TOOBUILDER question on plenums ?

Larry DeCamp · #1 05-23-2019, 06:12 AM

There are many comments regarding all the screws required to secure a plenum. Toolbuilder has referenced the use of piano hinge. This is very appealing.
The metal baffle wall component will probably need to curve a little. Is there a better way to curve a hinge than drilling and notching ?
Is there a better way to secure plenum than screws,1/4 turns or piano hinge ?

BillL · #2 05-23-2019, 06:27 AM

Quote:

Originally Posted by Larry DeCamp

There are many comments regarding all the screws required to secure a plenum. Toolbuilder has referenced the use of piano hinge. This is very appealing.
The metal baffle wall component will probably need to curve a little. Is there a better way to curve a hinge than drilling and notching ?
Is there a better way to secure plenum than screws,1/4 turns or piano hinge ?

I use #6 screws and nut plates spaced a little less than 3". I took a piece of fiberglass and screwed it to a wood block with varied spacing then pressurized the back side to determine the spacing. With a proper screw gun it does not take very long to remove it. It is 10 minutes, not 10 seconds. YMMV

Clint B · #3 05-23-2019, 07:32 AM

Hi Larry, I sent you a PM

Toobuilder · #4 05-23-2019, 08:11 AM

Concerning the curved hinges - note that only my rear wall is curved (and only the center 1/3). The front walls are straight (just worked out that way on my airplane)

...but no matter, the hinges lie flat anyway so they will follow a curve without modification. The "L" angles (.016 titanium in this case) on the front and back walls stiffen things up a bunch and provide a rudementary seal surface. The stiffness helps when installing the lid as even the relatively short side walls need a bit of "steering" to keep them in line when the hinge pin is sliding home.

Here's another view of the straight plenum end wall. And yes, the inlet is rigidly mounted on the wall. The inlet bell sits in front of the cowl and moves with the engine.

DanH · #5 05-23-2019, 09:17 AM

Quote:

Originally Posted by Toobuilder

And yes, the inlet is rigidly mounted on the wall. The inlet bell sits in front of the cowl and moves with the engine.

Somebody was studying a certain Boomerang shaped airplane....

Toobuilder · #6 05-23-2019, 09:35 AM

...well I do live right down the street.

DanH · #7 05-23-2019, 10:56 AM

Always thought that inlet structure could be the optimum for high Vi/Vo, as the diffuser shape be be fixed/perfect, not disturbed by internal couplers and torque misalignment.

scsmith · #8 05-23-2019, 11:37 AM

Definitely an advantage for internal aerodynamics, ease of fabrication and assembly.

A little bit of motor-mount sag and the inlet rings will conspicuously not match the cowl very well.

David Paule · #9 05-23-2019, 12:23 PM

Steve, if that happens, given the shape of the cowl in the vicinity and aft, would that have an adverse effect?

I see it as having no effect on engine or oil temperatures, assuming the oil cooler gets its air from the plenum, and having almost a zero change in airplane drag. But you'd have a better idea about that than I would.

Dave

Toobuilder · #10 05-23-2019, 02:34 PM

Engine mount sag can be shimmed as required to maintain alignment (just like most of us do to keep the spinner aligned). The degree to which any misalignment gets offensive depends on the treatment directly behind the rings. If you expect it to fair absolutely perfectly and look like one piece, it's going to be a mess. Treat them like two separate pieces and it gets easier. My cowl was built with much bigger rings in mind so I have a bunch of sanding and rework at the transition, but even in rough form it looks pretty good.

One other benefit to rigid inlets is that you can extend them far forward. Mine are breathtakingly close to the TE of the prop, but wont hit no matter how much the engine shakes around.