Chapter 23/24 – Energy & Angles

I learned from my Air Force fighter pilot uncle that dogfights are all about energy and angles.  Pretty sure that’s one of the principle themes of what is taught at Top Gun and other Fighter Weapons Schools.

Well, I’m clearly in a dogfight as well in getting this bird built… and just as the U.S. military did with Top Gun, I needed to refocus on energy and angles a bit in regards to my energy producing device, aka engine.

But first… I started out first thing this morning to get some glass curing.  I had planned on knocking out 3 sets of layups before throwing myself headlong into the upper cowling install.

I got this one set below knocked out before I started messing with the engine angle. Here we have the BID going onto the top side of the left winglet intersection fairing made up of carbon fiber on wood.

And here the initial top side layups are complete and peel plied on the left winglet intersection fairing.

And I I then did the same on the right side.

When I mounted the 3D printed mockup mod of the prop spinner flow guide and subsequent addition of the spinner, I wasn’t focused on the ANGLE of the aft blue face of the flow guide.  I texted some pics to my canardian buddies and in one reply, Marco stated that it may just be an optical illusion but that my spinner looked a little pointed upwards going aft.  I thought he was just being a bit snarky and passed it off with a snarky comment back.  Case of the snarky comment closed… or so I thought.

Clearly the vinyl record sized flat disc that now occupies the very aft of the airplane (with the spinner off) provides a much larger visual reference plane to observe than just that of the more diminutive prop extension mounting flange.

And then I saw “it.”

As I was heading back into the house I circled wide around the left winglet and paused to look at the top cowling.  Once I saw the angle of the bright blue disc on the aft side of the bird I could then clearly not “unsee” it.  It was glaring that my prop angle was off… notably.  I now understood what I initially thought as just a punky comment from Marco was actually a very spot-on observation of a detail that I had hugely missed.

Here’s a closer look at the prop angle.  I messed around with a 2×4 and other material to assess this forward leaning prop.

I knew that with a lot of weight in the nose and having not attended to the tire air pressures in months, that the nose was probably a little low… plus I know the shop floor angles down towards the front doors.

I checked the longeron angles and they confirmed the nose was in fact low. I put a 5/16″ piece of wood under the front tire and also filled it with air.  My longerons are always a hair of a degree off from each other, and at this point the left showed 0.1° degree down and the right flashed between 0.2° and 0.3° down… for this exercise I called it 0.2° down and averaged the total at 0.15° nose down (all this actually preceded the prop angle pics above).

I dug back into the plans to confirm that my engine mount was attached in the proper configuration.  I measured the top aft edge of the mounting cup on each side, from the firewall and also the bottom engine mount cup as well.  The F.S. reference point in the plans is from the front face of the firewall, which is F.S. 125.  The most important data point here is the difference between the top and bottom W.L.’s of the engine mount as spelled out in the plans: a 0.25″ delta.

My measurements, as shown here are about as spot as can be measured: 0.25″ difference on the left, 0.26″ difference on the right.

I took the bottom cowling off and spent a good hour rigging up the engine hoist, and removing the shims off the bottom engine mounts.

With the bottom engine mount bolts only finger tight, I rechecked the angle of the engine at the prop extension hub.  Interestingly, with a 250’ish pound engine —after removing 0.1+” shims— the angle only dropped from 1.5° to 1.2°.  That told me that engine weight alone was not compressing the Lord mounts to any real amount, which is what I somewhat expected.

After tightening the bottom engine mount bolts to spec —sans shims— I was now looking at only a 0.6° angle on the prop extension face.  But don’t forget the nose is about 0.15° low… so the actual angle is between 0.5° to  0.45°.

In researching engine mount shims and in also talking with the Lord Mount folks, it’s not surprising that engines tend to to drop a bit after the mounts get “exercised” and the engine settles into place after its operated.

Again, a closer shot.  At 0.5° aft engine up —clearly less than a degree— it’s close to the center mass target and the tendency in the future is for it to settle.  Better this scenario by far than that of the engine angle being too low.

Out of curiosity, I set the level with the water bubble just touching the aft line and measured the gap at the top between level and prop stud: 0.05″ engine drop until within the minimum “level range.”

With the bubble centered that gap is 0.079″… so at a hair over 1/16″ the engine will be exactly level.  Considering I need to still mount the prop, crush plate, prop bolts, Belleville washers, spinner and flow guide, not to mention the engine baffling, and that the engine hasn’t been run to settle into the mounts, I’m simply not going to worry about this angle from here on out.

What I do have to worry about is component clearance with the lower cowling. I will state emphatically that it is time to put this engine vs cowling clearance issue to a stop. If I need to start hacking up and reshaping the bottom cowling I will proceed to do so.

After the latest rounds of engine shenanigans, multiple hours later, I finally got around to pulling the peel ply and tape off the winglet intersection fairings.  I had planned on laying up a single ply of BID along the fairing’s underside, both along the wing TE to fairing seam and also along the winglet to fairing corner seam as well… but it was late and I wanted a break from the engine angle drama.

I will note that I weighed the trimmed carbon fiber/wood fairing structures together at 1.34 oz.  I then added the glass weight and doubled that to estimate the epoxy weight as well.  Currently the added weight is around 3.4 oz.  I expect with foam, micro and glass that the estimated total added weight of these fairings will be about a pound.

Once I add and shape pour foam on the top to blend the winglet intersection fairing into both the wing and winglet, with a pleasing shape radius, I’ll most likely significantly trim down the height of the mini wall that will create the outboard upright of the fairing, against the bottom inboard rudder.

And with yet another long (and dramatic) build day under my belt… yup, calling it a night!

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.