TECHNICAL SOLUTIONS for Large Scale Models. By Jan Hermkens.
With the examples;
The B17 FLYING FORTRESS : a.o. the EUROPEAN CHAMPIONSMODEL STAND-OFF SCALE '94+'95+'96+’99 (Europa Star Cup)
The SHORT SUNDERLAND : a.o. WINNER EURO CUP SCALE-WATERFLYING '94.
The P-61 BLACK WIDOW : a.o. the EUROPEAN CHAMPIONSMODEL STAND-OFF SCALE 2000+2001 (Europa Star Cup)
Built by Jan Hermkens from own Plans.
Specifications Models : (all Woodconstruction)
Boeing B-17 FLYING FORTRESS
|Northrop P-61 BLACK WIDOW|
Douglas A-26B INVADER
1/8th (only the 1/10th plan is available !)
4,00m = 158”
3,45m = 136”
|3,10m = 122”|
3,60m = 142”
2,85m = 112”
2,64m = 104”
|2,28m = 90”|
2,55m = 100”
NACA 4415 Root/ 2415 Tip
NACA 4415 Root / 2415 Tip
|Ownmix NACA 2414-2314/ 0014 tip|
Ownmix NACA 2414-2314/ 0014 tip
4x Laser 70 (11,5cm³) four-stroke
4x Osmax 46SF (7,5cm³) two-stroke
|2x Laser 240-V twin (40cm³) four-stroke|
2x Laser 240-V twin (40cm³) four-stroke
M.A. 13x6 (9000 RPM)
M.A. 12x6 (10000 RPM)
|Engel Carbon 18x11 (8000 RPM)|
Engel Carbon 18x11 (8000 RPM)
Own Construction; Electric
none (Flying Boat)
|Own Construction; Electric|
Own Construction; Electric
2x GR-24 Hott +2x Engel PMS Plus Powerbox
2x PCM double Superhet
|2x GR-24 Hott + 2x Engel PMS Plus Powerbox|
2x GR-24 Hott + 2x Engel PMS Plus Powerbox
My drawings are available (P-61, B-17 & Short Sunderland plans updated in 2010). The A-26B Invader plan is available now; 28 March 2017;
But from the B-17 only the smaller version 1/10th of 1983 (spw 3,16m).
For my 1/8th version I have cut out the necessary parts from my own 1/10th plan and copied them 125%, and have adapted the constructions a little bit for a fuselage-splitting and extra wing-splittings.
A-26B Invader. sp. 3,60m. Weight 21,5 kg
2x 40cc Laser 240V-twin four stroke
In relation to the transport and the manageable, the tailsection had to be detachable, and therefore providings were attached for the Fuselage-Tailsection joining and glued into some formers. At the joining so a dual former (Sandwich:2x 1mm ply with inbetween them 6mm balsa with plywood rings), these are bolted together with four socket bolts and blind nuts M4 (2mm balsa strips between them, this for.
The fuselage-joiner itself can be very light: flat springwire 6x1mm in flat brass tubes 7x2mm. The flat brass tubes are provisional in one piece and on both sides, glued in 2 formers (front- and tailpiece must be sheeted apart) and later, after the bolts have been removed, sawn through between the the dual formers (perfect fitting).
Fuselage-Tail Splitting Short Sunderland
Fuselage-Tail Splitting B-17 Fuselage-Tail Splitting B-17 (detail)
Tail-Boom Splitting P-61 Black Widow
Fuselage-Tail Splitting A-26B Invader
In the B-17 & Short Sunderland, all the Fuselage-servos are in the front of the nose (to avoid a lot of extra lead) and all linkages to the rudders are a closed-loop system (plastified control line wire), these linkages had to be disconnectable on the spot of the fuselage splitting.
On a 3mm ply strip, in the front of the tailpiece, there are 3 cranks (3mm axle), which all are connected with kwiklinks and a closed-loop system to the inner control horns of rudder and elevator. On these 3 cranks there are also, all on the outside, threaded ball links (M2).
The connection forwards take place with alu. ball links and and moved on security clips, naturaly also with a closed-loop system to the servo’s.
The kwiklinks on the servo’s have to be disconnected every time!
In the P-61 & A-26, the servos for rudder and elevators are as far as possible forwards in the tailsection (In the P-61 for both booms!).
Linkage-Splitting Short Sunderland.
Linkage-Splitting B-17 just like the Sunderland. Linkages-Splitting B-17 (servo’s all in the front of the nose!)
P-61 Elevator & Rudderservos (upsidedown) in front of Tailplane.
A-26B Elevator & Rudderservos in the front of the tailsection. Linkages elevators & rudder of the A-26B Invader.
STABILIZER- and WING JOINERS:
-On the Sunderland the stabilizer has been glued, in one piece, into the Tailplane, the elevators are direct linked, internal via 2 apart built in cranked pianowires (3mm bended pianowire, bearinged in plastic tubes) with control horns. The elevators they have glued in tight fitted plastic tubes, and are slided over these cranked pianowires, not glued, so they are always removeable. So also the rudder.
Left Stabilizer-part B-17
Stabilizer P-61 with carbon join tubes Stabilizer P-61 with the elevator control tubes (so also B-17)
Elevator- and rudder linkage on the P-61.
A-26B Invader Stabilizer Joiners. A-26B Invader Elevator linkages.
-On the 1/8th B-17 the wing consists of 2 halves , which are slided to the fuselage with wingjoiners. The wing joiner on the front is a loose thinwall
Dural tube Ø30mm and the rear a Ø18mm one. (Each winghalve on the 1/8th B-17 is also divisible in 2 parts ).
(Each winghalve on the P-61 & A-26B is also divisible in 2 parts ).
These tubes are thus loose and in a single piece and slided into fiberglass guide tubes (glued in to the fuselage and innerwing halves).
The wings, also provided with guide tubes, are slided over the stretched out tubes and secured with long “Parkers”, through abachi blocks
(glued on the guide tubes) into the tubes, which are provided internally with short and small ash-wood or hickory blocks (on her sides and so making
the tubes even stronger against nodding).
Winghalve B-17 (innner & outer part)
Winghalve P-61 (inner & outer part) Winghalve A-26B (inner & outer part)
-Each winghalve on the 1/8th B-17 , P-61 & A-26B is also divisible in 2 parts and also these are connected with wingjoiners, but now glued in, and not loose in one piece.
The outer wingparts;
On the B-17: in front a Ø18mm dural tube and the rear one a Ø11mm carbon tube.
On the P-61 & A-26B: in front a Ø25mm dural tube and at the rear Ø11mm carbon tube.
On the spot of the deviding there is of course a dual wingrib (for each part, one) and during the open construction, the joining has to been attached, this for an accurate fitting! The security is done, as with the fuselage-wingjoiners, with “parkers through abbachi blocks into the tubes, internally also provided with ash-wood or hickory blocks. The stabilizer joining with the fuselage consists of glued in carbon tubes Ø11mm, forward and rear, which are slided into polyester guide tubes glued in the fuselage. Here also the security is done with “parkers”.
Splitted Winghalve B-17
Dural and Carbon Wing & Stabizer Joiners Outer Winghalve P-61
P-61 Inner Winghalve with Nacelle + Outer Winghalve.
A-26B Inner left Winghalve with partly the Nacelle. A-26B Inner right Winghalve with Nacelle.
-At the Sunderland the wing only consists of 2 parts (stabilizer glued on), which are connected with each other by a wingjoiner of flat springsteel 1,5x15mm in gluedin brass flat tubes 2,5x16mm, forward with 2 of them above each other, a long one untill the First nacelle and a short one. The rear is a short one 1,5x12mm in a brass tube 2,5x13mm.
(In 2010 drawing updated, so now, loose thinwall dural tube Ø 30mm and carbon tube Ø 12mm. So also for the 1/10th B-17, loose thinwall dural tube Ø 25mm and carbon tube Ø 11mm, and for the splitted Stabilizer Ø 9mm: Lighter).
The Winghalves for the Sunderland are secured to each other with 2 alu. strips and parkers in abachi blocks, and than bolted on the fuselage with 4 long stainless steel bolts M6, in to the fuselage fixed metal inserts.
Over this than comes the long cockpithood, which stretch from nose to the rearend of the wing.
Wing-Joiner Short Sunderland
FLAPS & SPOILERONS.
B-17 Split Flaps:
The real B-17 has splitflaps and on the model these were built directly against the wing underside, with clearfoil between it as a barrier.
The special flap hinges were attached, recessed into the flaps and wing under-surface. There is a gap between wing and under-surface when these are lowered, giving more lift and less drag!
The flaps are controlled with internaly M2,5 rods., and because of their huge surface linkaged to an 8kg servo.
With the computer transmitter these servo’s were slow downed to 5 sec., so that the nose does not go up, because the model has so the time to adapt itself to the flap situation, every moment!
Splitflaps B-17 (still to construct against the wing!)
Short Sunderland Fowler Flaps:
This are Flaps moving simultaneous backwards and downwards, giving a lot of more lift in slow flying or allowing even a more slower landing.
At the Sunderland model that had to be max. 9 cm backwards and 40° downwards!
But how to make this complicated system practicable for a model?
The end result looks very simple and finally it is so! But before that it was a long way to go. A lot of experiments, hours! For certain, because the Flaps have a trapezium shape, so the travelway is on each outerside different, and adjusting of them did require a lot of time.
The Flap itself was constructed again, with clearfoil between it, against the hollow underside of the rear wingedge. Also here, each Flap is linkaged to an 8kg servo, and for the same reason as at the B-17, slowed down to 5 sec.
From the servo there goes a rod (M3 with ballinks) to both sides of each flap to large cranks. From these cranks there are leaving 2 rods, both connected on the same side of the crank. One (the outer one; M2 rod with kwiklinks) for the there and back movement. To this rod there (beared in hardwood blocks provided with a piece of plastic tube) the Flap is connected, hinge jointed,. The second rod (the inner one; M3 rod with ballink and kwiklink) goes from the crank straight to the control horn of the Flap (the Up and Down movement).
How does it operates than?
Because the inner rod on the crank makes a shorter stroke than the outer one, the inner one really holds up the flap during the slide-movement,
so that the flap starts to turn over on his hinge-joint on the leading edge. The more larger sliding movement, the more turnover movement!
The extent of the turnover movement defines thus the angle of which the flap gets lowered and is thus dependent from the distance differance between the two side by side seated rods on the crank!
After a severe waterdamage, by a hole in the hull (an under water hidden boulder), the condition of the balsawood rapid declined, and at one point it was not justified to fly any longer with the model.
Fowler Flaps Short Sunderland (upperside)
Fowler Flaps Short Sunderland (underside)
P-61 Flaps and Spoillerons:
The P-61 had conventional slotted Flaps (Not the ZAP Flap as described in some articles or books! These were only on the prototype XP-61 !).
The spoilerons are laminated, in a halfround and tapered moulding, of 3 sheets 0,4 ply, and between them carboncloth + epoxy.
They are hinged (almost at the rear, on the upperinside of the the outerwing), on a long 3mm pianowire in a plastic tube, so it can turn out of the wing (therefore in the ribs are sawed segment shaped slots),
Flaps P-61 (on inner- and outerwing)+ spoileron & tiny tipailleron Spoilleron-linkage P-61 Spoilleron + tiny tipailleron P-61
A-26B Invader Flaps:
The A-26B Flaps had Double-slotted Fowler Flaps, but for the model it was to complicated to make it scale! So I made normal Fowler Flaps (but the linkage is different as it is in the Short Sunderland model).
These Fowler Flaps are going backwards max. 50mm en max. 50° downwards.
An 8 kg servo (again slowed down to 5 sec.) controls a long 4mm pianowire via a controlhorn in the middle, on this axle, there are 2 more controlhorns en these are controlling 2 pushrods (through long guiding tubes), linked on the upperside of the Flap-leading edges. On the same controlhorns there are M3 rods, but a little bit closer to the axle and linked on the underside of the Flap-leading edges.
On one outerside of the flaps there is a third rod (also through along guiding tube), but not controlled, just for extra guiding!
Because these M3 rods are controlled closer to the axle, they make a shorter way than the upper pushrods, and so the Flaps are turned over, more and more, during pushing.
Very simple, but adjusting the whole, took a lot of time, because the flaps are not rectangular but tapered! So the push-movement is on both outersides different!
COWLINGS, MOTOR-DUMMIES, TURRETS, etc.
Cowlings Short Sunderland & B-17:
These have all over, the same cilindric dimensions. For the Short Sunderland & 1/10th B-17 ; Ø150mm (6") and for the 1/8th B-17 ; Ø180mm (7").
Therefore these can be made from aluminium stove- or flue pipes (available in many diameters), the rounded nose cowlring can be made from GRP, with the so called balloon method (see further on!). A plywood ring has been glued half into the pipe, on this the self formed GRP ring can be glued. For this ring you have to make a wooden mould, like the other moulds(3m ply bottom + some 3mm ply cross-sections + filling with balsa, and than planing and sanding).
Balloon method Polyester parts
Cowlings P-61 & A-26B
These are not the same diameter over its entire length, so the methode for the Sunderland & B-17could not be used (aluminium stove- or flue pipe with a GRP nose cowl-ring).
GRP Cowling P-61 with radial Dummy A-26 Mouldings & Plugs for Cowlings, Cockpit, etc. GRP Cowling A-26 with radial Dummy
Radial Dummy Engines;
After hardening, there are three possibilities:
2) Alternatively; use the negative mould to form the dummies with GRP.
Sometimes you can buy them, also for some purchased plans, but certainly not for your own designs!
Kitchen oven methode:
Bond to the plug, a block as a spool handle. Then make an increased framework of 6 to 10mm plywood, make an opening, 1mm around larger than the basic shape of the plug, place over this a cockpits appropriate 0.5 to 1 mm thick plastic plate (clear PVC or Buyrate), tighted around, and place it in the kitchen oven (± 200° C).
Tail Turret Short Sunderland
Nose Turret Short Sunderland
Chin, Sperry Ball Turret, and Tail Turret of the B-17, the Top Turret and Nosedome of the P-61 & A-26B were also formed with GRP on positive moulds, against which the large balloon was pushed again, as defined by the nose ring of the cowlings. For the round Sperry Ball Turret of the B-17, I have used the cup of one of my many trophies as a template! (just the right size).
Nosedome Plug B-17
Cheek Gunshields Plugs B-17
Nose Dome & Chin-Turret B-17
Cheek Gunshield left side B-17 Removable Cockpit Hood B-17 with Top-Turret
Sperry-Ball Turret B-17
Turrets & Nosedome of the A-26B Invader Removable Cockpit Hood A-26B Invader
The Sunderland was a flying boat and not an amphibian aircraft, so it really does not have an under-carriage for take-offs and landings on land!
Removable & extensable Beaching Gear Short Sunderland
B-17, P-61 & A-26B:
They have a retractable undercarriage, an own construction and works via an electric motor, gears, micro switches and an M6 screwjack. This spindle is mounted in 2 alum. head walls and moves a bronze part back- and forwards, which is attached with two 3mm alum. Strips, hinged into flange bearings, to a forked and swivelable strutholder, which can tilt through it.
The micro switches are pressed down by adjustable rods, which in their turn are pressed by the spindle back- and forward moving part. Thus, the up- and downstroke can be set in any desired position. The whole is jointly controlled and reversed by one third and double microswitch in the fuselage (for example from Graupner, which fits on a servo).
Own constructed electric Retract B-17, with 6½” wheel
Own constructed electric Retract B-17, down.
At the P-61, the nose strut (with 4½” wheel) is retracted backwards, which is also closing the welldoors. In the lowered position, the welldoors are kept open by self curved springs from 0.4mm piano wire.
During retracting the gear, all doors, 5 seconds delayed, get closed by two 180° Servos.
Own constructed electric Retract P-61
The own constructed Retracts with plastic cogwheels as a gear
The plastic cogwheels as a gear, on the inside of the head wall (B-17), was exchanged later on the P-61 by a gearbox directly to 6V. speed 400 motor, because here was the pressure going towards the same head wall. (Struts on B-17 are retracted forwards, and on the P-61 backwards, and here was some pressure than on the plastic cogwheels, where they could not stand it!)
At the P-61, the plastic gear has been exchanged now for a gearbox
P-61; the 180º Servo for the mainwheel well doors
Main Strut P-61 with 7” wheel
Nose Strut P-61 with 4½” wheel
At the A-26, the nose strut (with 5½” wheel) is retracted backwards, which is also closing the welldoors. In the lowered position, the welldoors are kept open by self bended springs from 0.4mm piano wire.
The main legs (with 7” wheels) are also retracted backwards, and disappear entirely in the wheelwells, whose wheel welldoors exists of 2 parts, which remains open.
During retracting the gear, all main-geardoors, 5 seconds delayed, get closed by 180° Servos.
A-26B Electric Retracts. Own Construction A-26B Retract of the right-innerwing with the wheelbay doors Self bended springs for the wheelbay doors
ENGINE LINKAGES & ENGINE MIXERS.
The engines at my multi-engine models have no side thrust and little or zero down thrust (nacelles are being built on at 0 degrees).
-At the B-17 (1/8th) that are 4x four-stroke Lasers 70 (11.5 cc) with MA 13x6 props (9000 RPM), these have a very small overall height.
on the servo (in the center) and control cables (sullivan) to the 2 throttle horns.
And the B-17 is, due to its enormous tail fin in the start, with the wind is in an angle or cross, will rapidly break out to the wind direction.
Controlling the throttles happens here on both sides with control cables of sullivan, linkaged to a servo.
Mixing Rudder / Engines, B17 - Sunderland – P-61 & A-26B:
For this we need 4 mixers, of which mixer 2-3-4 are on the same switch! (at my transmitter a switch on the throttle stick).
Never forget to switch off Mixer 2-3-4 after taking off! (otherwise the engines will react unintentional, when operating the rudder).