The Weighty Decision

Hey, since the end of January, we have been working up to where we are now. hahahaa! Of course! The landing gear was done and it was time to focus on systems. Such as the fuel system, electrical, oil, lights, etc. We installed the main fuel line in the fuselage from the selector valve to the firewall. Fabricated the selector valve mounting bracket and painted it. Made steel plugs for the firewall engine mounts to fireproof them as they pass behind the firewall. This allowed us to install the engine mount. We added the major components to the firewall as well (Ignition system, voltage regulator, air/oil separator, oil tank, the structural “tunnel”). We then decided to go ahead and mount the engine. We had to fabricate four spacers to move the engine 1/2” forward due to an error in the engine mount design which put one mounting puck support too close to an intake tube. This worked fine and we bolted the engine in place. Now we can look at the throttle installation and were all the systems and cables will route up forward. We had a cowling nose bowl that Rob Walty made for us and we had to fit it on the front. The sight lines are awesome!

Then, someone asked: “how much does it weight?” Well, that got us thinking, we really should do an in-progress weight and balance before we install everything - in case we need to move stuff. Well the wings were at the hangar and with the weather interfering, we needed to pick a good day to move the wings to the shop. We finally got a chance and brought them to the shop unscathed. Didn’t take us long to say - “ok, lets pin them on and see what she looks like”. Well they fit almost perfectly, but not quite. They had been assembled on the bench at the home shop, but we had replaced the inboard to outboard attachment brackets with more robust ones. Well, the more robust ones were a smidge too long and the inboard holes didn’t match the outbd holes. Scott spent a few hours filing the new brackets down to where the wings will fit together as intended. So we pinned the outboard wing panels onto the inboard ones and DANG! Looks like a fighter plane! We had finally achieved another milestone! We still need to drill and ream the holes to install the final bolts, but we have some other work to do first.

Back to the weight and balance, we did a relatively quick weighing of the plane on our bathroom scales. The weight seemed really good, 50 lbs under our target, but the balance was way off towards the tail. The whole thing felt fishy. Did some head scratching for a night or two, we then ordered better ‘industrial’ scales off of Amazon. Well as industrial as they can be for $40 each (They aren’t bad!) Importantly though, they have a digital readout display on a cable separate from the scale pad itself. We decided to try again with the hopefully more accurate scales. These new numbers showed us 90 pounds over our goal and still tail heavy. 90 pounds??? Turns out while bathroom scales can handle a decent amount of weight, they are designed to handle that from two feet standing on either side of the scale. Put all the weight in the middle and the plastic flexes… and bottoms out. Thus I can go on a serious 30lb diet just by changing my posture to a Flamingo stance! Jeeze, I thought that they may be cheap, but would only be off maybe 10 pounds or so. Nope! BEWARE! Even the new ‘Industrial’ scales showed a bit of flex from too much of a point load, so we decided to try and spread that more evenly on the scales.

So with some thick aluminum plates to distribute the load from the wheels, we rolled the plane onto the industrial scales: and at this point it was showing with the current engine, wings, structure, and all of that we are 775 pounds empty, with an aft CG which will only work with the lightest pilots. Ok, well we knew we were not building light and lean. We did not optimize this first-article’s structure for weight. This is a prototype and we know we can loose about 20-30 pounds with some design refinements, but we’ll definitely have to make sure to focus on that tail area next time. However, we also have been planning to at some point swap the engine for the heavier 7 cylinder Verner, or in-line auto conversions, such as the AeroMomentum AM15. Looks like now is a good time to do that!

The good news! The calculations show that with the Verner 7U or the AM15, or Viking 130, or basically any engine in the 200lb range, our empty center of gravity will be at 16% of our mean cord, with a total weight of approx. 829 pounds. A light pilot and low fuel is 22% cord and heavy pilot, full fuel and bags is 26% cord. A little heavier than planned (as is tradition it seems) but just about on par with published numbers for the RV-3, Panther, Onex, WAR Corsair and others, in terms of wing area, empty weight and hp. Each is a little different and we fit right in the middle of the lot. All told, we should have excellent performance!

So, which engine? We have been agonizing this bit over that for two weeks and they all have pluses and minuses for our project. We love radials! The 7U is an awesome engine with power and sound that just grabs everyone’s attention. We will be first to admit they can be a bit pricey. The AM15 is an excellent deal for price and power. Mark and the whole company are awesome to work with and they know their stuff. Most of our initial customers are going to want the P-40 model and we need to develop the firewall forward for it anyway. Sure, the reduction drive propeller shaft sits fairly low on the engine, and turns the “wrong” way; but that would still work for the early model P-40s (A, B, C) which is what this prototype is set up to be anyway. I have a lot of VW engine time so I and others like me are used to the p-factor being “wrong”. Then certainly it looks like the Viking 130 or even the new 150 would be options for the P-40E models and should have the power to make this aircraft perform. There seems to be plenty of options there. Some of the exhaust stuff on the P40 is going to be a bit of a tetris challenge to tackle, with getting the exits to all work. Radials aren’t immune to that either of course.

Of course, at the end of the day there was one obvious answer, and the only question was whether it could happen in time. We all want to see the P-36 fly, so we’re going to stick with the radial and get it flying as fast as possible. With our relationship with Verner, and the joy it would bring all to see our plane fly with a 7 cylinder on the nose, we talked with them and found a way to get a 7U within our timeframe. A toast for those guys for the effort they put into all of the engines and for taking care of us.

Of course, we need to still do a P-40 before there’s a riot; so the plan remains that we will do that, and use the auto engines for the next production prototype. On the next fuselage it will be a P-40E type, and will have all the improvements and lessons learned from the first go; including the retracts and a few other great adjustments to the lines. This version we will build concurrently with our early builders. And of course, for any P-40, we need to develop the engine mount, the exhaust headers, the radiator installation, the plumbing and cowling. Possibly utilize an inflight adjustable propeller as well. That will be a fun challenge - for later!