Pitot Measurement or AOI

Is there a published measurement for the AOI or distance from rib to orifice of the pitot? New plane, ASI is way off on the low end.

Thanks

TxAgfisher said:

Is there a published measurement for the AOI or distance from rib to orifice of the pitot? New plane, ASI is way off on the low end.

Thanks

Click to expand...

Check for leaks, will it hold vacuum? Recently installed new AV-30s in our Cub and on test flight the IAS was only 50 MPH, at cruise. Checked lines and found one not seated. Detected the leak by applying a "small" amount of vacuum 5 psi with a hand vac pump attached to the pitot tube. Before we found the leak no vacuum, after it would hold vacuum. Damage can be done, so only a tiny bit of vacuum to ensure system is leak free.

TxAgfisher said:

Is there a published measurement for the AOI or distance from rib to orifice of the pitot? New plane, ASI is way off on the low end.

Click to expand...

Where is/are the statics port(s)? Doesn't matter how accurate the pitot pressure is if the static pressure is wrong.

frequent_flyer said:

Where is/are the statics port(s)? Doesn't matter how accurate the pitot pressure is if the static pressure is wrong.

Click to expand...

That's a good question, where are they typically?

TxAgfisher said:

That's a good question, where are they typically?

Click to expand...

Top is pitot and bottom is static. This is the type on our Cub.

https://www.univair.com/piper/parts...zoh5H2PPLybDAf9ukv6pvcRGhMJ47rvMaAheSEALw_wcB

^^^^Make sure the static holes (small - 2) are open and the tubing not plugged with debris

Gary

stid2677 said:

Top is pitot and bottom is static. This is the type on our Cub.

https://www.univair.com/piper/parts...zoh5H2PPLybDAf9ukv6pvcRGhMJ47rvMaAheSEALw_wcB

Click to expand...

I have that same assembly.

Just for info; don't put a vacuum on the pitot side and don't put pressure on the static side. You will damage an instrument if you do. The safest way to check for leaks is to get ahold of the test box that puts a measured vacuum or pressure on the correct lines. The old school work around is to use a blood pressure cuff squeeze bulb. Look at a Cessna maintenance manual to see how its done. If you use any other source of pressure/vacuum, disconnect the instrument FIRST. And actually pressurizing a pitot system to 5 psi will damage the instrument. Ask me how I know.

That being said, I agree that the most likely cause of low airspeed is a leak. Nylon tube systems will leak at every joint after two or three seasons so sometimes just gently tightening all the connections will solve the problem. If all leaks have been fixed (zero leak down on the pitot system) then check the static system (if it's connected). The vast majority of fabric aircraft up here have no static system plumbed. In these cases the instruments are simply vented to atmosphere at the instrument panel and I've rarely found issues with this. However, if a static system is plumbed and is clogged or pinched it can cause a low airspeed indication (but only if there is a static port on that particular instrument).

The factory pitot and static plumbing (and in my opinion the best) is 3/16" soft aluminum tubing routed from the pitot tube assembly to the wing root. Then from the wing root to the forward side of the instrument panel. Each section is joined to the next with soft rubber tubing. Check the rubber tubing on these as the can crack with age and handling or get pinched during maintenance.

Let us know what you find out on your aircraft.

Web

Airspeed in a Super Cub doesn't work worth a dam at low speeds. Look where the pitot/static tube is. Notice it's proximity to the flaps which are deployed for landing. Guess what the air does in front of the flaps. Power, pitch and time in the seat.

Oh yea, pitot tube should be inline with the bottom of the wing on a Super Cub.

Improve the location and it works great.

This location under the wing is perfect at all angles of attack. The only error is below 40 indicated which is likely instrument error. Notice how the yarn is flowing at the high angle of attack. Straight back, within the +/- 10 degrees tolerance of relative wind to the pitot.

All great ideas for you experimental guys. People ask me all the time what airspeed I use for landing. I have no idea, I fly the airplane by feel, pitch and power. I can tell you the Top Cub has a more accurate air speed indicator. They did a lot of testing to find the best place to put the static port.

Steve Pierce said:

Airspeed in a Super Cub doesn't work worth a dam at low speeds. Look where the pitot/static tube is. Notice it's proximity to the flaps which are deployed for landing. Guess what the air does in front of the flaps. Power, pitch and time in the seat.

Click to expand...

After watching the PSTOL flap testing video yesterday I'm less concerned about the indicated now. I have been flying it way too nose high with these flaps.

And, FWIW, the ASI on that Cub in the video had a pretty rock solid airspeed indication the whole way down.

TxAgfisher said:

And, FWIW, the ASI on that Cub in the video had a pretty rock solid airspeed indication the whole way down.

Click to expand...

I fly the PSTOL flaps with a flat attitude where the pitot is in the slip stream but you are sinking as well. My airspeed is probably solid but I know it isn't accurate from glancing at the ground speed and I am more focused on the approach and adjusting my power and pitch.

Steve Pierce said:

All great ideas for you experimental guys. People ask me all the time what airspeed I use for landing. I have no idea, I fly the airplane by feel, pitch and power. I can tell you the Top Cub has a more accurate air speed indicator. They did a lot of testing to find the best place to put the static port.

Click to expand...

Under 40 for landing, that’s the last time mine works. The rest is by feel, which you get from going out and doing a lot of landings.


Sent from my iPhone using Tapatalk

Steve and Tom do it by feel. I just hold my breath, suck up the seat cushion and grip the stick as hard as I can. When everything stops moving I open my eyes.
It’s worked so far, but it’s early still.


Sent from my iPhone using SuperCub.Org

Lots of options here...set up stabilized approach to about 500 AGL at preferred V-speed and sight picture then forget the AS and watch/feel the rest to landing. Sight picture of LZ over cowl-hearing the plane-power to control rate of sink. I live near and drive a road with 35-40 mph speed limit most every day. That's what it looks and feels like when landing mine. My recurrency method in the dead of winter.

Gary

My pitot is the J3 type tube on the front jury strut. My Airspeed is the Helicopter style that reads down to 20 MPH. I have my choice of where I want it to be real accurate. High or low airspeed. If I adjust the tube for 95-95 mph dead on it will get me close to 40 mph accurate. If I try to get it dead on at 38 mph I will be off a bit at 95. I think the key is to not worry if it shows accurate mph at slow speed as much as it always shows the same speed when the wing stalls. Everybody likes to post how the plane stalls at some unrealistic airspeed anyway. As long as you under stand that that is not the true speed and just use it like a AOA indicator (I contend the small aircraft ones are just airspeed indicators with lights instead of numbers) just stay shy of the stall speed number it don't matter if it reads 30 or 40 mph. Compare it to you GPS at altitude and you will know the true speed then do the math in your head. I do tend to cross check my airspeed/gps on short final to make sure I have not missed a tailwind, but once I get into the flair it is all done by smell.
DENNY

OP asked about correct position of the pitot. I guess they start out parallel to the wing bottom. Until they get hooked on clothing or bumped somehow, then it's how many look after years of use. When on the jury strut I'd adjust mine to be more nose down as I could care less about what my airspeed was in cruise vs approach to landing or during takeoff. J-u-s-t--t-r-y-i-n-g to get to T/O airspeed on floats or skis when drag is high is critical, especially approaching terrain in a go or no-go situation.

Gary