IAS Issues at Stall- No static port (Airspeed wrong)

[Cardiff Kook]

I had a similiar post about this several months ago- but it came up yesterday while flying with another guy.


My airspeed gauges are dead wrong when performing power on/off stalls- by somewhere between 10 and 20 MPH too fast. One of them (AV-30) is dead right in straight and level flight.

Set-up on my 1950 PA-18:
Pitot: The pitot tube is located on the jury strut and has an aluminum cover that opens in flight. I think it is similar to that of a J-3.
Static: No dedicated static port. 2 airspeed indicators have open static ports on the bag direclty to the cockpit area behind panel.
Airspeed indicators: Av-30C (accurate in level flight based on GPS ground in calm wind) and UMA p/n 16-310-160. The pitot is plumbed to both. I basically ignore the UMA/analogue.

My pitot looks like this:





Yesterday I flew with a CFI/much more experienced pilot than me. He basically said that it was unsafe to have my airspeed be that wrong around stall (not the point of the post.) Yes- I know when my plane stalls.

I had an avionics guy check the accuracy with a meter on the ground. Think it was a water meter of some sort- and it confirmed the AV-30 was dead right, and the UMA was about 20 MPH slow. The issue must have to do with power on/off stall configurations- angle of attack or the flaps or something.

I did notice my AV-30 now reads about 40MPH when I'm sitting on the ground with the engine running- but again- it works perfectly in straight and level and when hooked up to a meter. Must be the static?

In any case- I really would like to have an accurate airspeed. I want to fix it. Any suggestions? I think the issue has to do with the pitot/static.

 

[mvivion]

AV-30 should NOT read 40 mph on the ground. Something haywire there. Have you opened/closed side window and observed the result on the A/S data?

And, by the way, the mechanical gauge is primary, at least as far as the FAA is concerned.

In my experience with Cubs, you can "tweak" the direction the pitot mast is pointing and modify the indicated airspeed at different AOA. That said, I would wonder why the error between the two instruments. As I noted, the mechanical is considered "primary", so should be as accurate as possible.

That said, at the stall, many of the Cubs I've flown indicated zero airspeed, or some very, very low (and nonsense) speed. That is a function of the pretty large variation in AOA in a Cub from cruise to the stall. Hard to keep that little tube pointed into the "relative wind", which is what we really care about.

Finally, Try removing the little flap on the end of the pitot tube and see if that changes things. It's just there to keep bugs out on the ground.

MTV

 

[wireweinie]

Can you verify that both gauges use the same pitot tube. Or is there a second tube for the AV-30?

Web

 

[BC12D-4-85]

Some airplanes plumb a static line into the left wing cavity away from the cockpit's varying static air pressure. Others like Piper later provided an adjacent dual static (a blocked tube with small side holes) and pitot line more to the rear of the PA-18's wing. That can be changed. For info tie a piece of yarn to the base of your pitot tub/jury strut junction and go fly. Note the angle of the trailing yarn at varying AOA vs the fixed pitot.

Excerpt from CAR3 1956:

FLIGHT AND NAVIGATIONAL INSTRUMENTS
3.663 Air-speed indicating system.
This system shall be so installed that
the air-speed indicator shall indicate
true air speed at sea level under standard
conditions to within an allowable installational
error of not more than plus
or minus 3 percent of the calibrated air
speed or 5 miles per hour, whichever is
greater, throughout the operating range
of the airplane with flaps up from Vc to
1.3 V., and with flaps down at 1.3
The calibration shall be made in flight.

3.664 Air-speed indicator marking.
The air-speed indicator shall be marked
as specified in 3.757.

3.665 Static air vent system. All
instruments provided with static air case
connections shall be so vented that the
influence of airplane speed, the opening
and closing of windows, air-flow variation,
moisture, or other foreign matter
will not seriously affect their accuracy.

Gary

 

[cubdriver2]

This will never lead you astray. Working fine for over 100 years



Glenn

 

[BC12D-4-85]

CK here's an excellent opportunity for you and your CFI to explore TCDS 1A2 and Civil Air Regulations 3 (CAR 3). Note the model and year your Cub was certified then use both to learn about what was expected for certification. Same for your maintenance. The Cub changed build specs over time and the pitot/static configuration was one of them.

Also, it appears that you take seriously this FAR. Excellent!

§ 91.403 General.
(a) The owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition, including compliance with part 39 of this chapter

Gary

 

[DENNY]

Do you mean on the back? "2 airspeed indicators have open static ports on the bag direclty to the cockpit area behind panel". My pitot tube is the same as yours and I also have static simply open to back of panel. Is everything set up the read the same IE Knots vs MPH. I flew with a pilot that said he was landing at 45 mph it just looked and felt way too fast from the back seat. We latter figured out the gps was set up in knots not mph. So that would be the first thing to check is everything reading in MPH. Next it is not uncommon for a airspeed to off on the top or bottom end because it is a dirt simple system. I have bumped into mine in the past and had to straighten it back out. I adjust it for the most accurate in the 40 mph range. I really don't care if it is off some at 110 mph. What stall speed are you currently reading on the gauges? Should be right around 39 mph.
DENNY

 

[stewartb]

If your indicated speed is consistent? Good enough. If you want to land at stall x 1.1? It doesn’t matter what the number says as long as you know what it means.

 

[Cardiff Kook]

Can you verify that both gauges use the same pitot tube. Or is there a second tube for the AV-30?

Web

One pitot


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[Cardiff Kook]

Do you mean on the back? "2 airspeed indicators have open static ports on the bag direclty to the cockpit area behind panel". My pitot tube is the same as yours and I also have static simply open to back of panel. Is everything set up the read the same IE Knots vs MPH. I flew with a pilot that said he was landing at 45 mph it just looked and felt way too fast from the back seat. We latter figured out the gps was set up in knots not mph. So that would be the first thing to check is everything reading in MPH. Next it is not uncommon for a airspeed to off on the top or bottom end because it is a dirt simple system. I have bumped into mine in the past and had to straighten it back out. I adjust it for the most accurate in the 40 mph range. I really don't care if it is off some at 110 mph. What stall speed are you currently reading on the gauges? Should be right around 39 mph.
DENNY

Av30 stalls at 47

Analog at esentially no registetered airspeed.

Everyrhing is MPH


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[stewartb]

Is airspeed says zero at stall it doesn’t sound like a static problem. More likely a pitot probe problem. The first thing I’d try is removing the trap door.

 

[stid2677]

I would remove or wire open the bug catcher, a stiff spring in it could be a cause, The AV30 and Airspeed Indictor should be sharing the same air source, the AV30 can be set up to show GS from GPS and IAS from pitot, also the AV30 Airspeed can be trimmed to match in the steam gauge in the "Install Menu".

 

[DENNY]

Every plane is different, but your cub should be stalling a lot slower than 47 mph. So I suspect you have an issue with the bug catcher or tube angle. Both are easy to fix.
DENNY

 

[BC12D-4-85]

My PA-11 had the strut mounted pitot adjusted somewhat perpendicular to the strut and parallel to the ground. I wanted landing accuracy more than how slow I was flying in cruise.

Gary

 

[KevinJ]

If you want a fly by numbers airplane, buy a Bonanza. Sell me the cub. Numbers don’t mean that much to me. I can barely count to 47….by 2s;-)

 

[LMartin]

This will never lead you astray. Working fine for over 100 years

View attachment 64610

Glenn

Perfect. Where do I get one?


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[BC12D-4-85]

Here's an interesting discussion of pitot placement and especially finding static source location. http://www.redbackaviation.com/a-pitot-static-system-for-the-homebuilt-aircraft/

Gary

 

[skywagon8a]

My pitot looks like this:


View attachment 64605

Not that it will make a difference in your airspeed reading but that bug preventer is not installed correctly. It should be further aft on the tube so that the little cover is flat against the front of the tube. Just not touching.

Also, the old mechanical ASIs are not accurate at the low end of the range, which is why the low number generally starts at 40.

Also, due to the aerodynamic shape of fuselages the air flowing past the outside creates a lower ambient pressure in the cockpit. Since your static ports are open to the cockpit, it is likely the ambient pressure is lower. An ASI balances two pressures. If the static/ambient pressure is lower the instrument will indicate the same as the pitot pressure being higher. Opening and closing a window or door during the questionable speed ranges may cause the speed needle to move. If it does, you have a static location issue.

Also, the air flow angle at the pitot opening should be +/- 10 degrees for accuracy. At higher angles the pitot pressure will be distorted falsifying the pressure information. Test this by tying a length of yarn to the jury strut directing behind the pitot tube.

Why your AV-30 reads 40 on the ground is a mystery to me.

In the old days pilots were taught to fly by the seats of their pants. So that the airspeed reading during landings and take offs was not important, nor was it noted.

 

[bob turner]

I run into that with experienced pilots transitioning to the J3 - “what airspeed should I see here? Then I cover the airspeed indicator.

Our J3s have a variety of airspeed indicators. One reads in knots, and can be flown at 28 indicated. My personal Cub got a new indicator in 1963, and with 15 hours total time I knew it was 10 mph off. So I rotated the needle clockwise 10 mph. For the last 60 years it has shown 38 at stall and 80 at cruise. Good enough for me.

 

[kestrel]

As others have said, the 40 mph on the ground is a curious point. I'd disconnect all instruments from the pitot line and blow it out. I've had problems with water in the line that resulted in non-zero airspeed while sitting on the ground and shutdown.

While your at it, inspect the as much of the pitot line as you can for breaks/problems.

 

[Cardiff Kook]

My guess is the 40 on the ground is due to a crakced window causing negative pressure in cockpit.

I dont need it to be accurate to fly- but when a more experience pilot/CfI tells me its not okay for it to not work and possibly a safety issue if someone else is flying I sit up and take notice.

 

[mvivion]

As others have said, the 40 mph on the ground is a curious point. I'd disconnect all instruments from the pitot line and blow it out. I've had problems with water in the line that resulted in non-zero airspeed while sitting on the ground and shutdown.

While your at it, inspect the as much of the pitot line as you can for breaks/problems.

That’s great advice. When you start having odd airspeed readings, verify the integrity of the pitot line. You could well have some water in that line. Disconnect the line at the instrument, and use low pressure air to blow it out. Then ditch that flapper on the end of the pitot. It’s just to keep a bug from crawling in there on the ground.

Finally, once you sort all that out, in flight try opening the window, while watching the airspeed. It will change. The Husky has an alternate static port inside the cabin (ice protected), and opening that in flight changes A/S some.

MTV

 

[mvivion]

My guess is the 40 on the ground is due to a crakced window causing negative pressure in cockpit..

Nope, not even close. Whoever installed that instrument needs to figure that out and fix it.

MTV

 

[kase]

Try moving your pitot to outside the flap bay area like Piper did on the 135 and 150 Super Cubs.

 

[skywagon8a]

Try moving your pitot to outside the flap bay area like Piper did on the 135 and 150 Super Cubs.
View attachment 64627

While that is a better area for accuracy, that is not the location where it was when the airplane was certified. That would be considered a major alteration. If it was an experimental airplane, that would be a different story.

 

[Cardiff Kook]

Pulled the bug catcher. No change in reading.

The tube does appear to point slightly up- wondering if that is an issue?

Looks real fragile so didnt want to try to bend it down or anything

 

[skywagon8a]

Pulled the bug catcher. No change in reading.

The tube does appear to point slightly up- wondering if that is an issue?

Looks real fragile so didnt want to try to bend it down or anything

+/- 10 degrees to the relative wind is the recommended angle. Tie a length of yarn to the strut behind the pitot tube for the answer. Bending it a small amount should not be harmful.

 

[bob turner]

Airspeed indicators in Cubs are like compasses. Use for reference only. When you fly close to the edge, your airspeed is repeatable, if not accurate. Go fly on the ragged edge for 15 minutes, memorize where your needle is, and use that position for reference. Add ten for a reasonable Vx.

My personal Cub appears dead accurate - ragged edge is 40 indicated. The instructional cub - same angle of attack, same weight, is 28. Makes no difference at all in how we fly them.

 

[sj]

Airspeed indicators in Cubs are like compasses. Use for reference only. When you fly close to the edge, your airspeed is repeatable, if not accurate. Go fly on the ragged edge for 15 minutes, memorize where your needle is, and use that position for reference. Add ten for a reasonable Vx.

My personal Cub appears dead accurate - ragged edge is 40 indicated. The instructional cub - same angle of attack, same weight, is 28. Makes no difference at all in how we fly them.

The only thing I would add is that if you have flaps, you need to do this with the flaps you intend to use for landing.

sj

 

[bob turner]

Absolutely. The Flight Review is usually the only time most local Super Cub folks go that slow. We do extensive flight on the ragged edge, inducing stall only after several minutes at stall airspeed. No altitude deviation allowed except for the full power stalls.