Welcome to my

KR2- project pages
 Kr2 in the foreground!

Project is at last at test phase. Fifteen first flights were flown last August and Sebtember. Testflights will be continued after winter, on March or on April 2009 depending on the weather. Next main targets are to finish adjusting the carburetor and geting acquainted with the flight characteristics. Flight testing includes 40 flight hours and constructing a Flight Manual, that is accepted by Finnish Aviation Authorities.

The plane is quite pleasant to fly in spite of being a taildragger. During takeoff the nose covers the horizont but after geting the tail off the runway visibility is good. Rudder is effective and it is easy to keep the plane in the middle of the runway. During flight stick forces are low. Adding control movement increases the force only a little. However, the effect of the movement is not too strong, you have to move the control to get result.

Landing is typical to taildraggers. KR2 shows no exess elevator sensitivity, but certain sensitivity in using it must be maintained. In principal during landing stick is moving backwards if moving at all (concerns the elevator). To save the tiny tailwheel it is useful to keep it just off the runway when main wheels touch the earth. Tailwheel is swivel and can be locked to straight forward position. For takeoff it is locked and opened after landing when speed is low enough. I also designed and built an airbrake, that can be seen in the photo under the belly. It has three positions. Usually the first is used in downwind leg, the second during base and the third during final. Force to use it is almost equally small for every position increassing hardly at all from notch to notch. The feeling is gentle and soft but at speed used for landing it is effective. It brings a minor nose down trim force when moved to notch three.

During construction I made some changes and additions to the Rand Robinson instructions. The following links lead to more detailed review of some of them: fuselage wings engine. Photos of building process and test flights are shown at photos (5.7 MB).


Fuselage

The fuselage boat is built obeying the instrutions, almost. The width of the fuselage in the cocpit area is widened. Top longerons are bent as in the instructions but increasing their distance on the widest area about five centimeters (two inches). The length of the fuselage is increased about two centimetres (one inch) due to the lenght of the material. Other additions and modifications are shortly described below.

airbrake

Airbrake

An airbrake was built instead of flaps. It is a plate of aluminum and is hinged to the bottom of the fuselage with one full length piano hinge. A wooden reinforcement block for it is glued on bottom plywood inside the fuselage and another made of 5 millimeter plywood outside the fuselage between bottom longerons. They are located a few centimeters behind the rearmost CG position to avoid any influence in yaw stability. I have restricted the CG area to 8-14" instead of 8-16" from inboard wing leading edge.
For attaching the airbrake extending mechanism a wooden reinforcement block is glued on both side plywoods near the bottom behind the main spar. To extend the airbrake a steel tube is located between these blocks and anchored on place in a hole of a nylon block attached on the reinforcements. On distance of one third of the length of the tube counted from sidewalls two perpendicular "brances" are welded to the steel tube. A push rod from both of them is used to extend the airbrake. The tube is winded by a third "branch" welded near the left end of it. A notched arc on both side of this lever acts as locking mechanism to positions of the airbrake.

tailwheel

Tailwheel

Tailspring is laminated using the same fiberclass cloth that is used for wing skins. Tailwheel is from Aircraft Spruce and Specialty. It is left swivel without connection to rudder cables. An aluminum collar is bolted on the cable attaching arms around the vertical axle. The two holes in it are for locking the tailwheel. The collar is located so that the locking bolts go into the holes only when tailwheel is at straight forward position. At 180 degrees position the aluminum is solid and the locking bolts only rest on the plate. Locking bolts are lifted and lowered using a thin cable from cocpit. This solution appears to be functioning well. During taxing toebrakes help on tight turns and the rudder is effective enough to maintain direction on straight taxiway. There is a bigger photo of tailwhel on the Photo pages.

rudder

Rudder

Three main changes were made to rudder:

1) the length of the lower rib of the rudder is longer than in instructions
2) the main spar is lengthened upwards and an additonal area of rudder is built above the vertical stabilator
3) a counter balance weight is located on that area.

Mass moment of the counter balance compensates 1/3 of the mass moment of the rudder in relation to hinge line. The plans have no mass balance for rudder. Additional skin cloth was laminated to vertical stabilator.

landing gear attachement

Landing gear

Landing gear is fixed. The gear legs are cut and bent out of a railroad carriage steel bow. They are attached to main spar outside the fuselage using self designed attachement parts. The distance between outer sides of wheels is 197 cm (77" ) when the plane is unloaded. Bigger photos at Photos-page.

Fuel tank pressure

There is some information in the internet, that there is an area on underpressure just where I have the fuel tank ventilation. Fuel flow is by gravity only and that's why I made these arrangements to measure the pressure on that area. The result is that there is overpressure prevailing on the mouth of the ventilation pipe all the time when the engine is running. The overpressure increases along with growing dynamic pressure during flight. Photos of measurement arrangements at Photo-page.


KR2 with long wing

Wings

The wing is built as instructed in the plans, however with some deviations. The airfoil is from plans RAF-48.

Spar

The caps of forward spars of the center wing are laminated using five wood blocks of equal dimensions. After epoxing them together the cap was planed off in a carpenter shop so that the thickness of the outer blocks is the same. The caps of forward spars of outer wigs are laminated of three wood blocks and planed so that the outer blocks are of equal width all the way. The glue joints are vertical for all joints.

Lenght of the wing

The spars of the outer wings are longer than in plans. They are continued outwards by adding one additional vertical support block to the outer end of the spars. The wing tip is thus a bit lower than in the plans. The length of both outer wing forward spars grew 18 cm (7 ") by this. Total wingspan is 56 cm (22 ") longer than in plans. The rest of the addition length is made by building a bit longer wing tip foam structure.

Excess rib

excess wing rib

In the plans outer wing has only one plywood rib, at the end of outer spars. I put an excess plywood rib between the spars inside the second outmost foam rib to make the outer wing more firm. Excess rib takes loads from skin-spar laminate joint and also from wing attach fittings. Both plywood ribs are 2,5 mm (0.1") birch plywood.


Foam strength

wing surface reinforcement

If a for example one inch thick foam plane has fiberclass laminated on only one side, it makes the foam stiffer but not much. If it has fiberclass laminated on both sides, it's bending stiffness grows remarkably. I laminated outer wing skin foams on both sides. Lower surface of wing was foamed first. After the foam blocks having been epoxied on place to wing I laminated fiberclass cloth on inside surface of the foam between the foam wing ribs going up to spars about one inch. For upper foam I first laminated a strip of cloth about 90 degrees angle on edge of a foam block. After fitting the foam blocks on place to the upper surface of wing I first laminated fiberclass cloth on their inner surface and after that a strip of that angle on that cloth to forward and aft ends of the blocks so that when epoxing them on place to wing the cloth angles became epoxied to spars. These additional laminations were made to foams between the spars and between the rear spar and the additional spar for aileron.

Geometric twist

The plans tell you to put washout of the center wing to 3.5 degrees and the outer wing to 0.5 degrees. So there is twist of 3 degrees for the outer wings. I made twist of only 2.2 degrees. Anyway the behavior of the wing is quite gentle.


place for ForceOneProphub

Engine

Originallly the engine was included in a "firewall forward" package from HAPI. After a delay the package was delivered by Mosler. Current engine is built using parts from different sources. The block, crankshaft, propeller hub (Force One Prophub), connecting rods and push rods are from Great Plains. Accessory case with starter and alternator, flywheel, engine mount, oil pump, oil cooler, cylinder heads with rocker arms and ignition system with four coils are from Mosler (HAPI). Rest of the parts are from local VW dealer.

Cylinders and engine block seats are bored to 94 mm diameter which gives with 69 mm stroke 1915 cc. Pushrod lengths are adjusted so that rocker arms are perpendicular to pushrods at half way of valve travel. This gives longest possible valve travel and minimum side forces to valve guides. I know there are other opinions about this. Oil pump is modified to give oil output to an external oil filter.

Adventures with hydraulic valve lifters took many years before I decided to build the engine using parts originally designed to VW 1600 engine. This is why test flights are goin on only now, after several years of the original date of finishing the plane.

Carburetor

The original carburetor was a HAPI ultra carb with float bowl and cocpit adjustable mixture. I changed it to Mikuni TM40-6 motorcycle carburetor in context of rebuilding the engine. The carburetor is designed for motorcycles and so it is not desinged to meet any considerable changes in the input air pressure. At first I had the air inlet in front of cowling and thus the changes of the dynamic air pressure in carburetor throat influenced in the mixture quite strongly. I blocked the front air inlet and opened another one on bottom of the air filter box. Now the input air pressure stays more or less constant and the mixture stays in tolerances. See photo at the Photo-pages "KR2 in winter".


Most interesting results will be written here when test flights are continued.                 Visitors:     after March 2009.

email:  aimo.paivoke@hotmail.com