Curious question - why do gas cap handles stick up?

k9medic

k9medic

Line Up and Wait
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ATP-H, CMEL, CSEL, CFI/CFII Airplanes and Helicopters
On one of my most recent long cross country flights I was left staring out the window and specifically at the 2 gas caps on each of my wings.

I have always been of the belief that if it doesn't produce lift or thrust then it produces drag.

Why would manufacturers not put recessed handles on a fuel cap and reduce some of the parasitic drag?
 
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My Velocity has flush caps.

Cessna used to have a type of flush fuel cap. The eventually became known as the "killer caps" as they could allow water in. So I'm guessing that potential to allow water in is the reason you don't see flush caps on GA aircraft.
 
My guess is they are cheaper and the resulting drag is inconsequential for those airframes
 
My Cub tanks cane with flush caps. I tossed them and had Atlee Dodge make me riser necks with standard umbrella caps. My Cessna still has flush caps on the outboard fillers (rarely used) and raised umbrella caps on the inboards. I wouldn't want to deal with flush caps for everyday use.
 
I have a flush cap ,behind the pilot door.
 
k9medic said:
Pipers
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The newer PA28 at our club is flush. Mooney are flusg

To answer your question though, probably just out of laziness and cost, and because for a 110 knot Piper or Cessna they really don't make a difference
 
As far as I know, all the Pipers with raised caps were built in the past. Perhaps that means they’ve seen the light?
 
Also, the Tigers I have seen are also flush
 
Tantalum said:
Also, the Tigers I have seen are also flush
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I think they changed in 1979. I think the ones before are umbrella style.

Also, DA40 caps are flush.
Edit: and they have key locks, too.
 
Beech has flush caps. Pull up on a little handle embedded in them to open it.
 
$ The caps originally came off cars. The more expensive airplanes got flush caps.
 
How much drag do you think they generate for a spam can than goes to yellow arc only during a nose dive?
 
donjohnston said:
My Velocity has flush caps.

Cessna used to have a type of flush fuel cap. The eventually became known as the "killer caps" as they could allow water in. So I'm guessing that potential to allow water in is the reason you don't see flush caps on GA aircraft.
Click to expand...

There was an AD against those caps: http://www.airweb.faa.gov/Regulator...7CA32CBFD7107A87862569B9004D1566?OpenDocument

And that wasn't the first AD. An earlier version regarding leaking caps applied to the 182 only. That one, 83-13-01, warned of loss of fuel and erroneous fuel gauge indications if the caps leaked badly enough. The low pressure atop the wing would suck the fuel out by removing the air from the tank faster than the underwing vent could replace it, and the bladder would collapse and lift all the fuel and squeeze it out past the cap's leaks. The collapsed bladder would hold the fuel sender float up, giving the pilot the wrong info about fuel in that tank.

And even then the ADs weren't detailed enough. They mention checking the cap seal, which is a large-diameter o-ring, easily seen, but neglect to remind us of the little, hidden seal around the locking shaft. Either of those seals are happy to let water into the tank if they're old, shrunken, or cracked. Being out in the weather like they are, they don't last that long.

Terminating the AD meant replacing the flush caps and their filler necks with an adapter to the smaller filler neck and raised caps with the big rubber gasket we're familiar with. Still not perfect, but better. The best systems I've seen use a sealed, hinged cover over the cap, and a drain from that cavity carries away any water that gets past the cover before it can find its way past the cap.
 
EdFred said:
They do. Just not on the planes you fly.
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Yup. Many airplanes have flush fuel caps, Diamond DA40s, Mooneys, Cirrus, Bonanza.

WAG: the lower end Cessna and Pipers don't because the flush ones have more moving parts (they have a locking tab) and and more expensive. It's a cost savings.
 
When a $8 gas cap costs $225, it makes sense. In the “real” world, the flush cap would only save $15.
 
The primary benefit that I see from the ‘handle’ on the gas cap sticking up on my Cardinal RG is that it makes it easy to see if the caps are properly aligned. If they aren’t lined up right they aren’t closed properly and some people need a step ladder or stool to reach them.

I can reach the cap from the ground and do a quick check to ensure they are tightly closed but it’s still nice to be able to see the alignment as I walk up to the plane.

Gary
 
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