WWII replica plans available in 2021?

[ScaleBirdsScott]

Random thought I had this morning but in lieu of an electrical drive motor driven by a generator, could the P-39 be a viable application for a hydro-somethingorother drive motor driven by, well, high power pump located in the midfuse? Basically replace the mechanical driveshaft with a liquid one.

Also say someone wanted to take one number off the project and do a P-38, a twin, could they have a single powerplant driving two hydro motors at each nacelle? Also would make it relatively easy to run a standard hydraulic constant speed prop I'd think.

Seems electrical could be a lighter setup and probably more reliable but is it really?

 

[vhhjr]

The only time was when I was getting some aerobatic instruction in a Citabra. Bailing out of a Sonex could be very exciting as it has a side hinged canopy and no telling what the aircraft would do if it wasn't ripped off immediately or if it ripped off and ran onto the horizontal or vertical tail. I have never heard of anyone actually bailing out of a Sonex and wouldn't care to the test dummy for the first attempt.

Vince Homer

 

[vhhjr]

Random thought I had this morning but in lieu of an electrical drive motor driven by a generator, could the P-39 be a viable application for a hydro-somethingorother drive motor driven by, well, high power pump located in the midfuse? Basically replace the mechanical driveshaft with a liquid one.

Also say someone wanted to take one number off the project and do a P-38, a twin, could they have a single powerplant driving two hydro motors at each nacelle? Also would make it relatively easy to run a standard hydraulic constant speed prop I'd think.

Seems electrical could be a lighter setup and probably more reliable but is it really?

There was a report last week of a four rotor unmanned drone using just that set-up. It had an engine driven hydraulic pump and four rotors powered by hydraulic motors. The system appeared to be reasonably light and greatly increased the flight time compared to an electic version. I see no reason why it wopuldn't work in a P-39 or something like a B-25 or other multi-engine aircraft. For some reason a 1/2 scale PBY comes to mind!! Or, one of those spiffy Pan Am flying boats of the thirties.

The P-38 may be a bit more difficult as there's not much room in the center pod for an engine/pump. Putting two smaller engine/pumps, one in each boom, seems more complicated than just using two engines.

Vince Homer

 

[Hawk81A]

Scott, Excellent idea. I had the same idea almost 40 years ago when I started working on hydraulic driven "slope mowers". These had a 5 foot mower blade driven by a hydraulic motor mounted on a 30 foot boom. It was driven by a hydraulic pump mounted to the tractor engine. I thought "now THERE'S a way to get my scaled P-39".
The down side: Because of the heat generated, those tractors have a 35 gallon hydraulic reservoir. They also have a pretty hefty oil cooler. With lighter components, it might be workable, but I'm thinking an ENCLOSED (safety) driveshaft may still be the best option. Gear reduction could be on the engine end and drive the prop through a CV joint type drive. Dennis

 

[ScaleBirdsScott]

Yeah heat of the fluid would be certainly an issue. But figure most of the motors wouldn't use all the radiator space that the original V12 needed.

 

[Tiger Tim]

Wouldn’t a mid-engine P-39 replica need some amount of nose ballast to replace the (I assume) absent cannon? Why not just pick an engine with a small cross section and put it in the nose? Maybe a rotary? Get someone to airbrush fake exhaust stains behind dummy midship exhausts if you must.

 

[ScaleBirdsScott]

Yeah but the scale weight of a cannon probably won't quite match up to the weight of the engine that would be going there regardless, but overall the pilot is moving and 20 other things so it's all up in the air and it can all be fudged.

 

[Bigshu]

Gear reduction could be on the engine end and drive the prop through a CV joint type drive. Dennis

Yes, I've been thinking about this setup! I picked up a CV drive axel off of FB marketplace, and am mocking up a driveline that includes a one way clutch to limit gyro loads back up the driveline.

 

[vhhjr]

The CG problem gets really bad if you put and engine in the nose or if you put the engine behind the pilot and add the PSRU to it. As with the real P-39 my study showed the best solution was to use a Casales drag boat "Z" drive in the nose. Even with the PSRU , the battery and the starter in the nose some ballast might still be needed. As Tiger Tim brought up, the gun(s) were heavy. In addition, there was a big CG shift between full and depleted ammo. I made an outline model with scaled components to play with the CG. I have attached an article from the defunct Expirementer magazine from a few years ago.

An interesting note: While working for Aero Uniion, a company that converted surplus military aircraft into forest fire bombers, we got many inquiries as to why we weren't considering the A-10 ground attach jet. It has the same problem, The minigun in the nose would have to be removed and replaced with a 1200 lb chunk of concrete.

Bigshu's comment about driveline loads is spot on. The combination of engien power pulses and gyroscopic loads from manuevering can induce torsional resonance problems that will eat driveline components. Any project like this will be a considerable development effort. Putting the PSRU in the nose, in addition to the CG problem relief, allows one to design a higher RPM drivshaft. This is good as the torque will go down accordingly and it's the torque that causes the problems.

Vince Homer

 

[cluttonfred]

I disagree on the balance issue with a scaled down P-39. Cheat the wing forward and the cockpit aft just a little and you’ve got the pilot sitting just ahead of the wing 25% chord line. Use a light engine mounted as far aft as possible with a prop extension and put the battery in the tail and it can work as long as your performance goals are modest.

 

[Hawk81A]

Again, "the issues" : People want to compare weight and balance to the real McCoy, and there is a WORLD of difference. Secondly, people want to design a replica that performs (aerobatics / speed) like the original.
I've mentioned elsewhere that I had a conversation with Mike Loehle (way back in the '80s) about the possibility of a "Loehle type" P-39. It could have used Rotax right behind the spinner just like the old Cox control line P-39 did. Rotax probably isn't much of a solution these days. Dennis

 

[Bigshu]

I disagree on the balance issue with a scaled down P-39. Cheat the wing forward and the cockpit aft just a little and you’ve got the pilot sitting just ahead of the wing 25% chord line. Use a light engine mounted as far aft as possible with a prop extension and put the battery in the tail and it can work as long as your performance goals are modest.

I can't speak for the others, but my performance goals are modest, just getting a good performer in the LSA box will make a good fun flyer. Figure a way to get a second body in the cabin and you double the number of people who might be interested.

 

[Bigshu]

Again, "the issues" : People want to compare weight and balance to the real McCoy, and there is a WORLD of difference. Secondly, people want to design a replica that performs (aerobatics / speed) like the original.
I've mentioned elsewhere that I had a conversation with Mike Loehle (way back in the '80s) about the possibility of a "Loehle type" P-39. It could have used Rotax right behind the spinner just like the old Cox control line P-39 did. Rotax probably isn't much of a solution these days. Dennis

I think the weight and balance issues are certainly easier to solve than the drivetrain issue. Putting a 100hp engine up front may or may not solve them, but the only way to find out for sure is model it, build a mocked up test bed or two, then maybe a large scale RC test flyer. I wouldn't want to go straight to pulling rivets until I had some indication I was on the right track. Not looking for high speed or serious aerobatics, but it needs to be able to do a barrel roll or what's the point? Might as well build a static display if all it will do is short XC work. And $1000 hamburgers!

 

[Saville]

Again, "the issues" : People want to compare weight and balance to the real McCoy, and there is a WORLD of difference. Secondly, people want to design a replica that performs (aerobatics / speed) like the original.
Dennis

I don't see that high a level of desired performance...300+ mph and whatever stress the originals were built for (9G?). I would think that +6/-4 G and reasonable speeds - say RV-8 level speeds would be very acceptable.

 

[vhhjr]

I doubt many would like a P-39 replica complete with the authentic aft CG. It had a reputanion for being prone to non-recoverable spins, not something that the average pilot should want to deal with.

The unique design of the P-39 dictates the layout both externally and internally. Speciiddically, an engine behind the pilot and guns and a reduction gearbox in the nose. If you put a light engine in the nose significant weight will be required to make up for no aft engine. If you have an aft engine and no guns you will need weight in the nose.

It was suggested to move the wing and pilot to deal with the CG. I sugest that a very small change in wing or pilot position will detract from the appearance of the aircraft as a P-39 replica. If you have the wieght distribution the same as the original, you will still need some nose weight to get out of the aft CG condition.

However you decide to move forward, I hope you have success. After all, the goal of all these projects is to safely get air under the tires.

Vince Homer

 

[ScaleBirdsScott]

Will say if its anything like the LiteFighters I'm working on, pretty sure the consensus has been that as long as it looks like one from the discernable shapes, few if any people care about the wing to cockpit to engine positions as pertains to the real vs 'replica'. Maybe if its much bigger and closer to real size there would be more scrutiny?

The secret weapon is details.

 

[Hawk81A]

Oh, if people INSIST:

 

[Saville]

Oh, if people INSIST:View attachment 124354

Doesn't one of the early Yaks look like that?

 

[cvairwerks]

Looks more like an XP-77 on steroids....

 

[Hawk81A]

While I knew that the original prototype had the pilot behind the engine, (other than these drawings), I was not aware that any were ever produced this way. I kind of question the accuracy or authenticity of these drawings. My dad was in the Army before WWII started. he told me about standing guard on a P-39 that had bellied in during the 1941 Carolina war games. Many years after my dad passed, I got the book "Bent and battered wings", and there in the book was a picture of the damaged P-39 mentioned as during the 1941 Carolina war games. It looked like any other P-39 I've ever seen.
Yes, I do believe there was a Yak with a very rear positioned pilot. It had the engine way up front though. Dennis
P.S. a P-39D actually crashed about 1/4 mile from my house Oct 14, 1942 (of course this was 14 years before I was born).