Resistance to Turbulence

[jchronic]

On our marine mammal surveys, we frequently circle in place for 20-30 mins at a time at 30 degree bank angles, so in light/no wind situations we regularly encounter our own wake. Last year, using a 'straight' 337, we got some pretty good jolts after several circles. This year we've been using a 336 with a Robertson (Sierra) STOL kit and the whole crew seems to think the wake turbulence jolts are much milder than last year (in the 337).

Strictly anecdotal and I wouldn't attempt to explain it aerodynamically, but thought I'd offer the info. In any case, I seriously doubt it'd be worth the cost of a conversion for whatever improvement there might be in the ride.
Joe C.

[tropical]

Originally Posted by jchronic

On our marine mammal surveys, we frequently circle in place for 20-30 mins at a time at 30 degree bank angles, so in light/no wind situations we regularly encounter our own wake. Last year, using a 'straight' 337, we got some pretty good jolts after several circles. This year we've been using a 336 with a Robertson (Sierra) STOL kit and the whole crew seems to think the wake turbulence jolts are much milder than last year (in the 337).

Strictly anecdotal and I wouldn't attempt to explain it aerodynamically, but thought I'd offer the info. In any case, I seriously doubt it'd be worth the cost of a conversion for whatever improvement there might be in the ride.
Joe C.

While you are circling do you have any flaps down? Doesn't the Robertson kit droop the ailerons slightly when flaps are extended?

[jchronic]

Last year with the 337, I arbitrarily put down 1/3 flaps as soon as we started the circle. Depending on the wind and sun angle, the circles can become ellipses and the bank angles get up to 45 degrees to get the pictures that the scientists want, and the focus gets to be on tracking the surface 'targets' rather than fine-tuning the airplane configuration.

With the STOL 336, I've found I can delay or even omit the flap extension, depending on how aggressive the maneuvering needs to be to get the data and photos they want. One thing we did notice, in straight and level flight, the airplane flies in a pronounced nose-up attitude, to where vis over the nose is almost marginal. I should note that we fly the survey tracks at 100mph GS; obviously it's better at more normal cruise speeds - but not much.

As I noted before, I can't imagine the $$ involved in a conversion would be cost effective strictly for ride improvement, or even for what we do; this just happens to be the airplane we were able to lease this year.

I can't answer the question on whether the ailerons are supposed to droop with the Robertson kit. Ours don't, and the airplane logbook shows that it's a Robertson conversion.

Joe C.

[John Hoffman]

Yep - the Robertson is interconnected with the ailerons so that as you add flaps the ailerons come down a bit to increase lift. Robertson also includes stall fences, leading edge cuffs and drooped wing tip caps - at least on my plane. Guess all the reconfiguration gives the STOL performance your looking for, cant say what it does for turbulence other than allowing you to fly real slow.

[jchronic]

That's interesting. Our airplane has all the things you mention, but I never noticed the ailerons droop when the flaps come down - but then I never really paid attention to that either. Or the logbook is in error - gee, I can't believe that could happen in general aviation...

We're flying tomorrow, and now I'm going to look!

[CO_Skymaster]

My aircraft has a really old STOL kit. Anyone remember OWL STOL? Eventually I had planned to replace it with the HORTON STOL kit. I suspect it will have better capabilities than the OWL (Leading edge only alteration). Also I want to add vortex generators. My reason for wanting those is that I fly out of airport that are typically 6 - 8000 feet. I would like as much performance as possible while climbing out.

I think my friend had several problems that made the flight uncomfortable besides turbulence. I plan to purchase a couple of the wrist bands to help with motion sickness and keep them in the glove compartment. Thanks for all the input on this subject.

Karl

[John Hoffman]

Mabey the things I mentioned are standard on a G model, im just running on memory that they may have been added with a Robertson. I never paid much attention to the ailerons either but just happened to carefully run the flaps thru the full down cycle after the last flite cause plane seemed to be draggy on the right side when landing - found the right was 8 gal. higher on refueling so that was probably what I felt. However in running the flaps down noted that first the ailerons droop in some proportion the flaps going down then as you approach full flaps the ailerons retract some from their lowest position - very intnteresting how they accomplish that. Be interested what you see when you check the aileron extension.

[jchronic]

We flew a 7.2 yesterday in two legs. I did not note the ailerons droop upon flap extension on any of the takeoffs or landings, although I was trying to watch from the cockpit and maybe couldn't see that well; don't know how noticeable it would be. Meant to cycle them on the ground and have someone watch from outside, but with other things, forgot to do so.

Going to the airport tomorrow to dig through the logbook again for a Form 337 or some other indication of what kind of conversion it is, Robertson or otherwise. Will also try a flap extension in the hangar and see what they do.