With the balance issues out of the way time to see if this engine runs!
First I chained her to the tree, not sure if my brakes have enough holding power and don’t need to taxi across the neighbors lawn. Made sure Sheri was holding our youngest so he’s not running around, until the engine fired all you could hear was him screaming. As soon as it started he was all smiles tho.
Moment of Truth
At first she was not starting, my son Charles pointed out that I’d forgotten to remove the plug in the exhaust outlet. Removed that couple cranks later she starts to spit and sputter and slowly comes to life. At that point I realized I had forgotten to set the choke, time to make checklists and start using them.
First sign of problems
When she first started I kept the throttle around 2000RPM until it was warned up. But when I slowly ran the throttle up the exhaust gas temperature would spike up to the mid-range limit of 1330F whenever I went over 4000RPM. I changed the clip on the carbuerator needle to the richest setting and while it helped it was far from acceptable.
Ask the experts
Andy Humphrey at heavenboundaviation.com http://heavenboundaviation.com suggested I should see what the max RPM was, for just a couple seconds, but to not exceed 6500RPM. He said the EGT can run high if the engine does not have enough of a load. It’s possible that my propeller is too small.
Reading the manual the manufacturer suggest that the propeller should provide enough load so the max static thrust RPM on the ground should be 100-200 RPM less than peak HP RPM of 6000. So 5800-5900 RPM. Well she easily revs up to 6400RPM so no point, the propeller is too small.
Correct the problem
Since the propeller I have is not adjustable I need to get a different one. Currently I have a Tennessee 60×26, I suspect a 60×28 would be about right, maybe a 60×27.
Now I might be simplifying a bit here but the only advantages I see to a wood prop is they are typically light and cost less than other options. They have downsides such as moisture in the wood migrating changing their balance.
After looking at the options I decided to get the Competition Aircraft Ultra-Prop. It’s cheap, pitch can be adjusted and you can easily cut them down. My friend David Gohn suggest I order it with 15 degree pitch blocks and trim the length until I get the desired RPM. Hopefully it will be here before the next weekend so I can try it out.
Sigh, so close yet so far away. Finally had a break in the weather and was able to put the airplane on the scales. The results confirmed my fears, the center of gravity is too far back.
Center of Gravity
For an airplane to have proper pitch control it must balance at a specific location. Too far aft and you can’t pitch the nose down, too far forward and the nose won’t pitch up. On this airplane the safe range is 21%-30% of wing chord with the ideal location at 28%. With a cord of 54″ the CG must between 11.35″ and 16.2″ behind the leading edge of the wing.
Measuring the CG
Measuring the CG is quite simple. The airplane is first placed on a level surface. A scale is placed under one wheel and wood blocks, the same height as the scale, are placed under the other wheels. This way the airplane will remain level as we move the scale between each wheel to measure the weight.
Next you must pick a datum, this is just an arbitrary reference point on the airplane. Then measure the distance between the datum and the center of each wheel, this is referred to as the arm.
With the airplane ready to fly, sans fuel, with pilot, me, in the cockpit we weighed each wheel. Each measurement was taken for times too ensure we got consistent results and then averaged.
|Location||Weight (lbs)||X Arm (in)||= Moment (in lbs)|
|Nose Wheel||34.7||X 11.8 in||= 409.46|
|Right Wheel||239.9||X 53.69 in||= 12,880.231|
|Left Wheel||240.85||X 53.69 in||= 12,931.236|
The CG is calculated as total moment divided by total weight
22,220.927 ÷ 515.45 = 50.87″ from datum
The leading edge is 32.75″ from datum placing the CG at 50.87″ – 32.75″ = 18.12″ behind the leading edge of the wing.
That’s nearly 2″ behind the limit, what’s worse is adding fuel will only push the CG even farther back.
How did we get here?
I’ve made a few changes to the airplane from its original design.
- Added the BRS for safety
- Added disk brakes, for safety
- Added a strobe on the tail, for safety
- Paint scheme, because it must look good
- Slightly heavier harness, for safety and comfort
- Added wing tanks instead of the fuselage tank, mostly because I hated getting my toes stuck on the fuselage tank when working the rudder pedals.
So did all of those changes cause the problem?
Nope, it’s easy to calculate, measure the arms of those items, subtract their weight and moments from the data above and she is still tail heavy.
The rest can be explained by knowing the history of this design. Originally it was designed for the now discontinued Rotax 277, which happens to weigh 20lbs more than the Hirth F33 I have.
Math says removing my changes and adding 20lbs to the engine resolves the problem. So she would have been tail heavy even if I had not made the changes. But I did manage to make it worse.
What to do about it?
Seems like I have two options, get a bigger heavier engine or add weight to the nose to make it balance. I posted on LonesomeBuzzards.com to get some opinions from the community. The community basically had two suggestions, add weight and give it a try or better get a bigger engine because you need the extra HP with that much weight.
Still not sure what to do I made the hour trip to visit my friend David Gohn. After looking over the numbers he suggested I should add the weight to make it balance leaving more money to build the next airplane. His assurance that the F33 was perfectly capable of flying this airplane safely was helpful.
Andy Humphrey from heavenboundaviation.com also felt the F33 would be fine. But suggested that if I’m adding weight, try to find a way to attach it directly to the engine since it might help dampen some of the vibration that the F33 is notorious for.
So I got a piece of 3/8″ thick mild steel to make a new motor mount from.
After cutting it with a reciprocating saw and drilling holes it weighs 8.264lbs, almost 1/3 of the weight needed to get the airplane balanced.
After a trip to the disc sander to cleanup edges and baking on some powder coating the new mount looks great and weights 8.208lbs.
Removed the lithium battery and replaced it with a sealed lead battery that weighs 26lbs
With those two minor changes she balances!
The CG range is 21-30% of wing chord with 28% being the ideal CG. After adding the weight she balances at 28.2% and 28.8% with empty and full fuel tanks.
Had I known that she would be tail heavy I could have made some small changes such as extending the nose a bit and building my own custom fuselage tank instead of using wing tanks.
I’m satisfied with the results, the gross weight is more than I wanted but within the design limitations of this airframe.
Just a few more items to wrap up and I she will be ready for the airworthiness inspection.
I’ve put the rear stab and fin on a few times in the past to check alignment and balance. But I’ve always removed it because the garage is just too crowded to leave it on. This time I’m mounting it permanently so I can soon do the final weight and balance.
The first issue was needing to rearrange the garage so everything fits. The left wing is placed against the wall then the right wing behind it. If this order is reversed the right wing tip makes it impossible to get the fuselage with stab into the garage. The wing arrangement leaves a small gap where the stabilizer can move over and line up with the door.
The other problem is making sure the strobe on the fin does not get chopped off by the garage door. To obtain the proper clearance the nose wheel needs placed on a dolly to lower the rear of the airplane. This also makes it much easier to manuver the fuselage in and out of the garage.
With the logistics resolved it was time to get started. I put some loctite and safety wired the teleflex cable bolts.
The vinyl tube and clamp safety was added to the end of the teleflex. This ensures that if the tube on the end of the teleflex was to come loose it cannot fall off and jam the cable.
With the fuselage prepared the fin was bolted to the stabilizer. This must be done before bolting the stab to the fuselage because its nearly impossible to install the rear fin bolt once the stab is attached to the fuselage. Unless you have help, this step needs done indoors. Until the braces are attached wind blowing on the fin could put too much stress on the bottom bolts and break something.
While tightening the bolts I discovered that either the bottom of the fin or the top of the stab was not perfectly flat. Not really sure how I managed to overlook this earlier but it needs addressed. I believe the issue is overlapping fabric at the front bottom of the fin. If the bolts are tightened the bottom of the fin bends a little causing the fabric to wrinkle. To resolve this I added a shim between the stab and fin at the front bolt.
The four angle brackets were bolted to the fuselage but not completely tightened. The stabilizer was bolted to the brackets and then the fuselage bolts were tightened.
Attaching the fin struts was easy except for the bolt on the fin because it has a bracket on both sides. I took a junk 3/8″ socket and ground the outside diameter to 1/2″ making it a super thin wall socket. That was the only way to tighten the lock nut, no other tool could fit into the bracket. To keep the bolt from spinning I jamed a flat blade screwdriver between the bolt head and bracket.
The wires for the strobe were attached and wrapped with self fusing silicon tape to ensure they cannot vibrate loose.
Finally the rudder and elevator were installed by inserting the hinge pins. I’ve not added the grease or cotter pin to hold them on yet since I may need to remove them when I put this on a trailer and take it to the airport.
Empennage competed, now I hope all my measurements are correct and it will actually fit through the garage door. That will need to wait for a warmer day.
The best place for the strobe is on top of the fin. Getting the wiring up through the fin will be difficult since I’ve already covered the fin. Looks like I need to make a 3 foot drill bit.
I got a 1/4 steel rod and sharpened the end to a point on the sander.
Then I used a dremel with a cut off wheel and made two groves 180° apart.
It’s a little slow but works well.
The hard part was getting the angle just right to drill right down the center of the nose ribs. To assist with this I made a small guide block, similar to the guide block used to drill the axle hole in the landing gear.
Once the hole was drilled the wires were routed through the nose of the fin. A hole was drilled in the bottom of the strobe and the strobe base mounted into the fin.
Terminal ends were added to the wire and connected to the strobe.
With the fin mounted the wires were connected and the strobe tested.
In the quest for an affordable strobe I found many suggestions. Some people have used fire alarm strobes, I considered this.
Others just say buy one so I looked into that. The Microavionics MM030 strobe is about $150 and has a xenon flash strobe powered with 20 joules and weighs about 9.5oz. Not really sure how tall or wide it is.
The cheapest pre-made strobe I found is the Skysports Bright Star Strobe avaliable from Aircraft Spruce. It has dual xenon flash tubes using 8 joules, weighs “under 5oz” and is 2.75″ wide and 4″ tall.
In my opinion the Skysports strobe is not bright enough. Sure it meets the need for twilight flying but 99% of my flying will be on sunny days. The main reason I want a strobe is to make my slow small airplane more visible to other faster approaching aircraft, this is especially important when taking off and landing.
I also wanted to avoid a xenon flash tube, the high voltage can cause radio interference. Sure properly grounded it should not be an issue but this is a wood airplane.
After looking I found a nice LED light that fits the bill. It weighs 7oz so it’s lighter than the MM030 but heavier than the Skysports. At 5.5″ tall and 2.75″ wide it has the same footprint as the Skysports just 1.5″ taller. Most importantly it is very bright. It also ended up being the cheapest at $40.49 with free shipping.
Unexpectedly this LED light did cause RF interference with the radio. Each time it flashed you could hear a hiss on the radio. Adding a ferrite core to the power wires resolved this problem. The core was salvaged from an old VGA cable and fits nicely inside the bottom of the light.
I’ll cover mounting it in a future post.
To drain the fuel from the airplane I installed some Curtis quick drain valves CCA-4850 and purchased the Curtis drain hose assembly CCB-39600-5. They claim that “Waste liquid flows through 5 feet of high grade vinyl tubing, preventing messy spills and making a cleaner, safer environment.” A better description would have been “95% of the fluid flows through the hose, the rest makes a messy spill”
At first I thought maybe the particular valve I got was the problem but looking on their site I see that my valve is listed with the hose I have. Upon further inspection of the valve I see the problem. There is no seal between the valve and the valve body when the valve is opened. So some of the fuel leaks between the valve and it’s body while the majority of it runs through the valve into the hose.
I removed the brass piece from the drain hose assembly and decided to make a simple modification. The brass is shaped like a little cup and can catch the fuel that leaks between valve and valve body. So all I needed to do was drill a small hole to allow the captured fuel to drain into the hose instead of overflowing and making a mess. I used a 0.09″ drill bit inside the nipple at a slight angle.
After trying out my modified drain hose I discovered that it does work much better but not perfect so I added a second drain hole on the opposite side. At this point I’m satisfied. Some fuel vapors will come out around the connection and sometimes a little will condense on the valve tabs. But fuel was not pouring down the outside of the hose and making a puddle on the floor like it did before the modification.
If it’s not obvious, it has one drawback, if the drain hose is obstructed in any way the fluid will follow out the holes, overflow the cup and make a big mess. Maybe a valve and a nipple to drain the fuel would have been a better choice.
Long ago I ordered some operating range decals from aircraftspruce.com so I can mark the air speed indicator. While the decals worked ok they are a little short. The more expensive decals they sell are full circles so no worries about being too short.
I labeled the stall speed with and without flaps, maximum speed with flaps extend, normal operating range, cautionary range and maximum speed. For three dollar decals they look very professional but you might need two of them to get the job done.
The air speed indicator requires some tubing that goes to the pitot tube and static ports. Previously I wrote about the static port installation, you can read about that here. This article covers the tubing for the pitot tube. I decided to use the removable pitot tube kit, sold by Leading Edge Airfoils, so I can remove the pitot tube when removing the wings.
I thought that putting it out the leading edge of the wing would look nice, trouble is I already built the wings so installing is not going to be a simple task. I created two wood blocks, one to fit inside the wing just below the leading edge stringer and another to fit in the outside of the wing to support the pitot tube mount.
The inside block was threaded to match the pitot tube mount. I added CA glue to the wood as I tapped the threads to ensure the threads remain strong.
The only access hole in the leading edge is where the strut mounts are located so that’s where I decided to mount it. I cut a hole in the leading edge ply and epoxied the threaded block in place.
All of that was done about a year ago, now that the wing is covered in fabric and painted, time to finish. I first run the vinyl tube inside the leading edge from the root to the access hole. There I slid the tube through a 3/4″ deep well socket, then the plastic nut and out the threaded hole.
The outside wood spacer block was slid over the pitot tube mount and then the vinyl tube was slid over the barbs on the mount.
The mount was threaded into the wing, I had to ensure that the vinyl tube inside the leading edge was rotating too instead of rolling up into a knot. Now for the hard part, I used some long forceps to get the plastic nut started on the pitot mount. To tighten the nut I dig around my junk drawer and found the perfect tool, an old useless screwdriver. After a trip to the disc sander and vice it now looks like this.
My newly designed wrench fits into the hole on the side of the socket allowing me to turn the socket about 1/4 turn at a time.
I used my $20 USB endoscope camera hooked up to my phone so I could see if the nut was indeed fully tight.
I retrieved the socket using a magnetic pickup tool.
Next I made a plate to hold the quick disconnect.
Here I forgot to take a picture. Before I drilled that 5/8″ hole in the plate above, I had only drilled a small 1/16″ hole at the center. While chewing some gum I installed the wing so I could mark the center of the quick disconnect on the fuselage.
Grabbed the hole saw and drilled a hole in the fuselage side.
The vinyl tubing was cut to length, disconnect plate varnished and disconnect installed.
The 90° disconnect inside the fuselage was attached to the vinyl tubing that runs up to the AIS.
I’m using the same valved disconnects I used for the fuel lines. So I don’t have to worry about bugs crawling inside my tubing and clogging it while I have the wings removed.
The final result looks great, hope it works well too!
Various parts of the airplane need to be locked for safety or plugged to keep bugs/debris out when the airplane is parked and not in use. These locks and plugs are usually attached to a flag stating “Remove Before Flight.” I purchased some inexpensive, well made, keychains for my flags.
The BRS ( aka ballistic parachute ) came with it’s own flag. This is for safety because you do not want to accidently activate it. It does not have ballistic in it’s name for show, that rocket needs treated like a loaded gun.
The static ports are used to measure atmospheric air pressure and are integral to the air speed indicator (AIS) and altimeter. I bent a small piece of music wire that attaches to the keychain and doors snuggly into the static port.
Maybe not necessary but seemed like a good idea I got a small rubber stopper and eye bolt to make a plug for the exhaust.
The pitot tube, for the AIS, was a little more difficult to block because the tube can be removed. We not to plug the hole the tube goes in or the end of the tube when it’s installed.
Drilled a hole in the side of a 1/4″ aluminum rod to attach the form.
The rod can easily plug the pitot holder.
Added a piece of 1/4″ ID tubing, this will be used to plug the tube when it is installed on the airplane.
With the pitot tube removed the hole is easily plugged and looks nice.
When the pitot tube is installed the same flag can plug the hole. The only downside is that it does add a couple inches to the length.
The droop wing tips I got from TEAM had some flaws. A few voids here and there were no big deal but not fitting the leading edge was a problem.
To fix the front I ground out the seam on the wing tip. Some wax paper was taped onto the leading edge of the wing. Polyester resin was mixed up and new fiberglass added to the wing tip. Another piece of wax paper was placed on top of the wet fiberglass and rubber bands were used to hold everything in pace while the resin dried.
The edge of the wing tips are not straight so I decided to mount them, drill the holes and then trim the edge based on the path of the screw holes.
I marked a line on the wing 2″ from the end around the perimeter, this will be used as a reference to drill the screw holes into the fiberglass tip. The tip was mounted and clamped in place then measured 1.5″ from the line and drilled holes into the wing tip every two inches. This places the center of the screws 1/2″ from the edge.
I would drill a few holes, remove the tip and enlarge the hole in the fiberglass and then bolt it back into the wing and drill a few more holes. Once all the screws were in place I used them as a reference to mark a straight trim line around the tip then cut off all the excess material.
A 1/8″ plywood piece was cut and fit to seal the end of the tip at the aileron.
It is held in place with some fiberglass cloth and resin from the inside of the tip.
The tip did not match up to the aileron as you can see here. Not really necessary but I wanted a better look.
Using a thin diamond cutting blade on the Dremel I cut a slot at the back of the tip.
Then I used some polyester resin to glue in a piece of 1/64″ plywood that I pulled from my R/C airplane parts bin. This was a little tricky to get right because it can easily flare in or out if your not paying attention.
Once that dried I mounted the tip onto the wing and marked where I wanted the tip to end.
The plywood was cut along the mark, then polyester resin and fiberglass cloth was used to build up the tip to the proper thickness.
After applying some filler and sanding the extended wing tip looks great.
Filler was applied to all the other imperfections, the entire surface sanded and primer was applied.
A couple base coats of white were applied and finally the red top coat. Installed they look great!
One last modification was needed to the tips so I can easily remove the aileron with having to remove the tip. I modified the SHCS that hold the bearings on the ends of the aileron so I can install a safety pin so I know the aileron cannot fall off.
To make it easy to pull the pin I drilled a 7/8″ hole in the bottom of the tip directly under the aileron bearing and plugged it with a solid rubber grommet.
Installing the pin is not terribly difficult through the.