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.

NOTE:  All bolts and other fasteners that are directly associated with the engine should be torqued according to the engine manufacturer's owner's manual.

1. Attach the engine mount to the fuselage using AN bolts and hardware.  Use fender washers between the engine mount and the firewall.  The resulting installation looks like this:

Figure 2 -- Engine Mount to Firewall on ultralight aircraft

2. Install the engine to the engine mount, utilizing the supplied engine rubber mounts, bolts and lock nuts.  The resulting attachment will look like this:

Figure 3 -- Rear of Rubber Bushing on Engine

Figure 4 -- Rubber Engine mount from side on Ultralight Aircraft

3.  Attach the muffler.  We use tangs and swaged cables, hanging the muffler from the bottom engine mounts locations.  It is critical that a cable failure will not cause the muffler to roll up into the firewall (although that is why the firewall is there.) 

Figure 5 -- Muffler

Section C. Installing Fuel and Control Line

NOTE:  Do not allow tubing or wires to chafe on fuselage.  Use standoffs built from scrap tubing and tie wraps.  The following pictures shows the use of standoffs and spare tubing to prevent chafing in several different locations:

Figure 6 -- Standoffs and anti-chafing techniques are visible in several places in this photo

1.  Drill the firewall for pass-thru grommets as shown in the illustrative pictures. Be sure to de-burr the holes after drilling. A uni-bit (or 'step drill') is the best tool for drilling good round holes in thin sheet metal.  You will need a minimum of four holes:  for electrical #1 (starter relay), electrical #2 (battery), throttle and also for fuel line.

2.  Run fuel line from the cockpit to the fuel pump.  We strongly recommend the use of approved fuel line, such as supplied in our kits or sold by Aircraft Spruce and by automotive supply stores. 

Put a hose clamp on the fuel supply line at the selector valve, and install the fuel filter just forward of the valve. Route the line from the fuel filter forward to the engine. The preferred routing is under the floorboard, around the left rudder pedal to the right center of the lower firewall through the grommet. Use a small piece of thin plastic tubing to chafe protect the fuel line where it passes under the fuselage tubing alongside the rudder pedal. Notch the floorboard as necessary. Be sure that the fuel line doesn't kink, chafe or pinch. Drill a small hole in the forward edge of the floorboard and tie-wrap the line at that point. Run the line through the grommet, pulling all the extra length through and leaving it hanging forward of the firewall.

CAUTION:  All hoses, cables and wires that attach to the engine must have enough slack to be free to move with the engine as it moves in its mounts, as well as secured against chafing due to vibration, as previously mentioned.

3.  Run electrical wires. We recommend using high quality electrical connectors, and soldering all termination points as well. 

At a minimum, you will need to make the following electrical connections:

a)     Battery Ground to firewall / airframe chassis.  (Use 6 or 8 Gauge, high temp rated wire.)

b)     Battery Positive to starter relay input.  (Use 6 or 8 Gauge, high temp rated wire.)

c)      Battery Positive to starter switch in panel.  (Use 12 Gauge.)

d)     Starter switch to starter relay.  (Use 12 Gauge.)

e)     Starter switch to ground.  (Use 6 or 8 Gauge, high temp rated wire.)

f)       Starter relay output to starter motor.  (Use 6 or 8 Gauge, high temp rated wire.)

g)     Starter motor to ground.  (Use 6 or 8 Gauge, high temp rated wire.)

h)     Kill wires to engine from kill switch in panel.  (Use 12, 14 or 16 Gauge, high temp rated wire.)

Figure 7 -- Starter Motor

Figure 8 -- Starter Relay

Figure 9 -- Battery Compartment

Figure 10 -- Ground Connection to engine

Figure 11 -- Starter Relay connections

4.  Run Throttle Cable.  We recommend crimping and soldering the carburetor end.

Position the throttle lever so that when the carburetor is at the idle position, the lever is aft and low enough to be clear of the door sill so that you don't snag the lever getting into and out of the aircraft. Ensure that the carburetor piston is at the idle position by pulling and releasing the cable several times, and then tighten the cable lock screw. Actuate the throttle lever and ensure that the system travels smoothly throughout its entire range.

Depending on your engine type, your engine may have a choke or a primer.  This particular installation has a throttle and a choke cable:

Figure 12 -- Throttle and choke cable

Figure 13 -- Throttle quadrant in ultralight aircraft

What’s missing in the above photo?  Answer:  the fuel filter.
 

Section D. Cowling Installation

NOTE:  Be slow and be careful.  Read everything about cowling fit-up before doing any trimming on the cowling or firewall top contour.  Get a second opinion from a friend who has done cowling installation before.

1. Always ensure that there is a large amount of clearance between the engine and the cowl. The engine and prop can move around quite a bit in flight and you don't want contact betweeen your spinning propeller and the stationary cowling.

2. Make a center line mark on the aft end of the cowling top as a visual aid to aligning the cowling. You can use the intersection of the two fuselage tubes above the instrument panel as a target for the center of the cowl.

3. Trim the top contour of the firewall so that the face of the cowling and the face of the prop drive pulley are as nearly parallel as possible (in side view). In other words, you will be lowering the aft end of the cowling (by trimming the firewall top contour) to make the front face of the cowl agree with the plane of the prop disc.  Once the contour has been correctly trimmed, remove slight amounts of material to allow for the installation of the cowling to the firewall.

4. Juggle all the parameters of the cowling fit until you are satisified that it is the best it can be. Usually you will not be able get everything to fit exactly right and the best fit will be a compromise. While fitting the cowl use spring clamps on the mounting tabs of the engine mount and fuselage to help hold the cowling in place.

5. Once the cowling has been fit, drill through the cowlings and the centers of the attach tabs and cleco the cowling in place. This is your last chance to make minor adjustments in the position of the cowling before drilling the full-sized screw holes.

6. Remove the cowling and enlarge the #30 pilot holes in the tabs to 3/16".  Install nutplates (not supplied) on the tabs using the following procedure: hold a nutplate on the OUTSIDE of a tab and start a screw into it from the inside; then use the nutplate as a drill guide for the two #40 rivet holes. Remove the nutplate and countersink the holes for the rivets. Squeeze or buck the rivets to install the nutplate on the INSIDE of the tab. Enlarge the pilot holes in the cowling to 3/16".  Alternatively, simply use AN3 bolts and nuts.

7. Trim the cowling as necessary to clear all components, including carburetor, muffler, etc.,  bearing in mind that the engine can move around quite a bit in operation. Use a rule of thumb of 3/4" clearance to the carb and 1-1/4" clearance to the exhaust system. Also use your artistic eye and trim the perimeter of the cowling as shown in the photos. After trimming, remove the cowling.

Engine Cowling on twin opposed installation

Figure 15 -- Engine cowling on inverted engine

Section E. Other Engine Considerations

1. You may also be passing other wires through the firewall – frequent examples include tachometer, cylinder head temperature gauge probe (CHT), exhaust gas temperature gauge probe (EGT), in addition to wires you have already run. In addition, ensure that the engine is grounded to the airframe, as is the engine kill switch.

2. The CHT lead is designed to install under the spark plug where it can make tight thermal contact with the cylinder head.  Remove the spark plug, install the ring probe, and then re-torque the plug. After mounting the CHT probe to the spark plug run the lead through the grommet to the instrument panel.

3. The EGT probe fits into a hole on the exhaust manifold near the ball joint.   It must be very close to where the exhaust gases exit the cylinder!!!  Insert the probe into the hole and tighten the clamp, and then route the lead through the grommet to the instrument panel.

4. Connect the tachometer sense wire per manufacturer recommendations.  There are many types of tachometers on the market.

5. Ensure that the engine is grounded to the airframe using heavy wire.

6. Ensure that the kill switch is functional!!!   A mistake here can cause you one of two problems; first, the engine may not start because the ignition is actually 'off' when you think it's 'on', and second, if the ignition does not properly ground out through the switch YOU MIGHT NOT BE ABLE TO SHUT THE ENGINE OFF 'TILL IT RUNS OUT OF GAS!

Obviously, it is worth some extra effort to make sure that the kill switch is properly and securely wired.

7.  Verify everything with someone who is very familiar with engine installations.

Section F. Propeller Installation.

1. Install the propeller. Mount the prop to the drive pulley using the centering plate and crush plate that came with the engine, and the hardware provided. Torque the bolts I/A/W with engine / propeller recommendations (EG, 175 inch pounds); and safety wire the bolts.

2.  Check your propeller tracking according to the instructions that come with the propeller. If the blades are out of track consult propeller manufacturer  recommendations.

Section G. Pre-run Checks

1.  Pre-run checks. Before attempting to start the engine for the first time there are several things you should check, or re-check.

2.  Mix the fuel and gasoline in the proper ratio for break-in. See the engine owner's manual for specific information about fuel types and grades, oil specifications, and proper mixing ratios.

3.  Ensure fuel system integrity. Double check the fuel tank vent line, and fuel feed lines.

4.  Check every connection for security and a properly installed hose clamp. Be especially prudent in the engine compartment because of the extremes of temperature and vibration present there.

5.  Verify correct electrical installation, especially concerning the kill switch.

6.  Verify the propeller is properly installed.

7.  Check the operation of the throttle lever and cable.

8.  Tie the tail wheel ofthe aircraft to a fence post. Ensure that the prop blast (there is more ofit than you think) will not damage objects behind the aircraft. Sweep the area around and under the propeller free from stones and other debris that could get sucked into the prop and damage it. Remember that if the throttle cable is stuck the engine could start and go immediately to full power, so be prepared.

9.  Start the engine and run it just long enough to verify that the engine gauges are functioning. Stop the engine and check for leaks. If everything is normal, proceed with the engine break-in procedures from the engine owner's manual.

Here's a photo of four Belite ultralight airplanes:  no cowl, bad cowl, and two good cowls:

A Bevy of Belites with Cowls