Low altitude high alpha with a park jet

Hi everyone -

I know the title might sound like a bit of an oxymoron, but in recent testing with a couple of planes working on my high alpha skills, I have noticed a few things with respect to low level high alpha that got me thinking about what is going on.  I could just be speculating here, but perhaps it will make for interesting discussion... :)

When I say low level, I'm thinking mostly here about where the tail and most of the back end of the plane are within about two feet of the ground or in the "ground effect".  So I had to do a little more research to better understand what is normally going on with a plane in ground effect.  A fairly simple explanation (at least one I can understand) is here http://en.wikipedia.org/wiki/Ground_effect_%28aerodynamics%29

So I'm not sure that being at a fairly extreme AOA (angle of attack) in the ground effect is taking advantage of what would normally be increased lift and reduced drag, perhaps I have lost those benefits.  Without benefit of a wind tunnel, I can only go with what I see, hear and feel is going on with my plane as I push it close to the ground in an already challenging flight profile.

So you might say why put myself in such a situation doing this type of flying so close to the ground?  Well, no reason really I guess other than it is challenging and pretty cool, getting that tail down super low and sometimes even touching it to the ground is somewhat akin to a 3D pilot dipping their tail in the water while hovering their plane.  Not the most scale of flying as you wouldn't see a real F18 or Mig pilot try a "tail touch", but fun to play around with.  Very high on the cool factor, but as often is the case, a bit higher on the danger factor.  Like with any extreme flying, the margin for error is pretty small, but fortunately the plane doesn't have far to fall if it does decide to reunite itself with terra firma.

So what I was noticing yesterday as I pushed the limits with the plane I was flying was that as I got closer to the ground, within about two feet, some very interesting things started to happen with my plane.  The closer I got to the ground, I found the controls starting to get "mushy" and sluggish to respond and any minor changes in upward pitch would set off wing rock at a much lower AOA than normal.  I could also hear the prop wash hitting the ground and the sound of the prop changing as perhaps it was chopping through dirty, turbulent air being bounced back up off the ground.

I have drawn this very crude diagram to help explain what I think might be happening.

So what I started wondering with how the plane sounded and handled as it got closer to the ground is whether a bubble of dirty/turbulent air was being trapped behind the prop and under the back deck of the plane, especially as the elevons got closer to the ground, blocking some of the flow of air moving backwards.  This now coupled with the prop being close enough to the ground that perhaps the prop wash was now also getting bounced back up into the plane and maybe even back into the prop, causing the difference in the sound of the prop.  

I could be speculating, but perhaps as the tail of the plane was now starting to "rest" on a bubble of turbulent air as it moved forward so close to the ground, that could help explain why the back end and the controls elevons and rudders were feeling "mushy" and slow to respond or in the pitch far more sensitive.  I know that as soon as I lost my nerve or needed to recover out of being that low, once I got up to about 3-4 ft off the ground, the controls and plane felt much more responsive again.

Anyway, it is interesting to speculate, I definitely think that the prop wash being so close to the ground with the unique mid motor mount/prop in slot park jet style of plane has in impact on high alpha performance.  With a normal puller type plane where the prop would be much higher off the ground, perhaps this would not be as much of a problem, I suspect what I an seeing, hearing and feeling is unique to the park jets I fly.

So how do I recover out of it if (or should I say when... :/) I get into trouble?  Well, again, high on the cool factor is to hammer the throttle and power out of it, but this requires a bit of nerve, especially if the plane is starting to wing rock or do other crazy things.  Hammering the power when the plane is already a bit mushy, slow to respond and with spoilerons still deployed can end up in a fairly aggressive maneuver very close to the ground resulting in less than positive results...been there, done that... :/  It is also crucial that I have a motor that has enough power and instantaneous acceleration to get me out of trouble, not enough power and things can go bad as normally the plane is heading towards something it shouldn't be with sloppy controls and not enough power... :/.  

As crazy as it might sound, normally the safest thing for me to do  is to just chop the power and let the plane "flop" to the ground.  This normally ends up in less catastrophic results, allows me to launch the plane and continue on.

So as I mentioned, I could be speculating what is happening here, but I think what I have said makes sense based on what I have observed is going on.  Anyway, low altitude high alpha might not be for everyone, but as I mention, it is high on the cool factor, allows me to push my skills and focus on really being smooth with the controls... :)

Cheers,

Scott