Thorp Air Command - T18.net

Hey Guys
If you are interested updraft cowls I have posted some pics of mine to the gallery. It's my own design but was inspired by the cowl that was on Burt Rutan's Defiant pushme pullyou. I discussed with Burt who had had the engine mapped for temperature effect by Lycoming engineers who advised that the temperature gradient accross the cylinders which is normally 135 degrees F was only 35 degrees F. The cylinders should never suffer cracking! I liked the logic of cooling the engine from bottom to top since hot air rises! I think the big benefit is that all the hot bits run in cold air and the complete fuel system runs in cold air. After an hours flight I can put my hand on the elec fuel pump, carb, gascalator and lines and they are cold. The air continues to poor out the top of the cowl after stopping and you can feel a draft at the intake. Never have a heat soaked fuel system like conventional cowlings.
Tony S
ZK-VMS T18C

Tony, I love it!!! I thought JT's design was a good candidate, But I thought the air would exit out the side. Why was that not used?

Bob MO

The proper updraft cowl design would exit on the top. Garrison picked the sides on the t-18 due to the shape of the thorp fuselage. I could never figure out why he did not go the route that Tony did, in the end garrisons cowl forced the air to make a 360 deg turn, where Tony's makes a single 90 deg turn. Well done Tony!

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

Wow!!! Good job Tony!!! That looks real Sharp. What do ya have under that slick cowl? Looks like it would also improve on cooling drag. Where have you put the oil cooler? Again, Good Job! Dan

Glad you like it! The oil cooler is in the bottom of the lower cowl and simply has an exit. Inlet air is just drawn from the cowl which is pressurised. Bottom of cowl location is probably not the best as exit is into high pressure area but there was no other logical place to put it.

A few facts on the cowl. The inlet duct is a 11 degree divergent duct, the theory being you drop 155 Kts down to about 50 -60 Kts which raises pressure and makes it easier for air to turn corners without losses. I had to invert the intercylinder baffles and build an air dam around the engine to seal to the top cowl (standard cowl seals). During initial test flying I had to increase the outlet size about 20% to get the temperatures to where I wanted them. The outlet design is every bit as critical as the inlet and is convergent, the theory here is to speed the outlet air flow to as close to airspeed as possible.
The upper outlets were a problem because of the very nose down attitude of the cowl in level flight which probably causes a positive pressure on the top of the cowl. The theoretical best postion for outlets would be on the very forward of the cowl where the radius starts but that is actually forward of the front cylinders! Peter Garrison found this to be the best position as well in some of the articles he did on cowlings.
Ain't experimenting fun!
Tony
ZK-VMS T18C

Ah, the other clever thing about this arrangement is that at hight angles of attack there is a depression on the top of the cowl and this aids cooling flow at low speed and conversly at high speed the positive pressure on the top cowl chokes the outlet stopping over cooling on descent. No cowl flaps required. Cyl temps sit between 180 and 210 degrees C in all flight regimes.
Tony

Thanks Tony, keep us up to date if you have more info on this piece, was,it hard to build? Dan

Tony: I am always amazed when someone can do the math and figer out how things work. My math skills are on par with Jethro Bodine and we both shake hands when we both come up with the same answer...Nuaght from naught is naught ! Your set up is a thing of beauty !

RB

RB, maths!? I just listen and gather ideas. I'm a visual person and the idea looked logical to me so decided to try it. You will note that I can't show you that it works better than conventional cowls. It works and I like it. My engine is a low compression O-320 150 HP driving a Sensenich 66"x76" woody and flat out at 3000' 2700 rpm I see about 165 Kts.

The NACA ducts on the side panels are my fresh air inlets, boy they work well and I only have the panel vents typical of the overhead vents in airliners.
TS

A tip of the hat to the man with no math !

RB