What's the best way to improve STOL performance?

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USMC227

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While musing around I've been looking at some pretty impressive STOL aircraft. Some of the most impressive are based on the Piper Cub design.

So my question is what are the changes made to the planes? Longer/larger wings? Bigger engines? Swinging larger props? Puttin' on those big ole fat tires and pointing the plane skyward?

Which leads to another question. Say you took a plane such as the Fisher Horizon II, which already has pretty impressive numbers listed and try to make it better. Would it take some serious design changes? Or could a some small changes make a big difference?

Paul
 

Battson

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That's an interesting question Paul, and I'm also keen to hear some good information on topic.



I am in the throws of building a Bearhawk, and am keen to know (other than the obvious weight factor) what little things can help STOL performance (and equally important, good handling and predictible behaviour near Vso).
 

orion

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The problem with taking a particular design and modifying it for optimal STOL performance (or any other for that matter) is simply that once you start changing the airplane, the modifications have to be properly engineered to make sure that you don't introduce some negative aspects at the same time. After all, you're really making a new airplane. For instance, one of the first mods used on the Cubs/SuperCubs for better short field work was the installation of slotted flaps. The problem though, was that the flaps dramatically increased the wing's pitching moment when deployed. Couple this with the slower speed and it turned out that the new configuration did not have sufficient horizontal tail to take advantage of the mod - it always had to come in faster than it really could so it could flare in ground effect. Furthermore, the allowable loading envelope was also shrunk as the forward limit had to move aft. Some lived with the limitation; others increased the horizontal's size.

If you're willing to do your homework on this stuff and do all the work necessary, then you can make several changes to improve the short field work, most of which will be formulated to decrease take-off length, increase angle and/or rate of climb and of course increase the angle of approach and decrease length of landing. Power is of course a simple mod, as long as you can live with a heavier nose and maybe, even a higher gross weight (assuming your structure can take it). But with a higher weight you may be making some compromises, especially where speed is at issue. So here you may want to increase the lift coefficient capability - this can be done with vortex generators, slats, better flaps, etc. You can also increase the length of your main gear (assuming a tail dragger) thus giving yourself a more nose high attitude.

Most of these mods and their combinations have been tried and incorporated into many aircraft already, and could be added to your projects too, as long as the mods aren't arbitrary and are supported by proper analysis.
 

addaon

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The main improvement you can make in STOL performance is pilot training. There are some incredibly good STOL aircraft out there, but to take full advantage of them requires a skilled, experienced, current pilot. This is one of the factors that convinced me to not install leading edge slats in the CH 701 I'm building; while they definitely improve extreme STOL performance by allowing higher angle of attack, I as a low-airtime pilot am not comfortable at that extreme edge of the performance envelope. People can do things with Supercubs (and 701s, and many other aircraft) that count as "extreme STOL" in just about everyone's book... before you try to go even further, see what's out there, and see if you're comfortable even at that edge.
 

USMC227

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Yeah, I was just reading about the Vortex Generators. Looks almost too good to be true but they seem to work.

Increasing the nose high attitude seems prevalent. I wonder how high does one have to go to realize an improvement? How high is too high?

I've noticed that there are some weight and horsepower ranges for most homebuilts. Take the Horizon II by Fisher. I emailed them and asked if the VW engine was suitable and it is. I may not understand this correctly but I've got to learn. If the VW is stepped down with a PSRU like what is found on the Torque Master, then would the gain in weight be offset my much better short field performance?
 

Battson

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What do you think about the Robertson STOL kit type of thing on the leading edge? Obviously that is aerofoil specific, once again costs some speed as its traded for lift.

The idea of a stall fence interests me. Possibly an 'easy win', as doesn't add much drag and looks simple to install?

Oh - and drooped 'winglets' (if you can call them that) help overall? I often see them on bush-working 185F's etc.
 
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orion

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Yeah, I was just reading about the Vortex Generators. Looks almost too good to be true but they seem to work.

Increasing the nose high attitude seems prevalent. I wonder how high does one have to go to realize an improvement? How high is too high?

I've noticed that there are some weight and horsepower ranges for most homebuilts. Take the Horizon II by Fisher. I emailed them and asked if the VW engine was suitable and it is. I may not understand this correctly but I've got to learn. If the VW is stepped down with a PSRU like what is found on the Torque Master, then would the gain in weight be offset my much better short field performance?

VGs work quite well however they can also bite you if improperly designed or installed. Although not common, it is possible that VGs may cause a more abrupt stall, which would require more altitude for recovery. Think of it as stretching a rubber band - at some point it will snap. VGs create a chord-wise vortex flow that sort of sucks away and energizes the boundary layer. As the aircraft slows down the flow still detaches from the trailing edge, albeit at a lower speed, so you still get a near normal stall. At times though the unsteady flow at the lower limits can also destabilize near the leading edge - in that case it can make the VGs suddenly not function, stalling the wing in a very abrupt manner, well below the speed at which the airplane normally stalls.

Regarding the nose up attitude (assuming tail dragger only here), the higher the aoa, the higher the available lift coefficient for take-off. The physical limit would then be when you achieve an aoa just below stall. That would be somewhere around 20 degrees (no flaps) however at that attitude you might have a hard time seeing over the nose.
 

orion

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What do you think about the Robertson STOL kit type of thing on the leading edge? Obviously that is aerofoil specific, once again costs some speed as its traded for lift.

The idea of a stall fence interests me. Possibly an 'easy win', as doesn't add much drag and looks simple to install?

Oh - and drooped 'winglets' (if you can call them that) help overall? I often see them on bush-working 185F's etc.
The Robertson STOL type leading edge essentially creates more camber curve at the leading edge, thus allowing the wing to achieve a higher angle of attack and a slightly higher lift coefficient due to the new shape. But getting this right would require a bit of design and testing so certainly nothing that would be recommended for a homebuilder to play with.

The stall fence would probably not do much for you. It functions best on swept wings where at low speeds and high angles of attack some fraction of lift is lost due to span-wise flow. I've seen it on some conventional wings however I question the effectiveness there.

Regarding the drooped tips, their effectiveness for slower speed is generally pretty questionable. Several 185 owners I've spoken with agree that the tips do seem to make the ailerons more effective near stall but as far as lowering the stall speed, it's apparently barely perceptible.
 

Jan Carlsson

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The drooped tips have the positive effect that you only scrape the wingtips lower edge (hopfully) at a ground loop :) according to the Kitfox guys
 

Dana

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Right to the point of getting on the backside of the powercurve...:shock:
...which is where that bigger engine you installed for climb helps...

-Dana

**FLASH** Eveready Bunny arrested, charged with battery!
 

autoreply

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Regarding the drooped tips, their effectiveness for slower speed is generally pretty questionable. Several 185 owners I've spoken with agree that the tips do seem to make the ailerons more effective near stall but as far as lowering the stall speed, it's apparently barely perceptible.
I only know those from pictures, but they work essentially the same as winglets, which do increase roll considerably, but indeed, the extra lift is marginal at best.

A mod not mentioned yet is triangular plates between the flap and aileron. No extra drag, much better roll response, slightly higher lift (on the flaps) and no other drawbacks:
http://www.homebuiltairplanes.com/f...amics-new-technology/8710-flap-end-plate.html
 

Battson

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It seems common to think about STOL solely in terms of getting Vso down to as small a number as possible.

If you take a look at a typical STOL kit, perhaps as many as half the modifications are mainly focussed on improving low airpeed handling and reducing pilot workload (wing tips, autotrim, wing fence, etc).

For me at least, some of the aircraft I fly dont stall until maybe 35kts with a light load.
But even in perfect conditions I tend to approach at 55 or maybe 50 if its a short strip - because the aeroplane takes on a mind of its own if you fly any slower, which worries me, and if it stalls unexpectedly on short finals you're stuffed.

My point being; if the plane handles like a dog, that is possibly a bigger problem. Not mentioning pilot skill.... :speechles

$0.02
 
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Dave Prizio

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To land and take off in as little distance as possible you need to reduce the stall speed to the lowest possible number. Micro vortex generators have been pretty effective at doing this for a relatively small cost. More involved aerodynamic enhancements can produce greater benefits but usually at a much higher cost. They include things like leading edge cuffs, stall fences, drooped ailerons, extended wingtips, modified flaps, leading edge slots or slats, etc. Many of these mods can be found incorporated into experimental Super Cub kits and some are even available as STCs for production airplanes. The other STOL enhancement, less appreciated but very important, is to reduce total airplane weight,thus reducing wing loading.

STOL flying is another topic in itself. It does little good to spend gobs of money on your airplane if you do not also develop the skill to use the improvements. You must be able to land precisely on a given spot, and you must be able to control your airspeed within a knot or two. You need to be able to fly your airplane around at 1.1 Vso comfortably. If you can't do those things you need to work on that first. Lastly you must develop the ability to read the weather and develop good judgment. Suddenly shifting winds can leave you short on airspeed at a critical time if you have not allowed for and anticipated such things. This is why bush pilots prefer flying in the morning when winds tend to be calmer and temperatures lower.

Airplane and pilot must be STOL-equipped to be effective.

Dave Prizio
 

USMC227

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I agree 100 %... STOL flying is a package deal..... Can't have one without the other...

Ben.

Total agreement. I'm not a huge risk taker and would certainly put in the time for training and practice.

So if I'm understanding correctly accepting the fact that the pilot must be as prepared as the plane, the following enhancements could be made to most any taildragger:

Listed in order of ease followed closely by expense.

1) Vortex Generators (properly or factory installed).

2) Lighten the load.
---a) Within the confines of the kit/plans
-------1. How much weight savings could be realized in paint/covering techniques?
-------2. Minimum instruments?
-------3. Exotic materials for non-load bearing surfaces such as Carbon Fiber instrument panel? Other?

3) Raise the angle of attack.
---a) How much could be accomplished on a Fisher Horizon II and not have to change the landing gear configuration?

4) Increase Power
---a) Add PSRU?
------1. Slower prop speed results in being able to swing a larger prop, correct? Doesn't this help short field performance?
---b) Add Horsepower up to maximum acceptable engine weight.
---c) Prop change? If so, to what?

Would these changes be considered by most to add considerably to STOL performance without adversely affecting the flying characteristics?
 

autoreply

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2) Lighten the load.
It's the small details they weigh the most. FP prop saves a lot. Going to LiFePo4 batteries saves a lot too. Hand-start saves even more. A small PFD (Dynon 100 or so) can save a lot of weight over conventional instruments. A light-weight seat can save you considerable weight. Big tires are the easiest way to increase aoa, but weigh heavily.
 
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