Hacking a Vetterman Subcowl

I got tired of all the back and forth misshaping the area around the exhausts, so finally just taped it into place and added expanding foam. I also extended the foam forward to the gap in the cowl behind the front gear leg, I plan to put milspec 1/4 turn fasteners there to suck up both of those edges. I intended the center portion of the extended forward part of this piece to be flush with the outside cowling and the more lateral portion to be flush with the inside of the cowl. You can see where I taped across this opening to give the foam a place to sit.




More soon.

DanH I am not, but I am learning...

Hans

Hans,
Any update on this? Thanks

subcowl fairing kit?

After reading Larry's results on this, doesn't everyone want one of these?
an Aluminum one would be sweet, but I wouldn't turn down a fibreglass one either....just don't want to try fibreglassing upside down while laying in the dirt at my tie-down, somehow just doesn't spell s-u-c-c-e-s-s.

so when someone is keen to produce a batch, count me in!

Looks like a half a wheel pant might be a good place to start from.......

pV^2/2?

I'd be a bit concerned about cooling air flow under worst-case conditions. Larry's data looks pretty good in cruise, but what about a long full-power climbout on a hot day, or a two-mile taxi downwind? I'd want to see some data on that.

temp control

No doubt, Bob, there's gonna be some need for temp control, when closing up the exit like this. IF not a 'cowl flap' of some kind, perhaps inside the louvers that Larry did, or just truncate the tail end of the ramp, so there's always another opening for the air to exit...it'll still be far smoother than dumping out with the exhaust.
..which brings to mind....do you need to extend the exhaust tips a bit to take some heat away from the new fairing?

Low or High Pressure Exits?

flytoboat said:

Hans,
Any update on this? Thanks

Click to expand...

Not yet, I have been doing some other things, but it is a little further along. I am going to build a water manometer like this to measure pressures inside and outside the cowl to determine placement of the louvers. You hot glue sponge to the ends of the tubing to keep from measuring speed:

Then put some food coloring in the water, and take digital pictures in different flight regimes


I have been trying to learn some aerodynamics, and I have a question that I need help with. When I measure pressures inside and around the cowl to determine best louver placement, should I go for low or high external pressure? Obviously low pressure will exit more air out the cowl, but low pressure outside the cowl means faster air, so putting slow exit air into the fastest flow will be draggier than exiting it into a slower flow, right? Ideal might be low pressure in a steep climb and higher pressure in cruise.

I wonder how a bunch #40 holes drilled into the cowl at a angle would work?

When I taxi in serious Texas-style heat I feel the need for a controllable flap on top of the cowl as my engine sputters from the fuel boiling in the injector lines. I was stuck on the control scheme for one of these, considering how often the top cowl comes off, but I just realized you could actually attached it to the firewall and make it part of the plane, cutting the upper cowl to fit it.

I have also wondered what to do with the exhaust tips, should I cut them in a line with the subcowl?

Thoughts?

Hans

nucleus said:

When I measure pressures inside and around the cowl to determine best louver placement, should I go for low or high external pressure?

Click to expand...

Low, as it increases mass flow (cooling capacity) and exit velocity (decreased drag).

Ideal might be low pressure in a steep climb and higher pressure in cruise.

Click to expand...

No. A local pressure increase outside the exit would indeed decrease mass flow, but it would also decrease exit velocity and increase drag. For cruise you want to decrease mass flow while further increasing exit velocity. The usual approach is variable exit area.

Sub Cowl

I have built a couple of these the first one had large louvers. It cost us 5 kts
so I went back and built one with no louvers, to set a base point. but here in North TX I may have to build cowl flaps before I can fly it. I will keep everyone updated as I go.

DanH said:

Low, as it increases mass flow (cooling capacity) and exit velocity (decreased drag).



No. A local pressure increase outside the exit would indeed decrease mass flow, but it would also decrease exit velocity and increase drag. For cruise you want to decrease mass flow while further increasing exit velocity. The usual approach is variable exit area.

Click to expand...

Is your airplane at the hangar yet Dan? I'm really interested in seeing some of your mods. Canopy and cooling to be specific.

Sid Lambert said:

Is your airplane at the hangar yet Dan? I'm really interested in seeing some of your mods. Canopy and cooling to be specific.

Click to expand...

Planning to haul the fuselage to the hangar in a week or two. With the booth clear I can paint the wings, then it's time for final assembly.