Pitot tubes

[flyerfly]

For those of you that know about the internals of pitot tubes I have the following questions...

1. If it is raining hard where does the water go that goes into the pitot tube? How much does it affect airspeed etc? The same goes for ice that gets on there and then gets melted...some of it turns to liquid and some turns to vapor but if it is always pushed back into the line what happens?

I am no expert on heated pitots so would appreciate learning more about how well they function in bad weather. I am not IFR rated BTW.

I know there was an airbus that had its pitot/airspeed indicator system fail because of heavy rain/water from ice etc. so I am curious about the solutions.

Thanks.

Jon

 

[Mustang]

Jon,
This does not seem to be an issue. There is no airflow really, into the pitot tube. The airspeed indicator is really a pressure gauge calibrated in knots instead of psi. The only opening into the pressure sensing part of the device is the pitot tube and there is no exit or bleed. Any water that impacts into the pitot head, just lays there and does not seem to block the tube and the airspeed indicator continues to operate. The water does not run up the tube and into the airspeed indicator. Consider that the oncoming air would consider the hole in the pitot head to be blocked or plugged off and therefore it would flow around the head taking some of the rain with it. The increased pressure at the head is what is measured by the airspeed indicator.

Ice however is another matter. Ice can form and build up, blocking off the pitot head. That is why IFR aircraft need heated pitot heads to be effective.
Hope that is of some help.
Cheers, Pete

 

[Highflight]

Jon,
Just for grins, why not go ahead and put up a poll to ask the yes or no question specifically for RV builders: "Whether you are flying or building; have you installed or are you going to install a HEATED pitot tube?
You might even have a second question asking the same thing, but only of those who intend to fly IFR in their RV's.

Then people responding, if they like, can comment why or why not in the thread.
That might give you some idea of how the majority think about the necessity of heated pitot's.

Highflight
RV7-A

 

[flyerfly]

heated pitots

Pete,

Yes I do understand that there is no flow in a pitot but I was thinking that any water inside the opening would perhaps create pressure pulses from turbulence. Of course the water may be back in where the tube goes verticle (on those that have a verticle leg) and it might be that there is no turbulence back there. I know that on my cessna pitot there is a very small hole at the bend (that I used for cleaning spiders out of that manage to get past the cover some how) that might let the water out.

Vern,

My intent for this thread was not to decide the need or lack of for a heated pitot. Indeed the water question could apply to a non-heated one as well. I have flown in heavy rain even though I am not IFR rated...yes the minimums in my airspace were ok. My intent was to learn more about pitot tube internals. In fact what was really in the back of my mind was why the heated ones are so stinking expensive! I mean a heater cartridge is not exactly expensive. I do want heat as a safety thing. My dad flies IFR and I intend to. The type of IFR I intend to do is for needed landing and take off. I have been grounded many times where I know the soup is only 1000 feet thick and clear on top and at my destination. I have no real desire to buzz along without seeing anything for hours at a time...seeing to me is the primary fun in flying. There are debates about everything...it makes for interesting reading I guess

Take care.

Jon

 

[Mustang]

Jon,
Yes we have the same design concept. VFR except for required/necessary cloudbreaking procedures to get home. Perhaps for getting out, but not usually. Single pilot/engine IFR is a whole 'nother thing, and until you have done it in anger, it sounds wonderful, but in reality, it is pretty stupid, if I do say so myself.

I had an IFR certified C-180 for a few years. As a really experienced IFR jet jock, I once found myself over the Rockies in the Cessna, at 12,000 feet on top of an overcast. Just a few peaks were poking up through the cloud deck. I suddenly had this thought, "OK Marshall, what the hell are you gonna do if the engine quits at this moment?" Remember, this was before GPS and there was no real way of finding a valley with a LORAN C. That was the point when IFR in a single began to lose it's allure. It's like night crosscountry in a single. Very silly really. The other thing that happened was that I was IFR into Vancouver International one really crappy day and it was down to limits. There I was on final, shooting an ILS at 130 knots, trying to stay ahead of a 737, switching frequencies, tuning radios, fumbling with charts, and talking to ATC while, OH YEAH, FLYING the AIRPLANE! I almost forgot that part! Single pilot, hard IFR really sucks. They would have to pay me for that.

So, that is why we want cloudbreaking capability but we are not planning on filing hard IFR to anywhere unless assured that we can cancel IFR in a very short time after takeoff, or file IFR to get back into home base when there is good vis and ceiling under a cloud deck.

And yes, a heated pitot would be nice. Maybe a used Cessna pitot would work.

Cheers, Pete

 

[alame]

a heated tube is the better way to go, gives you that what if solution, there is a drain hole at the bottom for any water that might get that far as well in the system(cockpit) there is a "jar" to collect any moisture before it gets to the instruments, should be drained on a regular basis.

 

[flyerfly]

pitot design

alame,

Thanks for that. I guess I have not looked under the dash enough to see that jar you mentioned. It does make sense that there is something to collect any remaining moisture that might get pushed up in there. Anyway you have answered my questions. Thanks.