The small cabin Citations from the FanJet 500 to the CJ3 (but not CJ4 or Mustang) typically have a main entry door that is tapered to be narrower at the top than the bottom. Here's a picture that shows the taper somewhat:

I couldn't figure it out, but I eventually found someone who knew why it was done that way and the answer is surprising and enlightening.

Anybody want to guess?

Mike C.

Probably one of the most complex, strongest , expensive part of the airplane. Tapering is less area and therefore less material and less expensive to build. Same reason coke cans and the like are tapered at the top. Bwtfdik?

Just large enough that the seats will fit for removal?

The illusion.door open, passenger steps on the narrower part and it gets BIGGER on the way up. They hardly notice the smallish opening at the top, as they scrape their ears into this HUGE tube.

The double taper is more attractive

Uses slightly less bondo on the top than the bottom to allow 2 extra gallons of Jet A? (if you've ever seen one of these getting painted, you will appreciate how sloppy these doors fit.

Hint #1:

It isn't cosmetic.

Mike C.

I like the theory of getting the seats in and out...

Mike,

I think it has to do with planning for overpressurizing cockpit? Bottom end would blow out first. Lifting it over wing. It is a real stretch. Right into the engine.

Looks like the door would interfere with the structure around the passenger window if it weren't tapered at the top.

At the top, you have the structural issue of containing and supporting the window. You don’t have that at the bottom. Having the door edges closer let’s you put vertical support structure that works well to stabilize the door edges and the window because it’s close to both.

Hint #2:

Not related to interior or seats.

Mike C.

Cross-promotion with Queen’s 1978 release of “Fat Bottomed Girls”?

Hmm, does it have anything to do with pressure distribution and fuselage cross section/shape?