mph fast indicated....I disconnected the static at the AS and same....when I open the window it goes down to where I believe it should be....I just have the static line run into my wing leading edge.....Any recommendations here? brand new gauge and new lines....cause its a new plane

How about a sketch of the entire pitot/static system. I'm having a having time understanding what you mean by "I just have the static line run into my wing leading edge".

Does opening the window also change the altimeter reading?

 

[wireweinie]

How about a sketch of the entire pitot/static system. I'm having a having time understanding what you mean by "I just have the static line run into my wing leading edge".

Does opening the window also change the altimeter reading?

On an unpressurized, 'leaky' airframe like a Cub, some guys just don't run the instrument static connections to a static port. Some just don't connect the static fittings to anything and some tie all the static fittings to one line but just leave it hanging.

Web

 

[Dan Gervae]

I have a poly hose run out along the spar and connected to the built in pitot tube on my left jury strut…J3 and PA11s have this…no static port. I just ran a similar poly tubing from my airspeed indicator static up into the leading edge along the spar and attached it in there….the end is not hooked to anything. When I pull it off the instrument there is no real change….when I open the door…it then goes down 8mph and more closely matched gps ground speed…and gave result more what I expected….close the door, up it goes…no effect on the altimeter…but I have no static hooked to that either.

 

[BC12D-4-85]

Dan does your current AS follow the GPS but a higher value at all indicated airspeeds or just cruise? My PA-11 had the static line in the wing as you describe but it was always close to the GPS regardless of airspeed. It could be your AS indicator is off adjustment.

Edit: One way to test> http://www.rstengineering.com/rst/articles/KP89JUL.pdf

Or.....borrow a replacement AS and confirm the errors are real.

Gary

 

[frequent_flyer]

Dan does your current AS follow the GPS but a higher value at all indicated airspeeds or just cruise? My PA-11 had the static line in the wing as you describe but it was always close to the GPS regardless of airspeed. Gary

With no wind aloft GPS GS should equal TAS not IAS. I suppose if you always fly low and with sea level density altitude TAS and IAS are the same. Not true here in AZ at this time of year. 5,100 ft DA at a 2,300 ft agl strip at 9am earlier this week.

 

[skywagon8a]

I took it off the back of the instrument with no change….open the window and instantly loose 8 mph. I laughed as I opened and closed the widow and watched the as indicator bounce.

This verifies the lower than ambient pressure inside the wing. Due to the shape of the fuselage the pressure is lower inside the cockpit than the outside ambient pressure. This is proven when you open and close the window.

Recall other discussions on other threads about exhaust fumes entering into the opening at the tail, traveling forward to the lower pressure cockpit.

 

[Dan Gervae]

Always follows gps speed (or shall I say consistently 8 mph higher) no matter what unless I open the door….then it is consistently about equal to gps in no wind

 

[Douten]

Dan in my PA-11 if I open my back window a little my speed picks up 2-3 mph on the gps so I guess I have too much drag air coming into the cabin.