Pitot tubes

[Mac the Knife]

The current brouhaha about AF447 and pitot tube icing got me thinking.

Instead of the tube facing forwards and deriving speed from the ram air pressure, why not turn it around 180 facing backwards and derive speed from the Venturi/Bernoulli effect pressure drop?

I think I'm going to take some plumbing and a mercury manometer for a ride in my car and see how the rise/fall in pressures correlates (young Tom can read the manometer, it'll make a good science project).



Mac

[lpokijuhyt]

Why, so you can icing on the other end?

[Mac the Knife]

No, O most exalted lpokijuhyt - as might be evident even to you, my thought was that you'd get less icing that way.

(if you ever have a medical question I'll try to be as sarcastic and dismissive)



Mac

[Piltdown Man]

You certainly can do it that, but I think that would be a higher drag option. But given the overall drag of a large transport aircraft, it might be worth the price.

PM

[Piltdown Man]

You don't actually have to have a "backwards" pitot. It probably wouldn't work anyway (and would still be susceptable to ice). But you could have an "open" venturi, ie. a built up section on the the side of the airframe. But the most difficult problem would be siting it in such a place as to get a reliable reading at all speeds and angles of attack.

PM

[john_tullamarine]

the most difficult problem would be siting it in such a place as to get a reliable reading

Generally, the pitot is reasonably tolerant of siting and doesn't cause much in the way of errors.

Now, the static source, that is quite a different animal and takes up a lot of certification time and effort.

[Piltdown Man]

I was actually referring to an "open" venturi (a lump on the side of the plane). As such it's a modified static vent so it would be subject to all the problems of a placing a source, plus a few more.

PM

[Mac the Knife]

Two questions (actually 3)

Why would it be higher drag? Just facing the other way.

Why do you say probably wouldn't work? The basic physics seems OK

Wouldn't it be less susceptible to icing facing backwards?

Thanks for answers

PS: Can you explain what you mean by an open Venturi?

[BOAC]

One of the major hurdles in any design is that if you are measuring any sort of pressure you have to have a little 'ole somewhere where wot it goes, and the trick would be to stop ice getting there.

I would have thought sticking with the current shape but with variable heat input would not be beyond the bounds of the designer - eg when the probe is sensing 'cold' or 'ice' is detected, a high current and vice versa.

[Brian Abraham]

Could always stick one of these on the roof. I know, I know.



Wonder though if some of the methods here could bear fruit.

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