Thorp Air Command - T18.net

Guys,

I was reading Lee's comments on the speed thread and it was mentioned that he worked on tightening up the baffles and the cowl where the exhaust was to gain 10-15 mph. I was wondering if any one else has modified their cowls with respect to air inlet area, shape, etc? Also is there much difference between the various cowl types with respect to speed capabilities? My plane is down for the winter and this would be the time to make changes.

Your thoughts/ suggestions??( I have the Ratray cowl)

Jim Mantyla

There is one heck of a difference.. the ratray cowl is a pice of s*^$!!! I have it on mt project, and the first thing I did was cut the fiberglass flange area out and radius the inlets.. you can minimalize it 20% for drag effect and gain a good 5 mph... suggest change your baffling if its old as well... I'll post a few pics of mine... getting close to hanging the wings for good and taking it to the airport!!

Jeff Maynard
SN 510 N118JM

I was wondering how the classic t-18 cowling stacks up as far as drag, and what the current cowling options are (Im trying to decide what to put on my airplane).

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Quincy Zlotnick
Sebastopol CA
plans #576

Hey Jeff,

Can you explain better what you did with your cowl. I can't seem to visualize what you did from your description.

Thanks,

Jim Mantyla

Hey guys...

Which cowling do I have? Can you tell from this photo? I know it is fiberglass.

Tim A.

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The Rattray cowling bottom half is a sucker to get off without pulling the prop. I cut the sides out of the bottom half and hinged it to the top half which makes it easy to get at the bottom plugs and accessory section but I still have to buckle the bottom half to get it past the prop spinner. If I had it to do over I would cut the side down t the landing gear strut opening.

I haven't been all that happy with my cowl & scoop after looking at some of the cowls in this topic.
My scoop is crooked and the trailing edge is much closer to the left exhaust pipes that to the right.
Is this supposed to be this way?
I don't get very good manifold pressure at high speeds because the air filter is too thick and too small. Also it's right behind the inlet, so no deceleration/compression can occur.
It looks like the scoop just barely fits over a stock Piper Cherokee air inlet? (not sure)
Can anyone identify my cowl & scoop? It's pretty old & wiggly fiberglass, which has some bulges at the left cheek top, and right cheek forward & low. Big Drag there.

Not sure what cowl and scoop you have, but it appears the scoop/bottom cowl was not mated properly to the carb. My scoop is offset to the carb which is NOT bolted to the center line of the engine. By offset I mean the entire scoop is displaced several inches to fit over the carb and place the entire scoop in line with bottom cowl. I also noticed the cowl cheeks may need a few more fastners ?

The attched pic shows my scoop and the offset.

RB

Looks like the standard Glass cowling don't it Rich? That scoop has been around for many years I know CSA has those scoops, and also one that is real shallow and that's the one I used. Is the air rammed Chief or does it just blow in the scoop and and onto the filter.....Dan

I would say the air is not compressed by ram effect.
There is no divergent duct, and the filter is so close to the front that I don't think it would be possible.
The carb air box is a close fit inside the scoop. Very little room for an air filter.
Bruce Finney has an all aluminum welded cowl. What a beauty. His air scoop is the old style, not very smooth (no compound curves) but has a huge Buik air filter element inside.
Thanks for posting a pic of your (very) red cowl.
I agree that more fasteners would be better. I'll look into adding a few, and reglassing some of the worst areas.
Now I have to think about that crooked scoop...

@SHIPCHIEF The scoop is not necessary "Crooked" but placed inline with the carb airbox. As you know the motor is canted 3 deg to the right. It looks like the original builder just bolted the Piper airbox on and aligned the scoop with the airbox.

If you want to make some work for yourself the vans rv air box and intake scoop is pretty darn good. Much like the original Thorp box (like Bfinney has) but much cleaner aerodynamically while still benefitting from the large filter.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

I will add a second vote for the Van's airbox and scoop, having put them on a several planes now (Tailwind, Thorp and Pitts S1), and I am sure that Gary Green can say the same. If you buy the Ellison version of the FAB (the airbox with filter kit) from Vans, it does not have the opening for the carb cut out, so you can cut the opening for the carb and offset the airbox some to compensate for the offset of the carburator (which is offset 1) by the location on the oil pan by Lycoming, and 2) by the engine offset built into the mount.) The total offset is about 1" as I vaguely recall, and I was able to take most or all of that out when mounting the airbox. However, if you look closely at Van's airscoops, they bulge out of the left side some which is how the RV's do it (I don't think there is engine offset in their mounts). Anyway, you can get the scoop to be exactly on the centerline, which does improve the appearance a bit. I used the scoop for the RV6/7, which is for a flatter bottomed cowl and joined up to my Thorp cowling quite well with just some trimming away of the rear air exit portion. FWIW.....

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Dave M
Sacramento
ex-T-18 owner

I also used the RV air box and 0320 scoop.
The box inlet angle had to changed.
The bottom cowl can be removed with not problem

forgot this picture

Box is off-set, but scoop has room to be centered.