BillHarris

One of my future projects will be to bash a BTE Flyin'King into a twin-engined bird. I want a photo-airplane with an unobstructed nose for a camera and a twin configuration is ideal. Not tomention, this will be a good twin trainer.

See http://www.btemodels.com/ for info in the FK.

Some of my ideas, yea or nay:

1. The FK has a one-piece wing with a flat center wing section and dihedral wingtips. The center section is about 30" in length and each wingtip is about 24" long. I propose a three-piece wing with removable tips for ease of transport. The engines will be placed in nacelles below the wing center section at 10-12" from the center.

Question: how far should the engines be placed from the centerline?

2. I plan to use a pair of Thunder Tiger 46 Pro BB engines (or equivalent 4-strokes) and size the engine nacelles to hold 10-12 ounce fuel tanks. Or I may use small header tanks in the nacelles and a large 20-24 ounce main tank located in a pod at the center of the wing. Or I may mount the tank under the wing so that it fits into the fuse when the wing in bolted on. There should be enough room in the fuse for that.

Question: the plane flies well with a single 90 4-stroke and is designed for a 60-size 2 stroke. I may be able to smaller engines in the 30-36 size range?

3. The FK has flaps in the center wing section and this Twin will retain the flaps even if it means splitting the flap panels on either side of the engine nacelles. I'd like to use slotted flaps, although the linkage will be more complex.

4. I prefer taildraggers and would like to make this a taildragger. Also, as a photo-plane, the nose wheel of tri-gear could interfere with equipment in the nose. But I've not seen many model taildragger twins (although the DH-89 Dragon Rapide was a taildragger twin, and a bipe to boot).

Thanks,

--Bill

BillHarris

Is there a general rule of thumb on the fuse-to-prop clearance on a twin? For a 10" prop, my "TLAR" criteria would seem to give me 2" minimum clearance.

--Bill

William Robison

My Feedback: (3)

BillH:

There is a certain small effect of having prop tips close to the fuselage sides, but so long as the tips are just close it's not to worry about.

All else being the same, the twin with closer spaced engines is easier to fly with an engine out.

Tanks? Header tanks are not really a solution, You see the header tank and the remote main, the carb still has to pull the fuel from the main tank to the header tank. The engine sees the total length of fuel draw, not just from the header.

Muffler pressure is not a solution - if one engine quits the pressure from the other blows through the tank and vents through the exhaust of the dead engine, and you then have two dead.

The Cline system works fine for a single main tank, the header tanks are not needed.

My favorite way is using pumped engines, or a Perry mump on each engine. Works fine. And still doesn't need the weight of the header tanks.

Your three piece wing should be fine, it's the way I would go.

And there's no reason not to make the plane a taildragger, but I don't think the nose gear brackets etc. would really interfere using a small format camera.

Engines. The TT46 engines should have enough power, but you're going to be adding some structural weight also, you might think about the Magnum 52 engines for that little extra.

Bill.

BillHarris

Good info-- this will get me started on kicking around ideas on these kit mods.

I may have a wrong mental image of how header tanks function. I was thinking that the since main tank is pressurized and the fuel was being "pushed" to the header tanks, the draw to the main tank was not a factor. Putting check valves in each muffler pressure line would keep the system from losing pressure when one engine quit. But header tanks and check valves is getting too complex, so I'll apply the KISS principle here.

I was a bit conservative on the engine size-- it would fly on a pair of 46's, but there wouldn't be much margin for cargo or engine failure. I'll add some displacement...

Thanks--

--Bill

William Robison

My Feedback: (3)

BillH:

This is still my favorite method. The pump is just behind the firewall, operating hose to the engine is just a little over 1" long, the fuel tank is in the main fuselage on the cg.

Bill.

BillHarris

Then a pair of pumps it will be, Wm. I like that pump and throttle servo arrangement in your attached pic.

--Bill

William Robison

My Feedback: (3)

BillH:

Thanks for the kind words. You can just barely see it, but the aileron servo is right next to the throttle servo.

Picture 1 is a better view of the servos, the throttle is a Futaba S3101 micro, the aileron beside it is a S148 with the EMS ball bearing conversion.

Picture 2 is a view from the rear showing the servos again, but what I want to show is the ball links on the push rod. Everything in the plane is totally ball linked except the throttles.

The airplane is eight years old, and I've never had an in flight failure - not even an engine out.

And the engines are a lasting testimony to Magnum, they are XL 46 engines. If I ever manage to wear them out (and not destroy the plane) they will be replaced with a pair on XLS 46s.

Picture 3 shows one of the things that has preserved them. They have never flown without their Bru-Line air filters. New element in the one shown, they turn black in service.

Rats. It's not letting me load the pictures, have to find out what's wrong. Don't really need pictures forthis one anyway, might get them up later.

Bill.

BillHarris

I'll look forward to seeing the photos when "we" got over our growin' pains...

We fly from a semi-grassed field, and for a long time I would fasten a piece of mosquito netting over the carb intake to keep gravel and other debris out. Your recommendation of the Bru-line filters is food for thought.

--Bill

BillHarris

Putting the preliminary design pencil to the Flyin'King plans:

80" wingspan, 16" chord, two 50-60-size 2-strokes with 11" props. Weight will be up to 12 pounds and may carry up to a 5 pound payload.

The engines will be mounted on pylons below the wing and will draw fuel via pumps from a common center fuel tank in the 16-20 ounce size. The engines will be mounted 8" ahead of the CG at 10" from the fuselage centerline. The props of the twin engines will be 8" behind and 1" above the location of the prop of the design single engine.

Questions:

How much right thrust and down thrust should be incorporated into the engine mounts?

What sort of added reinforcement should be done to the wing at the engine mounting points? I see tying the engine pylons to the main spar, extending the center sheeting out past the pylons and possibly putting in a diagnonal brace from the pylons to the TE at the wing mounting bolts. What forces are involved with the engines?

--Bill

William Robison

My Feedback: (3)

BillH:

I would leave the engine thrust line at zero-zero. By raising the prop centers you have added a little effective down thrust, and with the 80" span the offset thrust will not have a great effect with an engine out. What you might do is lengthen the after fuselage by five to ten percent, which will give greater rudder authority and help to balance the increased engine weight also. And one further aerodynamic note: You can never have too much rudder area on a twin.

My very first twin, the Tiggerkitty, has basswood mounting beams in the wing structure, with bass spars running from the wing main spar forward to the firewall of the nacelle. It is far overbuilt. Great Planes, on the TwinStar, cut the leading edge and sheeting away between two ribs, and glued the nacelle sides to the exposed ribs. From the Tiggerkitty I went to ply formers in the nacelle, extending into the wing structure glued to the leading edge and the main spar. This is the way the C-3/10 was done, you can see a bit of it in the picture showing the pump. From there to only one former going in and glued to the main spar. This is the way Northeast Aerodynamics has the Twin-Air 45 built. My latest designs are the simplest of all. I've adopted Dave Platt's method which I first saw on the Duellist 2/40. It's also the lightest. Sheet the wing normally, being sure of the fit and glue joints to the ribs, then just glue the nacelle sides to the wing sheeting, being sure the sides are aligned with the ribs. I've never done one completely under the wing this way, I've always had the nacell lap over the leading edge to the side opposite to the main attachement for a little added strength. And it looks better, too. You can see the overlap in the attached picture of the Tiggerkitty - it was done there for both scale reasons and appearance, it surely doesn't need the added strength.

Your diagonal brace wouldn't be a bad idea, either.

Bill.

twinman

My Feedback: (2)

There is a company called Super Circuits at http://www.supercircuits.com/STORE/home.asp
They have a video tape of just such a twin engine plane with an on board video camera.
Suggest you contact them about the plane and possibly the equipment to do this.
Do a search using "Radio Control".
Good Luck,
Twinman

BillHarris

Thanks, Wm, this is useful info. The Flyin'King has a short tail moment to begin with and it wouldn't hurt to lengthen the tail. On the engine mounting, my usual tendency is towards overdesign and your comments will help to apply some of the KISS principle to this bash....

Interesting link to Supercircuits. I have a copy of the Incab R/C video tape.

--Bill

William Robison

My Feedback: (3)

Okay folks, let's try the pictures now, the ones I referenced in post #7.

Picture 1 is a better view of the servos, the throttle is a Futaba S3101 micro, the aileron beside it is a S148 with the EMS ball bearing conversion.

Picture 2 is a view from the rear showing the servos again, but what I want to show is the ball links on the push rod. Everything in the plane is totally ball linked except the throttles.

Picture 3 shows one of the things that has preserved the engines in excellent condition for eight years of service. They have never flown without their Bru-Line air filters. New element in the one shown, they turn black in service.

Bill.

BillHarris

That is a good setup on the engine nacelles. My first thoughts were to mount the engine nacelles below the wing, but I'm starting to think along the lines of mounting the engines along the chord line, as you have done here. Except for certain special purposes, like mounting the nacelles low for landing gear or high in a seaplane to keep the props out of the water, there is likely no reason to mount them high or low, except for aesthetics.

One of the tricks to preclude tip-stall is to make the LE of the inner wing sharper so that it stalls before the outer wing. JOOC, does sticking engines on the wing alter the stall characteristics of that wing section? I can see that the airflow behind the engines can be markedly different than if the wing were moving through the air.

--Bill

Wayne22

My Feedback: (2)

Don't forget your mainwheels are going to be down there somewhere under, or in front of the props, and they will kick up debris.....

The FK has tons of fin area and is very stable laterally. I would just increase the rudder chord.

I was toying with a similar idea, but going one step further and make it a twin engine flying boat...it is still in the "maybe" stage.......

William Robison

My Feedback: (3)

Bill:
With a high wing, as you have here, you will find much less difference between power on trim and power off with the engines under the wing. For neutral trim with throttle has to be on or slightly above the line of the vertical center of gravity; if the plane is hung by the prop shafts it should hang vertically. At least that's a place to start, as, believe it or not, the ideal position changes with speed. Remember drag?

As the air speed goes up the drag will also increase. Since the wing gives a large percentage of the total drag, the high wing all by itself will tend to make the plane go to a nose up trim with increasing air speed. This gives an amount of automatic airspeed control, but at the same time it also gives you effective up trim to the thrust line.

The compromise that works for me is to estimate the drag center line and the weight center line, and put the thrust line 10 to 15% of this distance above the weight center line.
Increasing the leading edge radius from the root to the tip is a trick that works very well. it gives the effect of washout without having it work against you in inverted flight. The new Funtana from Hangar 9 is the first popular model I know of using this.

On an airplane not intended for aerobatics washout works just as well, and I think the wing with washout just looks better.

And the lost prop wash over the wing behind the dead engine is the second major cause of the airplane turning into the dead engine - the wing loses lift. the plane tends to roll.

Think I've covered it, If I missed something or wasn't clear ask again, please.

Bill.

BillHarris

Nony--

Good point. I hadn't thought of the main gear _vs_ the props. The FK has decent tail area already and doesn't need to be increased. My original FK is a test bed for STOL enhancements and one of the early modifications was to increase the rudder and elevator areas, keeping the fin and stab areas the same.

Good comments, Wm. No questions as of right now.

Since I am increasing the "dry weight" and the payload of this FK, I am toying with the idea of increasing the wingspan (and wing area). From 80" wingspan (1280 in^2) to 100" ws (1600 in^2) or 120" ws (1920 in^2). With a 3-piece wing, transport of even the 10' wing will not be a problem. Of course, increased weight and wingspan will require larger engines...

Never ends, no? This isn't the first project I've had get into a "snowball mode"...


--Bill