Killing the Kneecapper

October 2022

The Incident

Kinda spooky when you think about it.

Put power on for takeoff after a touch-and-go...and someone taps you on the knee. Neat trick, for a single-seat airplane.

Then I moved my thumb down to push the carb heat knob in...and it ain't there.

Carb heat knob was mounted on a piece of 1/4" wood glued to the side of the #3 bulkhead, below the throttle. The wood had snapped off, leaving the carb heat knob dangling.

Continued the takeoff (situation wasn't an emergency, and didn't want the distraction) and as I climbed out, I did a syringe-type closure of the heat knob.

Bad news? Had to come up with a new mounting. Good news? Was having my condition inspection in a week, so was already going to disassemble the airplane.

Better news? I've always *hated* that mounting setup for the knob...knife of wood sawing into my knee.  I called it a "kneecapper."  And as this picture shows, I originally had two of them.

Why was it like this?  Who knows.  Let's review what Pete says to do in the original EAA magazine instructions.  In this diagram, I've highlighted the carb heat knob (red), the mixture (light blue) and the primer (yellow).

One would assume that Pete built N500F this way.  The trouble is, by the time *I* was flying it, the carb heat and the mixture controls had switched places!  Had Pete built it that way all along...or had it been swapped during the plane's restoration in 1982?

Could be either way.

The one sort of marginal thing here is the location Pete shows for the mixture:  It's *right* on the edge of the inner curve of the STA 3 bulkhead. Take a look at this close-up of my cockpit sidewall.

Notice the carb heat on its Kneecapper support...and the relative position of the diagonal on the fuselage side.  Obviously, if the carb heat had been positioned on the STA 3 bulkhead, there would have been a chance that the fuselage diagonal wouldn't have let the knob come out all the way.

Apparently, you CAN position the knob successfully there...probably if you put it *right* on the edge of the bulkhead.  But the builder of my plane didn't like the looks of it, and installed the carb head control on a 1/4" plywood extension glued to the STA 3 bulkhead.

Note, also, the Very Comfy (tm) kneepad just below the carb heat control.  The previous owner of my plane had been about eight inches shorter than I; the pads were probably ideally located for him.  However, with my towering six feet of height, my knees hit the damn plywood support, instead.  This was very uncomfortable.  And, of course, it was like that on BOTH sides, with the cabin heat control on the opposite side (albeit on the other side of the bulkhead).  I had previously removed this one, moving the control to the bulkhead supporting the fuel tank.

Even worse, the two kneecapper plates had been installed with sharp edges pointing inward.  One of the first things I did after buying the airplane was to carve a curve into the inward-facing sides of both you can see in the above photo.

The Repair

So, how to fix it.

I'd like to have put it smack dab in the same location N500F had it.  But...was concerned about clearance (like the original builder) and I would have had to strip the entire old plate off the bulkhead to have any chance of making it work.  Plus I'd have to drill the new holes with the diagonal potentially getting in the way.

Move it to the fuel tank support bulkhead, like I did the cabin heat control?  Would work...but the area itself is out of sight from the pilot seat.  The carburetor heat control is pretty important on a Continental; I didn't want to have to fumble under the panel to work it.  Cabin heat gets turned on in the fall, and off in the summer.  Doesn't get a lot of work, doesn't hurt it it's out of sight.

So the ideal solution would be to move it to the lower edge of the top side of the Station 3 bulkhead, alongside the mixture and the primer.

Two problems there.  First, there's not really a good spot for it.  If you look at the first image on this web page, if I were to install the control equally spaced with the others, it'd be nearly center in the cockpit...and a good foot from the throttle.  That's bad cosmetics, and bad human factors.  On a Continental, the carb heat knob should be turned to "hot" before ANY power reduction.  Better to have it close to the throttle.

The other factor is the difficulty in making a new mounting location on the STA 3 bulkhead of a completed aircraft.  The control itself is designed to go into a ~1/2" thick panel, yet the STA 3 bulkhead is a sandwich that's about an inch wide.  Just drilling a hole for the control won't work, one has to go behind the bulkhead and rout out the back of the bulkhead, as the diagram to the right shows.  And that's just too tough to do, with all the structure, wiring, and piping of a completed aircraft.

The solution:  Install the carb heat knob where the mixture control is located.

What to do with the mixture?  Remove it entirely.

Now, some of you are probably going through the roof.  "You need it to kill the engine!"  "You need it to optimize the mixture for altitude!"

Allow me to point out that my aircraft has a Stromberg carburetor.  They were pretty much standard equipment on Cubs and Champs.

And on these airplanes, the mixture control was an extra-cost that not every purchaser chose to have.


It's pretty simple:  The Stromberg actually doesn't adjust very far.  In fact, one cannot shut down the engine using the doesn't lean it out enough!

Altitude-wise, the mixture on a Stromberg doesn't start to work until one gets above, say, 5,000 feet.  And, despite living in the shadow of the Cascade and Olympic mountains (on clear days, I can see three volcanos from pattern altitude), I rarely fly much above 3,000 feet.

So, I removed the mixture control, and ran the carb heat control in its stead.  My A&P would be coming by to perform the Condition Inspection, and I had him wire the Stromberg full rich.

Hardest part was getting the nut started to attach the control itself.  As I mentioned, the mounting location is recessed due to the thickness of the bulkhead.  I slide a socket over the sheath for the carb heat, holding the nut, slid it up to the mounting location, and tried to get the nut engaged on the threads.  The trouble was, the socket was too deep to actually shove the nut against the threads so it could attach.

My solution is on the right... a short piece of tubing between the socket and the nut to shove the nut hard into the threads.


The hardest part is getting used to the new carb heat location.  My airplane's was located below the throttle, in approximately the same location N500F had it.  Was pretty darn used that location, and the new location threw me off.  I'd reach down for the carb heat and find myself caressing my left knee.

Which was resting, quite comfortably, on the Very Comfy (tm) kneepad.

It's been a couple of months, and I'm starting to get used to it.  It's too bad I had to do the change, but I'm happy with it, now.

Ron Wanttaja