c172p

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With pitot covered, ASI should indicate wrong speeds but not necessarily 0 knots

Why shouldn't it indicate 0 kt? In a mechanical ASI the needle is moved by the air that is "pushed" into the tube. Therefore when the tube is covered the ASI can't indicate anything. At least I think so...

That means, IMHO the ASI should indicate nothing when the pitot tube is covered.

That's my opinion too. IMO, The pitot tube cover is not airtight (it is just against insects or dust). Thus, there is no noticeable dynamic pressure inside the pitot tube but the static pressure can equilibrate, even if the static pressure would (very slowly) change. Anyway, even if the dynamic pressure were not exactly 0, I think it would be too low to give a reading on the ASI.

Sometimes (not very reliable), it can be an iron wire put inside the tube instead of a cover. That may give a very small dynamic pressure left, but I think it is of negligible interest.

According to http://www.skybrary.aero/index.php/Unreliable_Airspeed_Indications#Effects:

• If the pitot probe is blocked but the pitot drain and static ports are free, then in straight and level (cruising) flight the displayed IAS will tend to reduce, eventually indicating zero.
• If the pitot probe and pitot drain are blocked but the static port is free then the IAS will increase during a steady climb and decrease during a steady descent.
• If the pitot probe, pitot drain, and static ports are all blocked then the IAS will remain constant despite changes in actual airspeed..

In addition to airspeed indicators, systems which rely on information directly or indirectly (via Air Data Computers) from the pitot-static system are also unreliable if the pitot static system is blocked in some way.

• If the static vent only is blocked, then the altimeter will freeze on the altitude that the blockage occurred, the VSI will show zero climb or descent, and the IAS will over-read in the descent or under-read in the climb.

Would you agree with that, @Juanvvc?

All of this makes sense. But do not forget to make the difference between only covered by a non-airtight protection (which IMO is your issue) and blocked.

Unless you would wish to simulate much more complicated failures...

And with icing it is fully blocked?

I think it can. Almost sure.

related to: https://github.com/Juanvvc/c172p-detailed/issues/50

So when the pitot develop ice, the airspeed will slowly decrease to 0 or drop at once when the icing completely covers the tube?

I have no experience of that, so my opinion is not better than yours can be. My reasoning makes me think that the decrease should not be instantaneous, but I don't know at which time scale. Moreover, the failing description above shows that it depends on which part is covered by ice first (drain at the same time or not?).

@Juanvvc @tigert @wezzer Do you guys know what happens with the airspeed (and at what time scale) when the pitot tube gets blocked with ice?

Fortunately I have never experienced this pitot tube icing, so I cannot really comment how quickly it happens. Once the ice completely blocks the pitot tube, I would say that it's an instant effect on the airspeed indicator.

I found this: http://www.boldmethod.com/learn-to-fly/systems/airspeed-indicator/

this is possible using generic pitot-static elements - this is how I did it in the A320:

see:

http://wiki.flightgear.org/FGproperties/Systems

@legoboyvdlp What would you like to implement exactly? Just non-airtight blocking (blocked pitot probe) or icing of probe and static ports as well?

Icing can be done using the script originally implemented by Torsten on the Seneca. I have a modified version on the IDG-A32X. I was originally just thinking of the cover, but I can do icing as well if you want.

It looks like this is already implemeted: see indicated-airspeed.xml.

I think this can be closed.

Edit: on second thought, icing should be added.

It looks like this is already implemeted: see indicated-airspeed.xml.

I don't see where this is implemented in indicated-airspeed.xml, what part of the code there is responsible for the wrong airspeeds with the pitot tube cover?

There is just a switch which turns it off if the pitot tube is cover, but that's exactly what we don't want: we don't want the ASI to display 0 kts when the tube is covered, we want it to show wrong speeds (e.g. multiply the speeds by a certain factor). There were a lot of talks about partially and fully covered pitot tubes in this repo if you feel like searching for it.

https://camo.githubusercontent.com/48e972d6093fc4c2a2fd6f209b2d7c0cc16fe13e/687474703a2f2f7777772e73706f727479732e636f6d2f6d656469612f636174616c6f672f70726f647563742f392f362f393635315f342e6a7067 I'm assuming thats the type of pitot cover modelled, as you said yourself in another issue.

Dynamic pressure will be almost zero - I don't see how any airflow could enter the tube. Static pressure should be ok - thus, airspeed will 'increase during climb and decrease during descent'?

@legoboyvdlp the pitot tube cover is not air tight and also the resulting effect on the AIS depends on other factors, see: https://github.com/c172p-team/c172p/issues/538#issuecomment-155847709

All those effects, are to the best of my knowledge already in the generic pitot-static systems / instrumentation - regardless, if the pitot tube cover is not airtight, in that case, dynamic pressure would be equal to static pressure multiplied by some factor - so you would be right.

@legoboyvdlp I don't think those effects are there at all as I don't think we don't simulate the static port. We simply have a switch saying that if pitot tube cover is on -> AIS is unserviceable.

Have you any numbers on how much the cover would impede airflow?

We definitely do support the generic static port - its in the systems file.

What we have is a switch that makes the dynamic pressure measured by the pitot tube get stuck or read zero (i.e. = static pressure) - sorry, I can't remember which at the minute. That is fed into the airspeed indicator along with static pressure which is seperate from the pitot system altogether, and simulates such things as sideslip effect on the static port.

Have you any numbers on how much the cover would impede airflow?

Not really. But now that I think of it, even though the cover is not air tight, it should impede the absolutely majority of the airflow.

Also, the pitot tube cover will only cover the pitot tube probe, not the drain. So in that case our aircraft is handling well the first scenario I had posted:

• If the pitot probe is blocked but the pitot drain and static ports are free, then in straight and level (cruising) flight the displayed IAS will tend to reduce, eventually indicating zero.

What we miss is to implement icing, and icing can block three things: the probe, the drain and the static port. I assume that if icing occurs on the pitot tube, it will likely affect both the probe and the drain, so the case above doesn't need to be considered (that is, only the pitot tube cover will affect the case above) and we just need to consider these:

• If the pitot probe and pitot drain are blocked but the static port is free then the IAS will increase during a steady climb and decrease during a steady descent.
• If the pitot probe, pitot drain, and static ports are all blocked then the IAS will remain constant despite changes in actual airspeed..

I can confirm it works as you expect - the measured IAS increased to 140 knots when I ctrl + ud at 50 knots / 10 feet to 1010 feet with the pitot cover on, and decreased to 50 knots, my actual airspeed when I removed the cover 😄

I'm confused, lego. I understood from that post I have made above (in which I am quoting an article on how these things work) that with the pitot cover the airspeed should always display 0 kts, regardless of the airspeed.

They are usually not aligned with airflow apparently (correct me if I'm wrong).

You are correct:

But they can still freeze as far as I know. Perhaps under icing conditions the probe would freeze first than the drain, but that's just a random guess.

"If the pitot probe and pitot drain are blocked but the static port is free then the IAS will increase during a steady climb and decrease during a steady descent."

Assuming this is the drain hole, the cover would probably cover that one?

Ah, I see - so the drain hole is at the rear. In that case, with the cover on if I understand it the dynamic pressure inside the pitot tube would slowly run down, resulting in an indicated speed of zero?

I don't think that's the drain hole, I think the drain is in the back of the tube, see my image above. It should be completely against the airflow, I think. Thus I don't think the pitot tube cover covers the drain, just the probe.

with the cover on if I understand it the dynamic pressure inside the pitot tube would slowly run down

With the cover on, it will always stay very low since the cover isn't put in flight :smile: So basically it's 0 kts since take off. My understanding of the situation.