Thorp Air Command - T18.net

Not to worry, Lee, I understand the scope of work. It might be a winter project next year, but I have to do the right thing. What I have now is only marginally better than no filter at all, and that can't be good for the engine.

The seal (boot?) is drawing 751, which happens to be missing from my set. I'd love to have a scan if anyone can help me.

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In theory, theory and practice are the same. In practice, they are not.

Hey guys,

The weather is gettin better now and one of my objectives before next winter is to get fiberglass work done, including induction on my 4-piece cowling. I just read through 23 pages on the subject of induction in the archives so I'm geared up with questions. Quite a few comments were made about inefficient intakes on cowlings and I'd like to know what's the right criteria? Surely there are rules of thumb for area, length, radius, etc. Do filters render ram-air useless? I'm using an MA4 carb on an O-320, and probably avoiding the banjo box, just for its size. I also understand the danger of sharp edges, creating turbulent flow, etc. It seems a person could get hurt doing the wrong thing, but maybe it's not too complicated. There's a lot out there.

The Lieder-style airbox looks really impressive and I always wondered if there were plans for those or at least a few dimensions to work with. On the other hand, I kinda like the idea of a smooth cowl without a scoop. From what I read, I didn't notice anything about carbureted systems running off of the rear engine baffles. Is that a bad idea, or is there that much to be gained by ram air? The Lieder style doesn't seem to utilize pressure recovery, but neither does it have a smooth elbow or turning vanes leading into the throat. Off of the baffle it seems would allow more lattitude.

A related topic I ran in to was cabin-air intake. I decided not to collect air or even run ductwork from the engine compartment. Mine comes in from the fuselage side, just in front of the main spar, with ducts in the wings. The idea was to guarantee fresh air at any time, when otherwise there might be a possibility of fire, fumes, or carbon-monoxide. That's just a personal choice, something to think about.

Thanks for your help.
Les Krumel,
Albuquerque

I ran my air intake off the right back cylinder baffle. Power does not seem to be effected. Some Pipers use this setup. Most installs are using a HA6 carb or FI. Some builders have used Van's airbox setup.
Fresh air is best when taken from a point away from engine heat. I agree that a big hole in the firewall might not be desirable. I have air intakes 6" inboard of the wing gap (S-18) and eyeball vents in the dash, you can not unfold a chart with them on.

Les,
There are drawings floating around for the Lieder box, I should have some around here somewhere. Dad and I used to prefab them for Ken Knowles. I still have the form blocks.

My airplane, like Bills used to run off the aft baffle, I liked that a lot, one of the big advantages is the ability to pull the cowl without disconnecting anything. Fraser has a standard updraft carb with the air box facing aft then running to an aft baffle pickup.

I will say though people swear by the Vans box, I have one in the garage that I pulled off a banged up Rv6. Looks like a pretty clean setup, look at Gary greens airplane, he has the RV box and scoop ( as does Joe G and Paul K) it seems to adapt to the thorp pretty nicely. Not nearly as offensive in size we normally see on our airplanes.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Les - as Lee says, I am picking up air from the driver's side rear baffle - 3" SCAT to the air box - I am 66% complete with the baffling - The previous set up was running unfiltered air - I am going to build a square box on the rear baffling and install a filter - same one they use on the RV-8 - the filter is 6" by 8" (I think) and it will be a tight fit, but so are some of my clothes, and I make them work.
The rear induction off the back baffling ran great for the 5 hours I flew it - my only criticism is it really clutters up the sparse area aft of the engine and She Who Eye Wership HATES clutter, but scat tube is easy to R&R. If I were redesigning from scratch, I would consider an elbow at the airbox and bringing in the air from the side, making for a little more area between the air box and the firewall. I remember a discussion a few years ago about 90 deg induction elbows. I thought it was when Col Highley was putting in the new squirrel cage.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

Efficiency, looks, style, speed, access, complications, robbing Peter to pay Paul (cylinder cooling). Tom Hunter provides a very nice induction system modeled after the RV. Ram Air or unfiltered Ram Air - I think Mooney believes in it. I personally went for efficiency, looks, access, and less complications. I previously had induction air off the rear baffle into a filter canister then into rear facing FI. I now have a NACA duct on the bottom of the cowl into a filter (K&N) airbox on the rear facing FI. Studies indicate NACA ducts are efficient for applications that are free flowing. Aperture needs to be equal to throat of TB plus induction losses from filter and induction design.
If you go with a ram design the radius of the lip is important in velocity/mass/turbulence and parasitic drag. Equally important if you grab air off the rear baffle, use a radius intake lip (bell mouth), not a hole and flange. FWIW you might want to examine the numbers for the CI at various RPMs of air used for induction displaced from cooling cylinders then calculate how efficient your cowl intake and outlet is just to assure yourself.

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[b]Terry[/b]

Well guys, thanks again for responses on induction. Looks like I'll be doing fiberglass work on the cowl sometime this summer. That should be a sticky mess.

First I'll be building the trailer for it this next couple of weeks. Just bought 700 lbs. of steel, and you bet - it's not for plane parts.

Gettin closer,
Les.

Frasier, others,

I'm thinking again about taking induction air off of the RR baffle as you mentioned a while back. Mine is an O-320 with oil cooler on LR baffle, and from what I've heard the openings on the cheeks are plenty big. I'm workin out how to add a carb heat damper and wondered also what others have done.

I heard of an RV that was lost when the intake tube collapsed from vacuum; I guess size matters. Don't have numbers for friction or losses for the filter, but it seems that 3" SCAT is the going size. Do I figure right that the engine uses about 4 cu-ft/s at max rpm? Looking forward to someday get it running and do some run ups to test my losses.

If you have an updraft carb did you add a blister to fair over it on the bottom cowl? A lot of FI jobs have that nice profile w/o intake on the bottom cowl and it may save a little drag. Don't see too many like that with carbs.

Les Krumel,
Albuquerque

Hi Les,

Here's a detail that some of the alternative engine guys (uncommon
intake locations) have learned: SCAT tubing is *very* draggy. If you use
very much of it at all, there's a significant pressure loss in the
tubing. To reinforce the evidence that this might be true, I'd look at
what Van did for the forward facing induction engines on the RV's.
There's a fiberglass duct shaped to marry the air filter (in the bottom
of one cooling intake duct) to the throttle body intake. It would have
been much simpler to use SCAT, but he didn't. Hard to see how it works
from this pic, but here's the link to the kit:
> http://www.vansaircraft.com/cgi-bin/cat ... horizontal
> ;

Charlie

*** admin posted for Charlie England ***

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Forum Administrator

Have you looked into the "smooth" silicone intake hose the race car guys use ? Would eliminate the "drag" inside the tube. Not sure about the weight/cost ?

RB

Les,
The air coming off the back of the baffle is already pretty darn turbulent, I doubt the SCAT/drag concern here is worth the effort, it still has to go through a banjo box or some sort of filter and then to the air box, so there's plenty of things more "draggy" than the SCAT tube. As far as carb heat goes you can still pull off the crossover for heat, then have the fresh air come in from the aft side of the box, this would still require a bump on the cowl but not near as much as a standard updraft setup.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

... And as a reminder, we want to use skeet not scat on intake systems. Skeet is the double layer stuff.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Les - I'm not ignoring you - am taking wing tomorrow and will get some pics posted and an eval of the intake performance, etc within a week.

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Fraser MacPhee
N926WM
Serial #279-1
Angel Fire, NM (KAXX)

RAm air induction via the Mooney was considered, but discarded since I did not want unfiltered air going into my expensive Lycon FI engine.What I finally settled on after earlier attempts at picking up air inside the cowl from the right baffle was a plenum chamber an airscoop combination with a high flow K&N Filter. Several T-18's are flying with this system as well as an RV-6. It does involve some fabrication and cutting of your lower cowl but it offers some distinct advantages over the RV induction system. It is much easier to install. And it employs a direct path into the FI servo, so air is not forced to follow a meandering path with loss of energy or distruptions by scat causing turbulent airflow. Therefore it is much more efficient. see www.finetuneflight.com for pictures and write up.

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Tom Hunter
805-202-4261

You know how it is, Frase:

Anyone who's taking a test flight has the support of all of us, spoken or not. We're all rootin for ya. You're kinda keeping the fun part of 'experimental' in all of it.

Good luck. Be careful,
Les.