Boot Cowl Construction

[jaredyates]

Thank you so much Archer39J for posting the pictures here, and for allowing us to add them to the builder manual entry on the boot cowl here:

https://bearhawk.tips/3136

Please keep up the good work!

[svyolo]

Reading some boot cowl threads, I decided to make sure my panel and firewall flange didn't move while fabricating. I wanted to make sure they were in the right place. I bolted a piece of plywood with big stringers to make the firewall flange square, and then used some scrap (I create a lot) angle from the firewall tubing to the instrument panel. Both are pretty rigid now.

[Archer39J]

Reading some boot cowl threads, I decided to make sure my panel and firewall flange didn't move while fabricating. I wanted to make sure they were in the right place. I bolted a piece of plywood with big stringers to make the firewall flange square, and then used some scrap (I create a lot) angle from the firewall tubing to the instrument panel. Both are pretty rigid now.

Looks good! Good idea for a planar firewall flange, my method of pulling the firewall to the frame is what kicked mine out a little. Shouldn't matter since everything is trimmed to fit in place, just annoying.

[Glenn Patterson]

Excellent work on the boot cowl. The fit up with the windshield is perfect.That is one place where it was left to the builders imagination. It would be nice if someone traced the instrument panel onto a one inch grid and did a station offset table like they do for boat building for one half of the panel. Offset tables gives the measurement for X & Y from the center and bottom at the points the curves cross. If the reference was the top of the tube along the bottom of the panel then that is a common reference. Any builder could draw a grid mark the points & easily replicate a panel from the measurements. One half of the firewall on an offset table would be a godsend as well. It would save a ton of head scratching. That was a detail that the drawings did not offer and was a major deficiency IMHO. Thankfully the firewall layout & developing the panel was in the Beartracks and/or manuals.

One comment on the roll at the cooling exit. The top side of the roll in the photos is flat with a hard inside corner that may cause air going down the fire wall to stumble. An efficient corner from my research is a half tear drop to wrap the corner is the best shape for a clean exit. There was an excellent article in Kit planes around 2005 that had a comprehensive article on coofing efficiency and pressure cowls. That drove me to do a lot of reading on cooling efficiency, followed the James Engineering group and checked out LoPresti speed mods for as many ideas as possible. Chris Zavatson had an article in Kitplanes on building a pressure cowl for cooling efficiency and drag reduction. He had posted some articles of his research on cooling efficiency and trying to get speed gains through cooling reduction for his air racing. He used sophisticated equipment to read pressure inside the cowl at various point to see what the effects of each change was. Schmidtbauer got a Mustang II to over 260mph with a lot of clean ups and has a book on it. Schmidtbauer made big changes to his cooling to get a lot of the speed gain that was also in Kitplanes. We installed a tear drop that projects out about 1-1/4" inch or so and about 6" high so it is a shallow slope down the fire wall that ends in a round corner.

I read enough in the EAA & Kitplanes that cooling is a major issue among homebuilders and some early discussion on the BH forum leaned that way. Deeply concerned about not getting the cooling right . Really really did not want to fall in the rabbit hole to have to struggle to get decent cooling. I researched cooling, cooling efficiency, cowl & nose design to quite a bit early in the build. Streamlined cowls & pressure cowls were not an option for the BH but could borrow or adapt whatever seemed practical. Some of efficient cooling seems counter intuitive like smaller air inlets. I closed in an inch on the prop side of both cooling inlets to cover the fuel lines on the right from the pump without a second thought. Aluminum half ducts were flushed in the lower nosebowl that go up to the top of the flat edge of the front baffle. The bottom half ducts provide clean flow over square engine front. The aluminum half duct is held in with screws so that they can come out and the winter fronts share the same nut plates.

Thin fiberglass half ducts were added to the top half of the nose bowl . When the 2 halves of the nose bowl come together they form a pair of ducts to project air into upper cowl cavity. The concept intent was to have good pressurization over the engine to promote even cooling. The ducted inlet is efficient in loading air pressure into the cowl. Apparently as much as 20% of the cooling air can come back out around the prop so steps to seal between the engine and the nosebowl to limit that is positive. We built new baffles to seal tight to the nosebowl and the upper cowl to maintain the pressure. The engine has been able to have good engine temperatures summer and winter. The one advantage to the Continental IO engine is that all the fuel systems are on top of the engine. The exhausts go to the outside so there is a nice wide open channel under the engine to the outlet. No fuel systems or airbox inlets to contend with to frustrate cooling cooling. The underside of the cowl is a wedge that may assist the air exit like the inverse of a pressure cowl in that the space under the engines expands as it passes cylinders to comfortably take up the added air all the way to the exit so perhaps the pressure & flow is uniform. If that is the case then it was just the way the cowl worked out and not by intentional design. The engine keeps good temperatures summer and winter so fingers crossed that that stays true.

http://www.n91cz.net/Interesting_Technical_Reports/106-111_BuildingBasics.pdf

http://www.n91cz.net/ Lots of good reading the Project Tab about cooling and air induction for improving air into the engine with reduced drag

[way_up_north]

I appreciate you taking the time to post those pictures...its threads like this that really help the building process.

[svyolo]

The engine cowling overlaps the boot cowl at the firewall flange. The lower portion is fixed, and screwed to the flange. Do you share fasteners with the boot cowl?
I am trying to make my side boot cowl easily removable. If I share dimpled fasteners would I be able to remove the screws, and slide the boot cowl side piece out, with the engine cowling still in place? Then reinsert? #6 dimples are pretty small.

I am making my boot cowl (at least for now) in 7 pieces. The top center,2 tops sides, 2 sides, 2 bottom sides. I want to be able to remove the side pieces, 15" tall, for maintenance. I hate crawling under things upside down.

[Mark Goldberg]

Usually the boot cowl fasteners are not shared with the cowling. The cowling is removed pretty frequently and the boot cowl top and sides very rarely. If ever. From my experience. Mark

[jaredyates]

If you really want to use screws for sections of the boot cowl, perhaps you could alternate one screw that holds both the engine and boot cowl, then one screw that only holds the boot cowl and is then burried under the engine cowl. This is how mine works, except there are rivets burried instead of screws. Keep in mind that the windshield will preclude being able to remove much of the boot cowl. I used #8 screws for the cowl and wouldn't want anything smaller there.

[svyolo]

I thought it would be nice to be able to remove the side of the boot cowls to work on stuff. I didn't want to have to remove the windshield for sure, and preferably not the engine cowl. My boot cowl side pieces are 16" tall, and the lower engine cowl will overlap the bottom 8" of the boot.

I will think about it a bit before I start drilling. I am a few months from building an engine cowl so no rush. Thanks

[whee]

I used #8 screws to fasten my engine cowl. The rivets used to retain the nut plates also fasten the boot cowl.

I considered removable boot cowl pieces but determined for my situation it wasn’t beneficial. It would still be a long reach with difficult visibility due to where the access panel would have to be located to not interfere with the windshield.

I think making the instrument panel in removable sections is a better solution. Last night we needed to access something behind the panel. Took about 20min to remove the x-ponder, pull the center section of the panel, make the wiring change and button everything back up. On this occasion access though the side of the boot cowl would have been of no benefit.

I suggest carefully considering your plane layout and what components will be more accessible though the side of the boot cowl.

[svyolo]

Whee;
I am not trying to get to the back of the instrument panel, as I am doing that similar to what you did, from the front. The boot cowl access I want is lower, below the avionics tray, but above the floor.

My instrument panel, not cutout for instruments yet, but very soon, is .125 2024 with a 3/4" angle across the top. It will mostly be a skeleton, with a 1" surround on the top and bottom remaining, and two vertical sections towards the middle. Most of it will be 3 large cutout sections, covered with 3 pieces (the middle piece being by far the smallest) that the instruments are mounted to. Probably hinged at the bottom.

I like the idea of using nut plate rivets to attach the other skin.

[Battson]

There are photos online somewhere of a Bearhawk with access panels in the boot cowl. These were done in the style of the access panels in the wing.
I have also seen photos of spotter windows installed in that area.

I think the boot cowl is one area which draws your eye, it's in the "middle" of the plane if you will. Close enough to eye level too.
In my opinion, it's an area which has an impact on the aesthetics of the whole aircraft.

[svyolo]

Thanks for all the inputs. I will think about it a bit.

[davzLSA]

Dave thanks for the great pics, I am getting closer the that stage in my build and you are most correct the manual does have a lot to be desired, I would really like to see more definitive information on many aspects of the build. I guess I was spoiled with the Vanns RV4 and building plastic models? Well I can count that as the fun part of scratch building it forces me to stretch my imagination. Thanks for all the good info you put out.

[JimParker256]

I was having a similar discussion with an EAA buddy, and he asked if I was going to install an EFIS. When I said "Yes," he pointed out that it only takes four screws to remove the EFIS, and then you've got a pretty large hole in the panel through which you can access pretty much everything else. Of course, his 10" Dynon EFIS creates a bigger hole than the 7" GRT Sport that I'm planning to use, but I think a 7x5 (ish) hole should be plenty big for my purposes.

[Bcone1381]

Have you seen the new GRT Sport 10.1?

[JimParker256]

Had not seen it. To quote Arte Johnnson's German soldier character from Rowan and Martin's Laugh-In: "InterestinK... Very interestink!"