Detlef Kunkel

Mitch,
nice work, and a nice deja-vu for me wih the prop blades.
I started roughly at 20" pitch ( at 70% of the blade length) and had 3600 with the new 300 engine.
Today it runs at 4000 ( same pitch) and makes a nice powerful flying, just as a Bearcat should.
In other word it really goes FAST ( 250 km/h) straight out


Can you tell the weight of the Sierra tail unit? doesnt look too light-weight for me, using a directly mounted Servo , too...??

BTW I sold my BC last week (of course without engine)
I am going to start a new one, with a more scale landing gear.

mitchilito

My Feedback: (7)

Detlef, it sure is nice having people like you and Rob to consult with on this project. You are going to save me a LOT of heart ache!

I went for a little more pitch because I think just like the actual Rare Bear I'm going to be going a little faster than you guys. Biggest difference is going to be the big old spinner I'll have on the front (and the other aerodynamic improvements).

The Sierra tailwheel retract is NOT light. I'll weigh it (with servo of course) and let you know. My thinking is, with that boat anchor 300 up front (not to mention my heavier cowling and spinner) the extra weight ought to balance right out. You intimately know the balance on this very aircraft: don't you think I'll be close? As for the scale accuracy, it' not really that close. I doubt it's going to meet your demands. I'm not really concerned about the little details (at all). I just insist on having an accurate (stand-off-scale) outline at a couple of feet away and a functional airframe that will take the abuse this one is going to dish out.

You must've loved your bearcat to want to build another one from scratch! We sure know the weak spots to fix. You MUST document your build on RCU for our enjoyment!

rbgetz

Hey Mitch. I took some pictures on how I pitched it. I don't have a real jig to make it easier, but will probably make one over the winter. I'm using the standard 30 X 18 pitch gauge. I normal just set the hub on a flat surface, like granite, but since the blades are wide, I set the hub on a prop drill jig to get it a bit higher off the surface or the blades will hit. I measure right at the edge of the yellow blade tips. The 18 pitch is a little too much, and I found out that if I lift the back of the blade 1/8 inch off the pitch template, it seems to be perfect for these. I know it's not scientific, but I tried multiple pitches and didn't have a jig made at the time. I haven't touched it since and the Moki 300 has good rpm and pulls that 55 lbs plane really nice. I rarely fly it over 3/4 throttle.

mitchilito

My Feedback: (7)

Rob, I appreciate you taking the time to post your prop measurements. I'm posting two pictures. The first is the pitch gauge that was supplied with my prop. It came with a variety of wedges (I'm using the one marked 20") and as you see in the second picture it references the prop right at 70% of span.

Looking at at how far out you are referencing I'd guess you are also close to 20 inches at 70% span.

rbgetz

That must be something new that SL is sending with the props. I never got that gauge. When I originally bought props a few years back, they just gave me a couple pf paper templates on that you had to cut out. I'm looking forward to seeing your finished model. This winter mine is going to need a refresh.

Detlef Kunkel

Mitch,

I think about the Bearcat just like you do. It is no superscale model, but close concerning its outline. I dont care about small details on this one.
Biggest fault from some distance is the main gear, and thus the upright position of the airframe on the ground. My next one will have the double folding long legs and some other small improvements. I will try to save some more weight, because the scale retracts will surely be heavier than he usual ones.
We have the 25 kg limit here in Germany. Above 25, we need certificaions, annual checks and some other nasty things more. OK for a really big model, but too much trouble just for adding a scale gear, understand?
With a new build I have a fair chance to stay under 25.
We even will have a mould for the new wing with the attatchment points for the scale gear, and again have the chance to build a light hightech wing.
One thing comes to the other.

Your thinking about the heavy tail gear and the "anchor" ( nice) in the front is roughly correct. Is your motor position still the same with your long cowling?
In my case the tail gear was made with the same thinking out of very thin steel sheet metal. The round main housing that holds the steering axle is not even 0.5 mm thick walled. But I insisted on a nice scale wheel ( thin walled but Aluminium with tiny ball bearings) and in the end I had 450 gr alltogether. No servos, as I use wires attached to the rudder servo . The previous owner made a nice scale cockit with scale Pilot, but weighing 1 kg ( behind CG- )
Finally I had to drop the idea to have my elevator servos mounted in the tail fins. It was already prepared so but had to be re- changed.
I moved the servos to the main section and run long carbon rods to the tail fins. Had I used servos in the back, my CG qould have been too far aft.

Detlef Kunkel

one other thing- please regard the full former of the tail gear
Some fuselages are quite soft. If you attach the elevator, let a friend hold the fuselage nose tightly and the go twist the fuselage with both hands outside at the elevator, you see the big fuselage get deformed at the beginning of the rudder fin.
It looks so strong and big, but the elevator is extremely tall. In a way taller than the fuselage is made for (!)

If you put in a 215- might be OK.
But with a 300 you have pretty good chances to get tail flutter.
A full former ( or other reinforcement) of this section helps a lot

mitchilito

My Feedback: (7)

Detlef, I'm REALLY looking forward to seeing what you do with your new Bearcat. New wings?? That should be very interesting.

I'm putting some strategically placed kevlar cloth reinforcement tapes in the aft fuselage to stiffen, but not too much. I don't need the weight. I feel pretty confident it will hold up.

My engine will be just a bit further forward (with a big spinner on it). Maybe an inch. Plus I will not have anything in my cockpit other than a small John Penny bust.

Rob and Detlef: do you guys have anymore flying video of your Bearcats? If so, I'd pay money to see them!

Detlef Kunkel

Mitch,
there are some videos, but I purchased a new cam and was still trying like a greenhorn to take some useful movies. There are none until today. Someone would need a lot of time to combine useful fragments to a useful video. People would only laugh at the cameraman ( me) if I load up in YouTube what I have here uncut. Sorry so far...

mitchilito

My Feedback: (7)

Rob, didn't I read earlier in this thread that your CG came out good? I know you are using the same engine, and the same Sierra tailwheel assembly. Are your elevator and rudder servos mounted in back? If so I should be fine. . . . . .

rbgetz

My servos for the tail wheel, rudder and elevators (2), are mounted about mid-fuselage. they are not located in the back. I have carbon fiber push rods for each elevator and I have a pull-pull for rudder and separate one for tail wheel assembly. I think I have a few pictures of the location, but I'll check when I get back home tomorrow night. If not, I'll take some photos and share.

Detlef Kunkel

In case this is of interest here, I would tell you a little Story.

You might remember that I sold my faithful Bearcat. Shame on me.

The reason was the main gear. Since I saw a Bearcat model for the fist time, I thought how nice it would be to have a scale retract gear on that powerful Cat
Now I started to work exactly on that

Some folks were trying and designing around the same case, but you rarely see one on the flying field or in the air.

You might also know that I really like the challenge of thinks like that, and landing gears really ARE a challenge, exactly when the bird is a Bearcat.

Went to Duxford this fall and asked some friends there if I could get access to the live Bearcat.
They were friendly and Patient enough to let me climb under the wind and do some measuring and take a lot of pics.

Back home, I drew lots of ideas and dimensions on some pieces of paper - a lot of.

Then I started to make a primitive wooden dummy, simple, but adjustable in any angle.

Detlef Kunkel

A friend borrowed me an old mould of the wing, so I could assure that everything would really fin inside the relative thin wing.

Took me about three weeks to develop this test gear.

At the beginning, every item was adjustable.
each design aspect that proved to be correct, finally was replaced by a non- adjustable item

Detlef Kunkel

Studying hundreds of pictures and my measurements brought me closer and closer to the final design.


Next was to fit everything in the wing.
Laminated the inner section of the wing, with some extra strong layers, just to have a solid home for the further gear evaluation

Detlef Kunkel

pics

Detlef Kunkel

The lock is one of the most important features of this gear.
My idea was a mono action function with auto-lock and unlock function, using one big air ram that does it all automatically

The mechanic uses 6 mm of the cylinder lift for the locking / unlocking, the rest of the travel is directly lifting the gear.

The longer I worked, the more fun I had seeing the thing getting better and better

Greg Wright

My Feedback: (6)

Nice work Detlef!!!!!!!!


Will you be making the wing and gear available to purchase or is this just going to be a one of a kind?

You've really got me excited seeing this project. The Bearcat is one of my ultimate dream planes for a scale warbird.

Detlef Kunkel

Hi Greg,

Sorry I did not ask you before I was going to kidnap your thread here. Glad you like it

I have not decided yet what I am going to do. Together with two other friends we are going to make 3 Bearcats, and hope we will find a machine shop, toolmaker or whatever who can produce 3 or maybe 5 sets of this gear, once I am done with the design.
We are going to redraw the gear in a CAD program, hoping that someone will do the machining job.
This mini- series of 5-axis milled parts will be an expensive enterprise, that's for sure.
For me, it would be nice to have just the main parts roughly machined, and do some finish work here in my shop. Only the 2 other guys are not able to do it that way, either I do it for three gears, or we all have to pay a lot of money to have it made perfectly. You see, nothing comes easy.


If not, I will make just one set for me as I am lacking the time to make a lot of parts for third parties. I would do so if I had more time, but it really does not look too good.
So I cannot answer your question with a yes or no, it just depends.



Going on with the construction.

On the pics I marked the non-parallel centers of the main bone, also the twist angle between upper and lower part. Not an easy part to machine.
Next is the lock and its operation. Hope the pics explain the idea. I try to stay as close as possible to the original Bearcats design. The only main difference ist that I am not going to power it by hydraulic means. I did that once, it worked OK, but you have probably oil wherever to look. I like pneumatics.

Detlef Kunkel

the strut support was first made multi-adjustable. Later was replaced by the final design, using an excenter and finding enough support in the main spar and the wheel well wall. Will be carbon parts later.
Access to the clamping of the excenter via 2 small non-scale holes in the wing. Can live with that.
Took a while to come to this design, as the forces in the strut can easily exceed 150 kg ( 330 pounds) (!!) when turning sharp corners on concrete. You will not just glue the bearing to the wall, for sure

Detlef Kunkel

evolution leaves its traces


But later the reward was the view of the Cat, sitting in the correct height and angle, on my bench. Yes, it took three or four weeks and a journey to the UK, but it was worth the effort. I am totally pleased with the result, very motivating

rbgetz

Hey Detlef,

I know this may sound crazy, but I was actually thinking of building another Bearcat at some point. My question is, based upon the 3W kit, do you feel the gear you're designing will fit with slight modifications to the current kit, or will it require significant mods to the gear mounts from the factory?

Rob

Detlef Kunkel

Rob,
you could modify the original 3W wing, but it is really a major project.
You would have to cut away half of the under wing panel, to get the main rib , main spar and wheel well wall ( all made of multilayer plywood / carbon) installed, and the re-glue the cut away planking.
The attachment point have nothing to do with the 3W wing.
The new main rib is positioned much more inward.
That's why we make a new wing mould, just because of that situation.
The mould I have here has several damages. It still works, but you would need to fill and putty some spots later. No big deal, but again not ideal for making 3 sets of wings

Detlef Kunkel

the 3W gear sits where this wheel well ends, that is quite a different position

Detlef Kunkel

Last but not least the cylinder- dummy is positioned and trimmed to the correct travel ( 77 mm).
Piston dia is 28 mm. This provides a lot of force to move the gear trough the prop wash safely.
My old Bearcat had problems with that using a 24 mm piston. I made bigger cylinders and from then on everything worked fine. I like pneumatic driven gears with big and strong cylinders. I build everything by myself, and a truly can say that such a system never ever failed since

mitchilito

My Feedback: (7)

Beautiful work, Detlef! You show a real dedication to the perfect outcome. My gear is more of an answer of how to fit a set of gear in the existing space. There's just no way I would be willing to make new wings!

The only real problem I worry about with my remanufactured Century gear is the small-ish cylinders. I had thought of going hydraulic but of late I'm really considering big electric linear actuators. Strength is not going to be a problem.