mtv wrote:BadBilly wrote:Good evening all. I’m new to the forum and we are currently shopping for our first tail dragger, a Cessna180. Being new to both this particular airframe and the forum, I have a great many questions. I have been, obviously mistakenly, of the impression that the Robertson STOL, the Sportsman STOL and the Horton STOL were three different takes on the leading edge cuff. Following this thread however, it appears that some have installed the Sportman cuff on an airframe already equipped with the Robertson STOL. My previous understanding of these mods was obviously “well off-course”. I fully understand the benefits of modification to the leading edges of the wings, but please help this neophyte in the “tail-dragger-backcountry” world understand the true differences and how, for example, the Robertson STOL and the Sportsman cuff work together and in harmony to produce better STOL performance.
Thanks in advance and Happy Thanksgiving to all, yes even our Canadian friends. Hell, we’ve all got plenty to be thankful for.
Blue skies
First, the Horton, Bush, Owl, and probably a few other "STOL Kits" were essentially just a re-contoured leading edge cuff, added to the wing to slightly change the camber of the wing, thus at least slightly improving low speed handling and at least slightly lowering stall speed. I know....a lot of "slightlys" in there, but frankly, the "improvements" afforded by those kits were pretty minimal. Some (but not all) of these kits also included a set of "stall fences" on the top of the wing, located at the aileron/flap juncture. The purpose of these was to slow or interrupt the progression of the stall across with wing as the stall propagated from wing root towards the tip, and to keep the ailerons effective as long as possible as you approach the stall.
Second, the goal of all of these modifications is to allow the airplane to fly at a higher angle of attack, therefore at a slower speed than a stock wing. Slower approach speeds MAY permit shorter landings, and maneuvering in tighter spaces.....please note the caveat there, however. May is a huge word in this context.
Robertson developed a system for older Cessna aircraft and some other brands (the Twin Comanche was also modified by Robertson) which took a different approach. Robertson STOL on early Cessnas also added a wing leading edge cuff, very similar to the Bush/Horton/etc cuffs, BUT in addition, the Robertson kit's primary function was a mechanism which droops the ailerons with flap extension. This, effectively, provides for full span flaps. The aileron droop isn't just a simple droop system: When flaps are selected to 10 degrees, the ailerons droop a few degrees, something like three degrees (sorry, been a few years and I don't recall the exact deflections, but the important thing is the relative amounts of droop). Then, at 20 degrees of flap, ailerons droop a bit more, like five degrees. At 30 flaps, the ailerons reach their maximum droop, I think 7 degrees, and finally, at full (40 degrees) flaps, the ailerons actually retract slightly, back to say, five degrees. Again, unless you're rigging them, the exact number of degrees of droop isn't that important, but the ratios are. Robertson also included stall fences on the top of the wings on most kits.
The Robertson kit on the Cessna 185, for example, was actually flight tested and DOCUMENTED to reduce stall speed from ~ 54 knots to 37 knots.....which is an amazing, and easily demonstrable difference.
The downsides: The Robertson kit is fairly complex and requires some significant rigging to get it right. It also adds some weight, which is not insignificant, since there's the weight of the cuff, the additional pulleys, brackets, etc, AND a substantial addition of lead balance weights to the balanced ailerons.....like a lot of weight there.
At some point, Cessna recognized that there were a lot of these "STOL" kits out there that were essentially just a leading edge cuff, that re-contoured their leading edge, and in the late 70s or so (someone will correct me, this is a ball park), Cessna introduced their "Camber Lift Wing". Essentially, that wing simply changed the shape of the basic Cessna wing to duplicate (or close anyway) the shape of the older wing with one of these cuffs attached. The Camber Lift Wing leading edge was cambered, in other words, to improve slow speed handling and reduce stall speed. That said, look at the flight test documented stall speeds of the earlier wing airplanes vs the camber lift wing equipped airplanes and you'll find little difference there. Assume your own conclusions.
But, because Cessna had effectively duplicated a part of Robertson STOL's modification (the leading edge droop), Robertson simply dropped the addition of the cuff from their kits for the newer Cessnas.
At some point, a smart aerodynamics engineer who worked for Lockheed's "Skunk Works" program developed what has become the "Sportsman STOL" kit. The Sportsman kit is primarily a leading edge cuff, although the change in camber of that cuff is much more aggressive than the old Horton/Owl/Bush/etc cuffs. So, the Sportsman cuff actually increases the wing surface area noticeably, and the "drooped" leading edge is more prominent. Also, the Sportsman kit includes aileron gap seals, which help the ailerons remain effective will into the stall. This kit does not incorporate "stall fences" on the top of the wings.
Look at the wing leading edge on a Cirrus aircraft or a new Kodiak. Those airplanes are equipped with what is now called "MOLE" technology, which is essentially a "stepped" leading edge, with the outboard section of the wing leading edge (the section immediately forward of the ailerons) "stepped". If you look at the camber of those outboard sections, they're similar in shape and serve the same purpose as the Sportsman cuff. In other words, they delay the stall of that portion of the wings...the purpose in these airplanes isn't so much STOL performance overall as to keep that outboard section of the wing and the ailerons effective at slow speeds.
The Sportsman kit does significantly decrease stall speeds, and significantly improves slow speed handling. Frankly, I've never met an aircraft owner who installed a Sportsman kit on his or her plane that wasn't significantly impressed with the slow speed handling of the plane. Not so with the earlier, much less aggressive Owl/Horton/Bush/etc kits. Those kits undoubtedly do slightly improve slow speed control, but the effect is small enough as to be difficult to measure, frankly.
Now, add a Sportsman kit to a Robertson equipped airplane.....and you gain the advantages of both systems, including a significant decrease in stall speed, MUCH better control at slower speeds, etc.
In fact, one of the things that many pilots don't like about the RSTOL kits is that the drooping ailerons somewhat decrease aileron effectiveness with flaps deployed....like when you're landing/taking off. That's because the amount of aileron travel is restricted because of the droop.
This is a minor issue in most situations, but can be limiting in a gusty crosswind, for example, when taking off. There are ways to operationally mitigate this effect, but not to eliminate it, with a standard RSTOL kit. In fact, next time you hear someone talk about doing that, ask them if they've ever flown a deHavilland Beaver or Otter.....both of which droop their ailerons with flap deflection, in a manner similar to the Robertson aircraft.
BUT, add the Sportsman kit to an RSTOL equipped airplane, and those aileron gap seals give you back quite a bit of the aileron effectiveness that was lost with the RSTOL kit. Not all, but a good bit.
In fact, I know of some operators of Cessnas who've removed or disabled the aileron droop on their RSTOL kits because they don't like the reduction in aileron effectiveness.....Personally, I'd never do that. First, I'd add a Sportsman kit, and see if that met my needs.
I have a few thousand hours working Cessna airplanes with RSTOL kits, and I love them, frankly. I'd take one of those airplanes over a stock wing airplane ANY day for work or play. And, yes, I've spent a few nights sleeping in my RSTOL Cessna after I landed somewhere and either the wind picked up or I realized I'd screwed up, and takeoff with a load wasn't going to be safe in gusty winds. Sorry, but that's a small price to pay for the capabilities of these kits.
That's my opinion, and we all know about those. Others can add to, refine or argue with what I've posted here, and they may be right.
MTV
