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 donít want see the airplanes get wrecked.

 

Most pilots that hold flight instructor ratings, shy away from homebuilt and experimental/exhibition aircraft for various reasons Ė lack of familiarity with the type, variations in construction and quality, etc.

 

The Murphy Rebel is not very exotic, though Ė itís 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 werenít six inches off the ground when that happened.

 

That didnít 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 isnít fatal with a long paved runway, but I knew something was horribly wrong.

 

But why?It didnít have a ridiculously short wing span Ė 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 Canada, we are legally forbidden to fly an aircraft with any ice or show adhering to any lift or flight control surface.

 

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 didnít really want to grind the rivets off and replace them with smooth countersunk rivets, like you would find on a certified aircraft.And you canít 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 Ė 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 Ė 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 couldnít 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 arenít just a pilot/instructor.You need to be able to figure out what is going on with the aircraft when it isnít 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