Thorp Air Command - T18.net

I've had terrible luck with spinners on my airplane, and I'm just getting ready to finish and mount my third one. I must have done something(s) wrong on my last attempt, so I have a few questions for the experts (Cubes et al) so this one lasts more than 20 hours.

As background, I bought a new backplate/bulkhead after my original one cracked. That bulkhead has been on for 150+ hours and I've flown most of those hours without a spinner dome. The last dome I had developed a crack just inside the first screw from the prop cutout.

The new dome has mostly-complete prop cut outs and I'm working on the alignment and hole drilling.

1 - what's a good way to ensure that the dome is true to the shaft? I've removed four spark plugs and have been adjusting the seating by spinning the prop and making sure the tip stays centered on a reference point. When I get this aligned, the spinner is not true on the backplate. It also overhangs by 1/4" or so to be flush around the circumference.

2 - How do I mount the front bulkhead? I bought the airplane with the bulkhead mounted with double-sided tape to the inside of the spinner, and I used the same approach when I cut my second spinner. How do I get the bulkhead centered and adhered well inside?

3 - How do I locate the holes for the screws into the rear bulkhead? I have a hole locator tool, but the fit between the dome and the bulkhead is snug.

4 - Do I need to install an additional support / screw hole on the prop cut out? I've seen some folks make an additional bracket to plug the big gap behind the prop and help support the dome there.

5 - How much clearance should I have around the prop in the cutout? 1/8"? 1/4"? Is it ok to have even clearance, or does it need to be biased based on the expected movement of the CS hub?

6 - What are the minimum steps to finishing the dome? Does it need to be sanded smooth and polished?

There are only a few more work nights before KY Dam, and I'm hoping to have a spinner on for that. Of course, I'd rather fly without one rather than rushing and breaking a third dome.

Thanks!

First, the disclaimer: a builder I am NOT, just a fly-boy. However, with Cubes' invaluable help plus all the work done by my A&P/IA, I have high confidence that what was done was done right.

I'll post all the pics I have including the rear bulkhead "before," plus a link to the following video so you can see the end result:

Example of a T-18 spinner front and rear bulkheads w/CS prop https://youtu.be/zzhYP5aPzXQ

Mechanic put one layer of electrical tape around the front bulkhead to prevent walking or chafing. This was recent work, end of August 2020 -- so far, so good. Probably last 100 years, but ask me then.

Original installation was rear bulkhead only. I'm told that's why it cracked. Add front bulkhead, they said, and it will last...well...100 years. (that's not actually what they SAID, but you get the idea)

_________________
Philip Mandel
N118BC
S/N 957
Beaverton OR

Brother,
Send me a text and I’ll help any way I can. There is a lot to it, but I need to ask some questions regarding what you just asked.
Cubes
702 232-4261

Philip, I didn't earlier for such a detailed reply. The descriptions, photos, and videos were very helpful - thank you.

Can you confirm that your front bulkhead is mounted to the propeller hub with a single, centered screw? What are the four surrounding screws for?

I'm amazed at how long this has taken me to finish. It took a year to do my first spinner, and more than that for this second one. I finally drilled all the holes in the dome for the rear bulkhead. Thankfully, all the holes line up. Now what remains is the front bulkhead and final surface finishing.

So continuing on Question #2 above, how do I actually mount the front bulkhead?

I have attached some photos of the front bulkhead on the prop hub and held up next to the spinner dome.

My assumption had been that the front bulkhead attaches to the interior of the dome either with strong/permanent double-sided tape or with rivets/screws. It seems like Philip's installation is secured to the prop hub and the dome just rests on the front bulkhead when it's installed. Which is the right choice? Should the bulkhead be attached to the prop hub and provide support for the spinner (without attachment), or should it be mounted to the spinner dome and slide over the prop hub?


I would love this spinner #3 to last more than a few hours, so I'm happy to take the time to get it done correctly.

I epoxied my front bulkhead to the spinner with JB Weld. Very similar to your setup. Been there for 35 years. The other must have (IMHO) is fillets behind the cutouts that tie the opening together. Might have a picture later today.

_________________
Bob Highley
N711SH
SN 835
KLAL

The proper constant speed forward spinner bulkhead has five holes in the center. The center hole is simply a clearance cutout. The four surrounding it are drilled for #4 bolts. Get short AN4-3 bolts and gently bolt it directly to the constant speed cup on the very front face. It will be perfectly centered. I use electrical tape as anti shaft around the flange. Any thin material will work.
When you install any spinner dome, the fit should be tight enough that you need to use an awl to pull it aft to the backplate holes. I do not recommend putting screws and nut plates in the forward bulkheads.
Cubes

_________________
Bob Highley
N711SH
SN 835
KLAL

Dr. Drift,

My installation is much like Phil's. The 4 screws guarantee the front bulkhead will be centered. I have a few washers between the bulkhead and the dome on the C/S prop which, as Cubes said requires an awl to pull the spinner dome back for the screw holes to align on the rear bulkhead. I measured the run out of my spinner and it was less than .005". I am real happy with the set up I got from Cubes.

Regards,

Jim

Are screws and nut plates in the forward bulkhead also not recommended for fixed pitch propellers as well?

That is correct Ryan.
Cubes