ASIs and static tubes

[BobH]

Went flying in the Spectrum today, but before I did so I fitted my brand new ASI from Skydrive. All seemed well till I ran the engine up and found the ASI reading 35mph while I was stationary (!). I assumed it was vibration causing it to do this, so I flew anyway as I have a Pilot 3 to give me groundspeed, and with very little wind I knew more or less how fast I was going. While in the air the ASI gradually decreased in value till it was reading 0mph! It stayed at this figure for much of the flight, but when I came in to land it rose to 70mph, and stayed at that even after I'd landed and parked the plane.

Needless to say I was a bit baffled by this, so I disconnected the static line from the ASI, at which point it dropped to 0mph. Aha! So that was it. It turned out the static line was blocked with rain water, so I blew it out and after that the ASI read correctly on the next flight.

The Spectrum has its static ports mounted one either side of the pod and the tubes are joined by a T junction to a single tube that then goes up to the ASI. As they are not protected from the weather when the plane is parked, some water must have seeped in and sat there, closing the ports. Now that I know this, in future I'll have to set up something that will allow me to blow these out again without having to pull the tube off the back of the ASI like I did today. Either that or I'll have to make up some kind of cover for the pod, which is something I don't currently have.

 

[randombloke]

This has echoes of Antoni's ASI test on the previous forum, so there was water trapped in a U bend, with the air between the trapped water and the ASI unable to exit?

I must work out Antoni's maths again, I have the concept in my head now...

 

[Martin Watson]

Beware blowing into either static or pitot ports while they are connected to the asi instrument. They are very delicate and easy to damage that way

 

[BobH]

Thank you for the warning Martin, I guess I'll have to make up a cover for the pod then rather than having to go through the rigmarole of taking the tube off the back of the ASI before each flight.

 

[BobH]

randombloke,

Yes, the two plastic static tubes are not supported inside the pod, so tend to slope downwards toward the middle, which is where they meet the T junction and the single tube that then goes up to the ASI. I pulled the tube off the back of the ASI and the reading dropped immediately, so it was obviously something to do with the statics. I blew through the static tube, then went round to see the outside of the ports and noticed water marks down the sides of the pod below each static port. I also disconnected the pitot tube and blew through that as well, to make sure there was no water in it, but as I'd previously put a piece of fuel tubing on the pitot head when the plane was parked, it was dry inside.

I guess the thing to do is either block the static ports when I park the plane, or just make up a pod cover. I think it will probably be easier to make up a pod cover. That way I'll be covering the pitot tube as well, and if I make it big enough I can cover the windscreen as well.

 

["SMOKE ON GO"]

Would it not make sense to just route the air pipe above the static & ASI port so that the water can't lie in the pipe & restrict the airflow.

 

[BobH]

It would indeed, but trying to get in to the front end of the pod isn't an easy task. It's not like a flexwing pod that has a seat mounted a long way back. In the Spectrum the seat is bolted to the main A frame that the pod bolts to, so I can't just move it out of the way. Even with the screen removed it's not an easy task trying to get anywhere near the front end of it, and I find that the only way in is from underneath, with the front wheel removed, and even then it's a bit of a stretch, with lots of stuff in the way.

 

[BobH]

Here's a couple of pictures showing what the pod looks like from the inside. One shows it removed from the plane so that you can see the pitot and static lines. The other shows what's in the way when the pod is on the plane.

 

[Gentreau]

Do the instruments really need a static port, with the associated risk that you just discovered?
How much difference is there if you just leave the instruments open to the air inside the pod?
Sure there will be some difference, but is it really enough to be significant?

 

[BobH]

Good question Gentreau, I really don't know. The next time it's flyable I'll try disconnecting the static and see what difference it makes. I must admit that in my XL and my Q I don't recall there being a static tube, just the pitot that was mounted at the very front of the pod, just like on the Spectrum. So you may well be right, and there may be very little difference in readings. I'll let you know how I get on when I'm able to check it.

 

[randombloke]

I must admit that in my XL and my Q I don't recall there being a static tube, just the pitot that was mounted at the very front of the pod, just like on the Spectrum.

In a lot of older flexwings, there is no static tube, the ASI just vents under the dash which is not bad for static. Neither my Q nor my Quantum had a static, however, when you get into Leaps and Hypers with much taller screens, the pressure start to change, in the Hyper there's a low pressure area in front of your chest that may pull your flying suit forwards a little. So there needs to be compensation or a static tube.

 

[Aerial]

I recently completely re-plumbed my pitot and static systems because the previous installation was very unsatisfactory. The clear plastic "fish tank air line" tube had never fitted properly to the instruments. I happened to look on youtube for inspiration and found it here

The speaker advocates quarter inch plastic push-fit plumbing, very similar in operation to house plumbing so I then looked for a metric system (6mm) and found all I needed here https://wrekinpneumatics.fluidfittingsshop.com/Products/0049000a
I bought a whole system of instrument connectors (one eighth NPT in my case), 6mm tube in different colours for pitot or static and a whole bag of elbows, tees and an in-line connector for wing-folding times.
Having a good static to the ASI is just as important as the pitot as has been shown above. The new system is absolutely air tight and I'm very pleased. My static ports are 5mm pop rivets with the mandrels knocked out and I use some silicon rubber hose, probably surgical hose? to adapt from the 5mm rivet bodies to the 6mm air line.

 

[ginge]

Not having a static tube isn't limited to open cockpit flexies, the trusty ole AX3 worked well that way with its enclosed cockpit, so probably well worth trying.

 

[Gentreau]

Not having a static tube isn't limited to open cockpit flexies, the trusty ole AX3 worked well that way with its enclosed cockpit, so probably well worth trying.

And the french Aeroservices Guepard range, a very sucessful aircraft, never used an external static port either.

 

[Aerial]

Not having a static tube isn't limited to open cockpit flexies, the trusty ole AX3 worked well that way with its enclosed cockpit, so probably well worth trying.

I suppose that's true Ginge. I think that where a static system is called for in Build Manuals or TADS etc, then it's probably for very good reasons and therefore worth making the best job you can. F'rinstance, the X'air HADS states that "An exterior static vent is to be fitted immediately forward of a leading edge wing strut at least 8 inches below the wing lower skin, serving both the ASI and altimeter, + if fitted, the VSI. If a 2-tube combined pressure head is used, the static must be shorter than, and directly below, the pitot tube."
The SkyRanger HADS with which I am now familiar doesn't go into any such detail but the Build Manual give very specific measurements on a cowling for the static port locations.
Some ASI errors can be directly laid at the door of poor static system as we have seen. I think it's well worth checking all the documents for your airframe, of whatever type.

 

[Gentreau]

The burning question is:
How much difference in pressure is there going to be between the cockpit of an enclosed aircraft and the absolute static pressure, and how much error can that introduce?
Plus, is that potential error significant?

 

[deluc]

My Air Creation Trek had no static tube simply open at the back. Seemed to register very close, as to make to difference to real as far as I could ever confirm

 

[MadamBreakneck]

When I calibrated the altitude sensing on my Zaon MRX which took cockpit static pressure in the enclosed T600 pod, I needed to set an offset of (IIRC) about a couple of hundred foot to get correct readings when in flight.

There's a video on the subject at

It's apposite to this discussion for about 10 minutes then I got bored.

 

[Gentreau]

I guess the acid test would be to set your altimeter to zero before engine start, and then take-off and perform a low pass over the runway at cruise speed and have someone observe the indicated altitude. That would give a clear indication of any pressure difference during flight.

 

[randombloke]

I think that where a static system is called for in Build Manuals or TADS etc, then it's probably for very good reasons and therefore worth making the best job you can. F'rinstance, the X'air HADS states that "An exterior static vent is to be fitted immediately forward of a leading edge wing strut at least 8 inches below the wing lower skin, serving both the ASI and altimeter, + if fitted, the VSI. If a 2-tube combined pressure head is used, the static must be shorter than, and directly below, the pitot tube."

It's interesting that two aircraft who are cousins in the family of old shitters have such a different approach.

Both fusetube, but very similar all round, you'd wonder what the difference was if time was infinite and there was no flying to be done...