Friday, November 15, 2013

Pitot cover

In this post a couple of candidates 8 mm pitot covers are considerate.

A pitot cover is a simple device that have the finality of protect the pitot unit at rest or during the storage. Entrapped moisture and condensate can lead to wrong measurements and to premature sensor failure. A good cover must ensure that rain and dust cannot enter the pressure lines nor yet allow the insects to go live inside the unit. The pitot cover must also be highly visible because if it is not removed the airspeed indicator will not work once in flight. Visibility is assured by the so called “remove before flight” ribbon. This ribbon is becoming a quite common gadget for RC electronic devices. Effectiveness is warranted by a strict pre-flight checklist, indeed for RC planes it's sufficient to count the ribbons that have been collected before the flight, if they are the number expected then is safe to proceed.

Figure P22.1 Remove before flight ribbon on a Concorde plane
There are many pitot cover producers, pitot tip dimensions are quite standardized so it is easy to find a pitot cover for full size airplanes neither cost is a problem. Commonly used pitots have a separated static port located on the fuselage, so only the pitot total pressure port should be protected by the pitot cover, on the static port there is a pin. There are three main available cover typologies. The first is a hinged pitot protector that opens automatically during take off, the mechanism is composed by a target aerodynamic surface linked to a spring as per following figure.
Figure P22.2 Automatic type cover. Retrieved from this link
The higher complexity of this device is justified by the fact that closing is not operator dependent.

Second type is composed by a rigid cap that fits on the tube and a before flight ribbon.
Figure P22.3 Cap and ribbon type cover. Retrieved from this link
Third type is composed by a cloth that is slided over the tube
Figure P22.4 Fabric type cover. Retrieved from this link
First type of cover have been excluded because in our pitot static ports are integrated on the tube and this kind of arrangement impact too much on the aerodynamic behavior.
The fabric type cover require sewing skills, instead 3D print construction is chosen.
Is easy to conceive a cap that cover static and total port, have a look to the following figure for the first virtual prototype.
Figure P22.5 Pitot cover with ribbon virtual prototype
After modifying some shapes I reach to the following solution
Figure P22.6 Pitot cover, ribbon will be attached on the left side
I chosen the second solution and now is en route to the 3D printers. As soon as I've tested the cover I will let you know how it works.

The next post will come with photos and 3D model of completed pitot cover, so you can print your own piece.

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