D

dustman

Well Known Member
Here is one for the experts, Is it necessary to have a baffle seal where the air box meets the cowling? I have about a ? of clearance between the air box and the cowl. Why wouldn?t a bead of silicon at the junction of the two surfaces work, if a seal is needed? I?m going to put the baffle seal in the old fashion way but was curious about other alternatives.
 
There is a lot of relative motion between the airbox and the cowl - I think that you need the flexibility of baffle seals to let things move around.

Paul
 
And you'll probably want more clearance or getting the cowling on and off is going to be a pain. I've got about 3/8" and am thinking of trimming the cowl a bit more.
 
Here is a trick to help...

flion said:
And you'll probably want more clearance or getting the cowling on and off is going to be a pain. I've got about 3/8" and am thinking of trimming the cowl a bit more.
Click to expand...

Mount your baffling material on the sides and bottom of the snoot only, wrapped around like a "U". Attach a straight piece to the Air Box to seal the top. This makes it much easier to put the Cowl on and off.
 
What is the best attachment method to get the material fastened to the snoot?

JonJay said:
Mount your baffling material on the sides and bottom of the snoot only, wrapped around like a "U". Attach a straight piece to the Air Box to seal the top. This makes it much easier to put the Cowl on and off.
Click to expand...
 
Varies....

Brantel said:
What is the best attachment method to get the material fastened to the snoot?
Click to expand...

I believe the plans call for large washers and pop rivets similar to the Engine Baffle and Cooling Ramp. I used screws with a nylon washer, large washers on the back side with locknuts. I used pop rivets on the Air Box piece.
 
Bolts and washers....Ok.

I doubt one could get a pop rivet gun in there so that must be the way to go....
 
Not if you want to follow JonJay's advice.......

His idea is to have a U shaped piece on the cowl snout and a flat piece on the airbox top. When together they makeup the seal. This makes the cowl easy to lower down away from the airbox.

The plans and directions show the seal attached to the airbox but the smart people here at the VAF have deviated from the directions.
 
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<<The plans and directions show the seal attached to the airbox but the smart people here at the VAF have deviated from the directions.>>

Assume pressure at the intake opening of the lower cowl is higher than pressure inside the lower cowl (and it probably is). Why install a flap seal backwards so it leaks under pressure (loss of manifold inches), and raises lower cowl pressure (bad for cooling) in the bargain?
 
Is it really backwards?

DanH said:
<<The plans and directions show the seal attached to the airbox but the smart people here at the VAF have deviated from the directions.>>

Assume pressure at the intake opening of the lower cowl is higher than pressure inside the lower cowl (and it probably is). Why install a flap seal backwards so it leaks under pressure (loss of manifold inches), and raises lower cowl pressure (bad for cooling) in the bargain?
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Raise's good points but doesn't the Air Box get saturated with ram air? That pressure is going to go through the seal regardless of direction. One could argue that the factory method is a tighter seal, if you do it perfectly, but I am not to sure about that either.
I have a 3/8" gap between Air Box and Snoot. The baffle material is quite rigid and there is no way that Ram Air could get past the seal without saturating the airbox first, equalizing the pressure, and then the pressurized air is going to find a way out, probably through the seal, in either installation method. Makes sense to me anyway.
Not my original idea. It either came from here years ago or from an Rvator. Can't remember.... It has worked well for me.
 
DanH said:
Assume pressure at the intake opening of the lower cowl is higher than pressure inside the lower cowl (and it probably is). Why install a flap seal backwards so it leaks under pressure (loss of manifold inches), and raises lower cowl pressure (bad for cooling) in the bargain?
Click to expand...



Not sure I follow you here seeing as how the seal in both methods mentioned above is still installed on the same side "outside" of the snout and airbox.

The stock method just creates a wrapped around piece of baffle material attached to the outside of the airbox and the snout slips into that pocket.

JonJay's method is exactly the same just split into two parts and the bottom part is connected to the snout extension and the top part is connected to the airbox.

I am sure someone has most likely installed it where the fabric sits to the inside of the airbox but I have never seen one done that way.
 
the way to reduce leakage

Following Dan's observation, the way to minimize leakage is to have the seals rest on the interior of the air box intake and the exterior of the cowl snout. This is equivalent to the way the seals go on the cowl cooling lip to baffle intake floor. The idea is that the higher pressure tends to make the seal 'seat' against the inside of the air box intake and reduce leakage.

Another method you might try - and here I'm not that familiar with the detailed geometry, so if it can't work, discard the suggestion - but mouse pad neoprene foam, or an old wet suit, can provide a somewhat stretchy rubber seal that you can make fit tight on the mating surfaces so it doesn't leak. In this case, I think you can get away with making a seal that stays on the exterior of the air box intake, because it is pre-tensioned (stretched) slightly onto the surfaces and seals pretty well. this works really well on round ram-air intakes.
 
Brantel said:
Not if you want to follow JonJay's advice.......

His idea is to have a U shaped piece on the cowl snout and a flat piece on the airbox top. When together they makeup the seal. This makes the cowl easy to lower down away from the airbox.

The plans and directions show the seal attached to the airbox but the smart people here at the VAF have deviated from the directions.
Click to expand...

Got it. I gotta read more carefully. Still, I wish there was a better way.
 
I think this is an area where you really have to play with your finished cowl and air box in order to understand the problem of sealing the intake versus the ability to get the cowl on and off. A seal attached to the cowl that went "inside" the air box would give the best seal, but probably be impossible to install over and over. Any really tight interface is probably going to rip itself apart with engine movement and cowl remove/replace cycles.

The good news is that you can easily re-do this part of the airplane once you have it flying, and can see how it works - then you can work on the "better mouse trap"....

Paul
 
baffle

Well so far the info has been good but I still have questions. "I have the older style cowl which is really not much more than a hole cut in the lower cowl. From reading the different comments i assume that i would attach the seal material to the airbox then the baffle material rest somewhere in or on the air inlet hole in the cowl. how should the baffle mterial contact the air inlet on the cowl?
 
I have left the seal completely off with a 3/8 inch gap. Flown the plane and checked the airspeed and manifold pressure all at WOT at various altitudes. Then gone back and sealed the air intake to the cowling. Test the plane again and found no changes in airspeed or MP. If there were any changes I couldn't measure them and maybe only a puritist could.
 
More work to do...

dustman said:
Well so far the info has been good but I still have questions. "I have the older style cowl which is really not much more than a hole cut in the lower cowl. From reading the different comments i assume that i would attach the seal material to the airbox then the baffle material rest somewhere in or on the air inlet hole in the cowl. how should the baffle mterial contact the air inlet on the cowl?
Click to expand...

You have to lay up fiberglas around the "hole" on the inside of the cowling to form the intake properly and match it up with the Airbox. When your done, you will have a "tube" extending from the "hole" to the Airbox. This is done with a foam block that comes with the FAB kit. The instructions are part of the FAB kit. It should make more sense then.
 
Not surprised

Norman CYYJ said:
I have left the seal completely off with a 3/8 inch gap. Flown the plane and checked the airspeed and manifold pressure all at WOT at various altitudes. Then gone back and sealed the air intake to the cowling. Test the plane again and found no changes in airspeed or MP. If there were any changes I couldn't measure them and maybe only a puritist could.
Click to expand...

I dont think the seals are capable of overcoming the pressure. If they are the Ram Air Pressure overcomes the Airbox Pressure as the engine reaches its maximum manifold pressure. You just cant cram anymore air in there. There are other posts on speed mods that discuss this area and what might be possible if a complete seal could be maintained but I think they just dead ended the idea.
I find it interesting that the vertical induction systems, carb or FI, have the snoot and get a lot of ram air, but the horizontal induction intake is taken off of the inlet ramp where there should be much less ram air in comparison.
Bottom line; I thnk we are blowing, (or sucking), more than enough air for these systems to be operating at near max efficiencies. There is a limit to manifold pressure and at some point it does not matter how much air or pressure you force down its throat. It can only take so much, so fast.
If all of this is true, you are not sacrificing anything by making the seal as easy as possible to get the Cowling off, maybe not even by elliminating it although I would not recommend that.
 
Dustman, you need to glass in a tube to mate the opening with the airbox. IIRC, the plans tell you to trim the airbox back a couple of inches from the opening. Then you glue in a supplied piece of foam board between the airbox and the cowling (glue to the cowl only). Fit the cowling in place and carve out the channel between the opening and the airbox. Now you have a mold to glass to; I used about three plies laid up diagonally. Once it sets, clean up the layup around the cowl opening and trim the aft end so that there is a gap between it and the airbox. Don't forget to remove the mold by either chipping it away or dissolving it with solvent. Now you can apply the baffle seal. I've got pictures here.
 
Dustman, another tip (I think on the plans but not sure) is to lay up the glass in the snout and blow up a balloon on the inside to squeeze the glass against the foam block. Not only does this keep the glass in place, it actually makes a pretty nice surface with not much sanding required. Blow up the balloon just enough to have a bit of extra bulb sticking out either end of the hole.

greg
 
Ironflight said:
A seal attached to the cowl that went "inside" the air box would give the best seal, but probably be impossible to install over and over. Any really tight interface is probably going to rip itself apart with engine movement and cowl remove/replace cycles.

The good news is that you can easily re-do this part of the airplane once you have it flying, and can see how it works - then you can work on the "better mouse trap"....

Paul
Click to expand...

How about this: Make a tube-shaped seal and attach it to the outside of the cowl scoop, tuck it forward into the scoop (inside-out), then install the cowl and flip it right-side-out into the airbox. It seems like this could give a very good seal if sized correctly and make for easy cowl R&R. I'm thinking a wetsuit sleeve could be adapted.
 
May work for you too.....

After struggling to get bottom cowl on and off. A 3 time RV builder showed me how to beat this struggle.

I made the scoop on the cowl detachable from the main bottom of the cowl. To do this simply draw a good line around the scoop, use a dremel cutoff wheel to cut the cowl, but leave areas (tabs) in place where scoop is not completely detached from main cowl at this point. Use mold release on inside of cowl on the scoop portion. Lay up fiberglass lip overlapping the cut line. Drill holes for final attachment. Finish cutting the tabs to detach cowl scoop. Install nutplates on main bottom portion of the cowl. Do Not fully detach cowl scoop before laying up glass and drilling

This mod works so well, fits like a glove. Saves much rash on cowl when installing removing. Also will pay dividends when changing oil, carb adjustments and inspections as major portion on the lower cowl stays in place. From an appearance stadpoint, unless you are under the plane, you can't tell that the cowl is two pieced. Really makes it easy to get a good fit on air intake.

Works so well for me. Your results may vary.
 
Chuck, great idea, and I really like the method. And Steve, I like the sleeve idea too.

Finish up the job with a good air-tight airbox...which I'll bet Chuck did.
 
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