400% Green Toys plane

[Tench745]

Maiden attempt 2. Promising start, but not a good ending.

Between the impact damage and the water it landed in, everything from the cockpit forward needs rebuilding. It takes up too much space on my table in pieces, so I'll get to that sooner than later.

 

[Tench745]

Today I remembered I own a cheap-o tachometer, so I pulled that out and checked RPM at full throttle. If I believe the tach (it was hard to get a consistent reading) The 12x7 prop was turning 6300 RPM, giving the prop a pitch-speed of about 42MPH, that's only twice the estimated stall speed, so not much of a margin. Running the numbers through eCalc, it looks like a 10" pitch would get me a better safety margin. My 14x7 prop would have more thrust, but top speed would still be pretty limited. I may have to look into a 14x10 prop before the next flight test.

The rebuild is well under way. The fuselage is reskinned, the gear is glued in, and the wings are in process.

While trying to get the gear in straight I noticed that the tail is twisted a couple degrees relative to the wing. That may be the result of the last crash, but it looks like I might have built that twist in when skinning the fuselage originally. It's hard to tell. It's no bid deal, but I'm sure it'll take a little trim to correct once I get the plane flying.

 

[Inq]

Coming from theoretician side as I'm no expert so just a random comment or two... as your speed increases, it'll unload the propeller so 6300 rpm and 42 mph won't be the top-end once underway. And increasing the pitch... would seem to make things worse at low speed. You'll have less margin of thrust at static and when close to stalling when you need it most. It will also make it take longer to take off... which I see is already difficult.

 

[Tench745]

Coming from theoretician side as I'm no expert so just a random comment or two... as your speed increases, it'll unload the propeller so 6300 rpm and 42 mph won't be the top-end once underway. And increasing the pitch... would seem to make things worse at low speed. You'll have less margin of thrust at static and when close to stalling when you need it most. It will also make it take longer to take off... which I see is already difficult.

You're right that it will unload some, but ecalc was giving me warnings that a 7" pitch was low and even my initial back-of-the-napkin calculations using motor KV recommended 8".
You're also right that a higher pitch will have trouble at slower speeds. A 10" pitch will actually be partially stalled until it is moving forward fast enough. But, it also offers slightly more thrust than the 7" pitch. My LHS (local hobby shop) had a 13x10 that I picked up today to try out. That should give me a higher top speed but also more thrust than a 12" diameter prop. This should still be within the capability of my power system, it will just reduce flight times a bit. I'll run a test sometime today and see what kind of numbers I get.
I may also do some high-speed taxi tests before next flight so see how the different props affect takeoff roll.

 

[Tench745]

Threw the 13x10 on today. 21.2A draw at 10.7V with a static thrust of about 1.2 lbs on my fish scale tied to the tail. I am sure the scale on the tail thrust measurement is not accurate.
Interesting to note that, at full throttle, the prop became noticeably louder probably due to turbulence from the stalled blades.

 

[Inq]

I'm just thinking out loud... so take it with a grain of salt. My previous comment was kind of out of place and didn't take into account the huge cowling. You're obviously needing the larger prop just to get some blade sweep area outside. And all the blast heading toward the cowling has to divert around. You've got to be losing all kinds of efficiency, not to mention adding turbulence to even get a small portion of forward thrust. Is there any opportunity without spoiling the cosmetic theme to include internal ducting? Something like a Bearcat or FW-190 might have but exaggerated for passing blast? It might even aid motor cooling. Something like...

 

[telnar1236]

It's a cool project and definitely brings back childhood memories, so I hope I'm wrong on this, but looking at the plane, you might need to increase the size of the tail, unfortunately for the toy plane look. The combination of the wheel spats and fat fuselage probably result in poor lateral-directional stability, and, looking at the video, the plane seems to have some real wing rock and poor tracking.

 

[FlyerInStyle]

I'm just thinking out loud... so take it with a grain of salt. My previous comment was kind of out of place and didn't take into account the huge cowling. You're obviously needing the larger prop just to get some blade sweep area outside. And all the blast heading toward the cowling has to divert around. You've got to be losing all kinds of efficiency, not to mention adding turbulence to even get a small portion of forward thrust. Is there any opportunity without spoiling the cosmetic theme to include internal ducting? Something like a Bearcat or FW-190 might have but exaggerated for passing blast? It might even aid motor cooling. Something like...
View attachment 233481

I did something similar on my p26 and it improved my thrust by a significant margin, atleast I believe so.

 

[Tench745]

This is where things sit now, all put back together.

It's a cool project and definitely brings back childhood memories, so I hope I'm wrong on this, but looking at the plane, you might need to increase the size of the tail, unfortunately for the toy plane look. The combination of the wheel spats and fat fuselage probably result in poor lateral-directional stability, and, looking at the video, the plane seems to have some real wing rock and poor tracking.

You may very well be correct about that. That's part of the experiment here. My goal is to keep the outlines as close as possible to the toy, so I'd like to exhaust other options before enlarging the tail. That said, I think I could increase it 10-20% before it started looking odd.
I would expect the large spats to destabilize yaw more than pitch or roll and yaw didn't seem to be an issue in the air. It's difficult to tell when you've got less than 30 seconds to figure out what the heck is going on.
Before the maiden I had set up triple rates in my radio so I wouldn't be over-controlling all over the place. The ailerons seemed pretty effective in the brief time I had this bird in the air, but the elevator was just barely enough. That makes sense since the ailerons are in clear air and the tail is not. About half way into the second maiden I switched to mid rates to get more elevator authority, you can hear my radio say "Mid Rates". As soon as I did, the elevator felt much closer to my expectations, but it also upped aileron rates, making them far too responsive. This may be some of what you're seeing in the video. Also, if I was close to stalling, that could also account for some wing rock.

I'm just thinking out loud... so take it with a grain of salt. My previous comment was kind of out of place and didn't take into account the huge cowling. You're obviously needing the larger prop just to get some blade sweep area outside. And all the blast heading toward the cowling has to divert around. You've got to be losing all kinds of efficiency, not to mention adding turbulence to even get a small portion of forward thrust. Is there any opportunity without spoiling the cosmetic theme to include internal ducting? Something like a Bearcat or FW-190 might have but exaggerated for passing blast? It might even aid motor cooling.

I had thought about adding a couple cutouts at the rear of the cowling to improve airflow, or if I want more of a warbird look I could model some cowl flaps. Again, I don't want to make that kind of modification just yet. What may be hard to see in the pictures so far is the amount of airflow that is possible through the fuselage. The firewall has 4, 1-3/8" diameter holes in it to allow airflow into the fuselage and the cockpit is open to the interior of the fuselage. Maybe cutting some holes in the sides of the central box at the first bulkhead would let the cowling de-pressurize some without changing the external looks. I'll explore these options if the next taxi tests don't look promising enough.

 

[FlyingTyger]

Baffling the cowl/nose might help.

I built a warbird years ago that had some similar control issues. Initially the cowl was open on the front with little area for air exit.

In the air the tail didn't have a lot of authority and needed constant correction. I describe it as feeling like the tail was always trying to pass the nose. I added a baffle in the cowl at the back side of the dummy radial so that the only air that could get into the cowl was the small amount over the engine needed for cooling. The plane felt much more comfortable in the air like that. I figured that the air was collecting in the cowl, pressurizing, and causing turbulence. The baffle forced it around without compressing into the cowl area.

Just a thought.

 

[Tench745]

Going to try flying this one again today. Turned the Elevator rates up from 40% to 60% with 20% Expo. Also upped mid rates from 60% to 70%.
Aileron rates got turned down from 50, 60, 70 to 40, 50, 70, and low rates got 10% Expo.
CG is towards the forward end of the range. I'll report back tonight.

 

[Tench745]

Maiden 3 went very well until it didn't. I went to fly at my usual park, but there was a disc-golf tournament going on, so I headed over to my secondary field at the middle school. There isn't a great pavement area to fly from at the school, so I took off from the grass. The tail skid behaved much better on the grass than pavement, letting the plane track straighter. There seems to be a lot of gyroscopic precession on this plane as the tail swung around quickly as it lifted. I had room to continue the takeoff on the new heading, so I kept going. It lifted off reasonably well and was entirely comfortable to control. It needed left aileron to fly level, and as I was adding trim, the airplane slowly decended for the ground. For whatever reason I wasn't able to stop it, I don't know if I just didn't catch it in time, or if there was another cause. Damage is relatively minimal; left wing sheared off, cowl crunched, left spat wrinkled, broken firewall, and dented fuselage skins.

This flight showed me that it can fly and can fly well, so I am relatively happy with the results. I'll repair it and try to fly it one more time; maybe see if I can land. The next crash will probably be time to dismantle it. This thing is pretty chonky and I don't want it taking up space on my table if I have to rebuild every time I fly it.
If people are interested I can share the cut files and y'all can try it for yourself.

 

[Tench745]

Sick of having this plane sitting on my table taking up space, so I took it out for one last attempt at flight.
This flight was only 17 seconds long before the crash, but I learned things.
This was my second flight from grass The fields had just been freshly mowed so it was nice and short. A little power, get the tail up, and she starts rolling, feed in some rudder to keep it going straight and for the first time I felt like I was keeping ahead of the airplane. Elevator and rudder authority are were perfect, but the climb-out was slow. I didn't quite have full throttle in, but I think a bigger motor or prop would make for a more confident climb. I had to bank to the right to keep away from the nearby shed and pond, but over-controlled. Cue pilot induced oscillations as I repeatedly over-corrected trying to level the plane. Meanwhile I am losing altitude in the turn until wingtip meets ground in an almost wings-level attitude. Ripped the starboard wing off and crunched the firewall. Very repairable, but I want my table back, so I'll be dismantling this one until I can rebuild better.

Lessons learned:
-The ailerons are still very touchy; big ailerons in clean air and not much wing means an impressive roll rate.
-More power would aid climb performance.
-A reinforced firewall might be a good idea too, this one keeps cracking at the webs between the ventilation holes.
-A better/stronger wing attachment needs to be figured out. The stubby fuselage carry-through is not enough to keep the wings on in a crash, and gluing the wing root to the skin always results in a crunched fuselage side.

If anyone is interested I can put together a set of plans and y'all can experiment with it too.