VI
MosquitoBite Planes Robin
Forward
A few notes from Andy Lennon
The Robin is a versatile model airplane, with a wide speed range, that
can take off and land in either of two modes: the conventional mode or
the STOL mode (short take off and landing) – made possible by several
unique features which are described below. It is a “fun” airplane.
The wing features large slotted flaps that cover 65% of its trailing edge
and when extended 40 degrees, virtually double the lift of that 65% of
wing area. The wing’s airfoil is the Eppler E197. It’s aspect ratio (ratio of
span to chord) is 6. It has low profile drag and a gentle stall at 20
degrees AOA (angles of attack) out of ground effect. Its zero lift AOA is
minus 2 degrees.
Ahead of the ailerons, the E197 airfoil is modified to NASA development
leading edge extensions and droop that do three things: the stall is
delayed a further 9 degrees, aileron control at high angles of attack is
effective and there is no tip-stalling, important for STOL landings.
The fuselage, when the model is on its three wheels, is inclined upward
10 degrees. The wing is set at plus 2 degrees to the centreline and is
therefore at 12 degrees angle of attack; moving forward, the wing lift is
close to the maximum, but below the stall. With flaps half extended and
in ground effect, the stall is at 17 degrees. This permits the short take off.
The landing gear in conventional or “tail dragged”. With all three wheels
firmly on the ground, the model is directionally stable and is
maneuvered by the steerable tail wheel, linked to the rudder control.
With the tail wheel off the ground, however, the model is directionally
unstable; this calls for careful rudder action to prevent the model from
veering which it is prone to do!
The recommended power for the Robin is the OS MAX .46AX engine that
turns the recommended APC 12X7 prop at 10,000 rpm. producing thrust
estimated to equal the model’s fuelled weight. This provides a steep
climb. Minimum level flight speed, flaps fully extended, is 20 mph and 80
mph in full speed level flight, flaps up. For low drag, the engine is
enclosed in a ducted cowling. The lower half of this cowl is easily
removed and replaced for engine servicing, such as glow plug changes.
A remote jack permits glow plug lighting safely away from the lethal
prop. Engine cooling has not been a problem.
The horizontal tail is positioned low in the fuselage where it is in the
prop’s powerful slip stream. The elevators are 40% of the tail’s area and
when raised, cause the tail wheel to rest firmly on the ground under slip
stream pressure. To taxi, hold “up-elevator” which provides good steering
and prevents “noseovers” should the main wheels meet an obstruction
like long grass. The flaps, when extended, create considerable
turbulence. The large dorsal fin ahead of the vertical tail provides good
directional control despite the turbulence.
A 6 channel transmitter-receiver is recommended with a 3 position snap
switch for flap control: up, 20 degrees down, and 40 degrees down.
The ailerons have heavy differential (20 degrees up and 10 degrees down)
and are top hinged with modified Frise features that eliminate adverse
yaw. The model turns on ailerons alone without rudder input.
The Robin is spirally stable. Put in 20 degree bank and controls centred, it
will return to level flight on its own. It will fly inverted, but requires a
healthy amount of down elevator (inverted down elevator is up-elevator).
The two modes of take off are conventional and STOL. In the
conventional, the model is pointed upwind, flaps half extended, and the
throttle slowly opened. As the model accelerates, the tail comes up and
the directional instability raises its ugly head. Torque and gyroscopic
precession cause a swing to the left, calling for just the right amount of
rudder. Over control results in a swing to the right and danger of a
ground loop. This is not imaginary – it has happened! At 20 feet of
altitude, raise the flaps, level off, and the plane is up and away.
For the preferred STOL take off, point the model into the wind, lower half
flap, hold enough up elevator to keep the tail-wheel firmly on the
ground and open the throttle wide. On a calm day, take off run is under
4 feet; on a day with wind of 10 to 15 mph, the take off is almost
instantaneous followed by a steep climb. At 20 feet, raise the flaps, level
off and the plane is away. Directional control on the ground is good and
torque and gyroscopic precession are avoided.
There are two types of landing: the low wind STOL landing and the high
wind landing. On a low wind day, do not try to land flaps up. The glide is
fast and flat; overshooting the landing area is probable. Instead throttle
to idle, lower full flap; the added lift and drag permits steep approaches
at relatively low speeds. There is little or no change in the model’s
attitude with flaps lowered fully – just a noticeable reduction in speed.
The objective is to have all three wheels touch down simultaneously at a
speed just above the stall, which is (flaps fully down and in ground
effect) 14 degrees. At an AOA of 12 degrees, the plane is just below the
stall. Ground run 4 feet. This is when the “no tip-stall” feature is so
valuable. The wing area ahead of the flaps may be stalled but the two
outer panels are still lifting with effective aileron control.
On a windy day, land flaps up with a bit of power. A model flying at 30
mph into a wind of 15 mph has a ground speed of 15 mph. Rather than
a full stall landing, land on the main wheels first, closing the throttle to
idle just before touchdown.
The Robin will spin but very reluctantly; the spin has to be forced by full
up-elevator and full rudder offset. After 3 or 4 turns, the spin will convert
to a fast spiral dive, speeds of well over 100 mph are possible - the model
has low drag - so do not omit the mass balancing of the ailerons,
elevators and rudder which will prevent “flutter” at high speeds.
Enjoy the Robin and happy landings!
- AGL
In a later note, Andy offered more information about landing with
full flaps:
Lowering flaps while at altitude will create substantial downwash. The
downwash will hit the horizontal tail and influence it’s attitude. Pilots
must be careful to note that while landing with full flaps, you will
eventually come into ground effect. At that point, the downwash over
the horizontal tail will be reduced due to the pressure from the ground.
The Robin will pitch down slightly as the downwash pressure eases off
the horizontal tail, so pilots should be ready for it when approaching
with full flaps.
