Another alternate air thread

[Blain]

The idea of using a Robinson Helicopter style air door has intrigued me. Spring loaded automatic operation. No pilot activation required. In rotor craft both hands are busy in a power failure.

Some have dismissed the idea of having hardware inside the filtered area. I agree, however Frank Robinson wasn't concerned in his certified AC. I think the possibility of parts moving loose can be mitigated with liberal use of lock tite.

So here is my results after a order with Robinson. All factory parts for a R22 with 360. I know derated HP, so I closed up the opening by about 10% to compensate. Here is my assembly before cutting into the finished air box. Thought I would ProSeal the plate to the floor of the FAB. No extra hardware to come loose.

Really wrestling with this mod. FAB is done. If I get this wrong I'm building another

[Robin8er]

I could be wrong, but with derated horse power wouldnt you want a larger opening, not a smaller one? then you would have more surface area to pull on the same amount of spring tention.

I like this idea. New cessnas have the same thing, but I dont know how exactly they work other than they are a spring loaded door of some sort.

[Walt]

Looks very nice, but I'm not sure I would trust Loctite alone to retain the hardware

[RV8Squaz]

Could you reverse the orientation of the bolts so there's less stuff (nuts & washers) on the inside? All metal locknuts would negate the need for Loctite. I would avoid a cotter pin because the tails could break off.

[Blain]

Originally Posted by RV8Squaz

Could you reverse the orientation of the bolts so there's less stuff (nuts & washers) on the inside? All metal locknuts would negate the need for Loctite. I would avoid a cotter pin because the tails could break off.

I considered that but the way the cover floats on the guides would change. In this configuration the cover can tilt by probably 20 degrees based on the direction of airflow. I suspect there is a flow or pressure pattern that isn't constant around the perimeter. The cover and guides would need to be fixed together and float in the bottom plate. Wouldn't allow for that tilt. Changes Robinsons engineering.

[erich weaver]

Why the monster skid plate on the bottom? Seems like a much smaller ring would be just fine to act as a backing plate for the bolt heads.

erich

[Blain]

Originally Posted by Robin8er

I could be wrong, but with derated horse power wouldnt you want a larger opening, not a smaller one? then you would have more surface area to pull on the same amount of spring tention.

I like this idea. New cessnas have the same thing, but I dont know how exactly they work other than they are a spring loaded door of some sort.

This one kept me up last night...So the cover opens by pressure differential between outside or ambient air pressure and a lower pressure inside FAB.

Ambient at SL obviously 14 PSI (rounded).
Effective area of the opening is 7 square inches (rounded)
Spring pressure is about 7 lbs (again, rounded to keep the math simple)

That looks like a 1 PSI differential will start to open the door. Springs are progressive so that will increase proportionally.

Doesn't seem like much difference. But I guess factoring the ram air effect maybe there is always a greater than 1 PSI differential to keep the door closed?

Now I wonder about what the door would do in the case of pulling power back on the approach? Will the door be pulsing? Or will it close shut?

Will the engine produce enough "vacuum" with the prop wind milling to hold the door open?

In the R-22 you are always producing power even on approach (There are exceptions-but for the purposes of this discussion...)

So maybe, this isn't the best answer for our application?

[Blain]

Originally Posted by erich weaver

Why the monster skid plate on the bottom? Seems like a much smaller ring would be just fine to act as a backing plate for the bolt heads.

erich

"Skid plate" I like that. The fiberglass is a known fail point for the Vans install. My protrusion was a bit on the narrow side so a .040" plate was justified.

[BillL]

Interesting design. All metal locking nuts, jamb nut, and then pinch the end of the threads with vice grips to deform the end of the screw. Oh - and locktite might just get it done. Has the pressure drop vs flow been measured? Will inlet pulses have it dancing? Might do some experiments to see what it is doiing in phase I.

I abandoned my spring designs to go with a pull cable on my james mkIII inlet. I finally thought about a door with cam to allow adjustment of trigger then low force when it is open. The idea if instant opening is really appealing, but shall be delayed in favor of first flight.

[Tom Martin]

Is there room to put the springs under the box? This would lower the hardware count after the filter. Nut plates on the door covered in epoxy would almost eliminate the risk o loose hardware