Murphy
Rebel
I do a fair
amount of flight instructing in homebuilts, mostly
because no one else will. I get stuck
doing it because I don’t want see the airplanes get wrecked.
Most pilots
that hold flight instructor ratings, shy away from homebuilt and experimental/exhibition
aircraft for various reasons – lack of familiarity with the type, variations in
construction and quality, etc.
The Murphy
Rebel is not very exotic, though – it’s an all-metal 2-seat tailwheel
aircraft, constructed with thousands of pop rivets. I was told they have a nasty habit of making
the lift disappear all of a sudden resulting in hard landings if you weren’t
six inches off the ground when that happened.
That didn’t
make sense. Looking at the cross-section
of the wing, it had a deep camber, with the maximum camber well forward, which
will not be a fast wing, but should develop lots of lift in a friendly manner.
However, in
some instruction that I gave on a Murphy Rebel, sure enough that wing was
nasty. Lift was binary. I suspect that if you pasted a bunch of tufts
on the top of the wing, as the angle of attack (AOA) was increased as you
slowed down, at a certain point all the tufts would stand up, indicating total
separation of the boundary layer and lift almost completely disappearing. Not a very friendly Coefficient of Lift
curve! This required that the airplane
be approached faster than necessary, with the power kept on longer than normal,
which really increased the landing distance.
This isn’t fatal with a long paved runway, but I knew something was
horribly wrong.
But why? It didn’t have a ridiculously short wing span
– think of the rag wing pipers with their low aspect ratio, which are famous
for hard landings.
It dawned
on me what the problem was. Up here in
But the
Murphy Rebel has HUNDREDS of pop rivets sticking up on the front part of the
top of the leading edge. I theorized
that these rivets, like ice or snow on a wing, would disrupt lift, especially
at high angles of attack.
The owner
didn’t really want to grind the rivets off and replace them with smooth
countersunk rivets, like you would find on a certified aircraft. And you can’t really blame him.
So I told
him to order some vortex generators.
They are gravy for most of us, but I told this homebuilder they were
necessary because of the pop rivets in a critical area of the wing.
He ordered
the vortex generators from microaero.com which were very economical because
there was no FAA/PMA paperwork required for a homebuilt, and even had them painted to match his red wing. Note that you need a kit specific to your
type of aircraft – they are carefully located.
14 hours of
labour later, he had them installed, and we did a test flight.
What a
difference! The vortex generators
transformed the behaviour of the wing at high AOA – the lift hung in there, and
allowed the homebuilder pilot to reduce power in a normal manner during the
approach, as you would expect for that aircraft. A much nicer coefficient of
lift curve.
I googled and oddly couldn’t find any hits on the internet
for discussions of Murphy Rebels and vortex generators, used as a cure for
their nasty stall characteristics, which is why I wrote this article. Hopefully it can help some other Murphy Rebel
owners out there, whom are unhappy with their firm landings. Get vortex generators!
Also, this
goes to show why flight instruction on homebuilt and experimental/exhibition
aircraft can be tricky. You aren’t just
a pilot/instructor. You need to be able
to figure out what is going on with the aircraft when it isn’t performing well,
and figure out how to fix it, in addition to creating simple procedures for the
builder/pilot to follow, to operate his aircraft.
--
acboyd@gmail.com Sept 2011