How to attach an engine to an ultralight airplane

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NOTE:  This post describes the process for mounting a Hirth F33 (28HP) engine to a Belite Ultralight aircraft.  We support several different types of engines, all are similar in mounting techniques.  Call us (or the engine vendor) if you need installation information for any other type of engine.

The F33 is the exact engine that was used on the MythBusters Duct Tape Plane episode, which aired in October of 2011.  :-)

It provides awesome performance in our base aircraft.  This is due to the many aerodynamic improvements we've made to this airplane and our lightweight construction.  It is easily possible to fly a Belite with less than 210 pounds all up weight, including the engine!!

Let's get on with our engine installation...

Section A.  Firewall Assembly

Cut the firewall to the pattern supplied in our blueprint data.  We recommend using either 0.032 or 0.040 aluminum.   (If you don’t have the blueprint PDF for the Firewall – email us!  We will send it to you.) 

Fabricate the firewall from the aluminum sheet using the blueprint PDF template sheet. Cut the template sheet to size with scissors and then use spray adhesive on the template sheet to temporarily attach it to the aluminum sheet. Cut the aluminum with metal snips. Mark the templates' top contour on the sheet metal with a felt tip pen, but cut the top contour a little long (1/2" or so) in case your cowling does not fit up exactly as ours did. Any extra material will get trimmed away during the cowling fit-up sequences to come.

Using a brake, bend the bottom lip to fit under the fuselage, and drill holes at the engine mount locations.

Deburr all edges.

Section B. Engine Mount Assembly

Figure 1 -- Completed F33 Engine Installation in Belite ultralight airplane.

Hirth F23 twin has a redundant cylinder

After reading this, some of you will shoot me down.  It's OK -- I'm vowing to share what I know, no matter how embarrassing.

On this two cylinder engine, one cylinder is optional.

It seems like months ago -- but in fact, it was only two weeks ago:  we had two days of glorious weather.

Our airport was alive with activity.  72 degree high temperatures (in January) brought out one helicopter, some gliders, along with their towplane, our ultralight, and a buzzing crowd of powered paragliders.

My crew was working hard on getting another plane ready for delivery; we'd just swapped a brand new F23 Hirth twin boxer into one of our Belites.  (This particular plane is headed down to Texas and Florida to be a dealer demonstrator on floats... stuff for another post...)  The F23 had already been running for a couple of hours, as we slowly cranked up the power and prepared the engine for first flight.

(Another person was performing the extended runup).

I did a careful preflight, ensuring that everything was OK.  The airframe had already flown, (briefly), but it would be a first flight for this particular brand new engine.

The electric start on the engine kicked it to life very easily.  A check on the 4 point safety harness, and I taxiied out to the active.

A quick runup, and I was accelerating down the runway -- albeit slowly.  I shut it down and taxied back to the hangar.  Realizing that power was low, I quickly (and incorrectly) concluded that the propeller had the wrong pitch.  I had Gene fetch another with a better bite from our inventory of props, and a few minutes later it was torqued down and ready to go.

I taxied out again.  I applied full throttle, and the engine powered up smoothly.   Something was still wrong; RPM was quite low (at 800 RPM below full power); but the bird still wanted to fly, and I did to.  So I let it take off.

I made a couple of circuits of the field, enjoying the perfect weather, and also enjoying harassing the powered paragliders that were hanging around the east end of the pattern.  I sure wasn't happy with the power, though - it just didn't have nearly the snap I'd come to expect from our F33 installations in our Superlite models.

I landed the bird and taxied in.  As the bird rolled to a stop, I told Gene I still thought we had a real problem with the engine.

He pulled the top off each carburetor in turn, inspecting the linkage.

Well, it turns out one of the ball swages on the twin carburetor cables had failed (they'd passed a pull test in the vise!) and one of the two carbs wasn't advancing past idle.

So I'd been flying with one cylinder.  With a F23 Hirth, the second cylinder is not necessary for a good flight.