View Full Version : P337 STOL / VG Options

06-09-02, 07:36 PM
Does anyone have info on the current STOL options for a P337. Horton and Robertson (Sierra Industries) had been two options. Are they still available and has anyone done one recently? Cost?

Also, is there a VG option (not to improve VMC rather for short field performance)?

06-09-02, 08:45 PM
To my knowledge, Horton and Robertson are both still available.

Horton web site is http://www.airsport.com/hortninc.htm

Robertson is called Sierra Industries now, web site is: http://www.sijet.com/rstol.html

Also, Owen Bell (Aviation Enterprises) has STOL mods. We created a web page for him on this site, since he has not gotten his up yet. URL is


rick bell
06-09-02, 11:20 PM
2 /12 yrs ago tried to get sierra to do the mod (robinson has the flap extensions, which is great) however they were too important as they were now modifiying citations ect. tried to talk them into it with 10's of calls but no avail; im my opinion they suck. went with owen bells and it works great, the kit and installation was just under 10k

06-09-02, 11:50 PM
Rick, I agree. So far Sierra has been the same with me regarding the Robertson conversion, no return calls, apparently no interest. You have to wonder why they bought the STC's (probably for the 400 series mod's). Maybe they'd sell the 337 STC!

At any rate, I noticed you have a lot of Owen's mod's. What is his STOL mod (leading edge cuff or ???)? Also, what is your feedback on quality of Owen's work, etc. I've heard mixed reviews on the tail skirts and winglets. Do you know anyone that has done one of his big engine upgrades?

Mark Hislop
06-10-02, 10:09 AM

I have the Horton STOL on my P337. From what I understand, it is about a third of the price of the Robertson mod. However, it does not come with any modification to the POH (I think the Robertson does) and it probably is not as effective as the Robertson (the Robertson connects the ailerons to the flaps, so probably has a much greater effect on short field performance).

All that being said, I like the Horton. It does have an positive effect on slow flight characteristics and takeoff/landing distance. My stall speed is about 6-8 mph less than book values. Cruise speed seems to be the same as book values. Slow flight is very stable and contollable. Stalls are difficult to achieve, but are very gentle and they recover fast.

The Horton was already on my aircraft, so I don't know exact prices. I think the kit was about $3500, and the installation was about the same. The kit consists of leading edge cuffs, VG's under the aft fuselage, and droop wing tips which add about 2 feet to the wing span.

I think Skyking has the Robertson mod. Perhaps he can tell you more about that.


rick bell
06-10-02, 04:30 PM
if it is a risems, horton can't do it. owen bell kit slowed mine way down. 1600' strip approach at 80 final falir 70 at full gross. stable as a baby(no heavy controls) regular land with 25 mph direct crosswinds with no sweatty palms afterwards, crab then kickkout just before touchdown with wing low. before it was a disneyland ride shooting 95mph approaches, full breaks, raise the flaps and hope the brakes would not catch fire (use all 1600' at gross, if no headwind 50% of the time require a go around). stall kit includes leading edge and four stall fences. stall near gross is around 56 mph, stock it was 71.

Ron Ball
06-11-02, 03:10 PM
How is top end changed? Do winglets increase top speed? or is kit really low speed? What is weight increase?

rick bell
06-11-02, 05:54 PM
i believe they claim around 7-10 kts. i have not bothered to check it i usally fly 24x24 (45-50%) and indicate 160mph. as far as i can tell they be correct, does'nt slow it down. can't remember the wt increase; but the p337 is like a 206 whatever you can cram in it will get off, just can't get enough in because of no cargo door. winglet and boom fairing makes it rock solid on the low in (slow flight) totally responsive with no sluggishness

Jose L. Ichaso
06-11-02, 11:33 PM
I have the Horton Stol kit in my 69ยด 337D ( without the horton droop wingtips ), and is amazing how low can you fly without an abrupt stall; actually I think your limitation is the very high angle of pitch and elevator travel.
I use a 1200' feet strip quite often, at sea level, even with five adults plus some beach stuff ( Since the installation of a cargo pack ) , with better performance than many singles that operates the same place.
I have some performance numbers I added to my checklist, that were taken from a performance table from Horton some time a go, which came in a "Skymaster Newsletter" , and it claim a gross weight increment of 100 lbs, and some very low numbers for approach and stall speeds.

I tried to attach a photo file without success!

Ron Ball
06-12-02, 01:44 PM
Rick Bell, what exactly did you get? Did you add the wing extensions? I'm looking at only winglets and tail and vg's. My hanger is not wide enough for wing extensions. My e-mail is rball@goodmanball.com, for more details, if you like. I have 1973 P-337-G

06-12-02, 08:46 PM
For inquiring minds, R/STOL, using Robertson's technique for "NORMAL" and "STOL" takeoffs & landings goes like this:


1) 2/3 flaps DOWN (or option 1/3)
2) Full throttle/2800RPM & Power Check
3) Elevator Control -- LIFT NOSE WHEEL AT 53 Knots
4) Accelerate to 65 Knots over 50' obstacle
5) Lower nose and accelerate to 87 Knots after clearing obstacle
6) Brakes -- Apply momentarily when airborne
7) Wing Flaps -- Retract to 1/3 after reaching 87 Knots
8) Landing Gear -- Retract after reaching 87 Knots
9) Accelerate to Climb Speed -- 96 Knots to 105 Knots
10) Wing Flaps - Retreact (if extended) after obstacles are cleared


1) Wing Flaps -- 1/3 below 143 Knots -- 1/3 to full below 94 Knots
2) Final Approach Speed -- 65 Knots
3) Touchdown Speed -- 53 Knots



1) Wing Flaps -- 2/3 DOWN
2) Brakes -- SET & HOLD
3) Power -- Full Throttle/2800 RPM & Power Check
4) Brakes -- RELEASE
5) Elevator Control -- LIFT NOSE WHEEL AT 44 Knots
6) Accelerate to 57 Knots over 50' obstacle
7) Lower nose and accelerate to 87 Knots after clearing obstacles
8) Brakes -- Apply momentarily when airborne
9) Wing Flaps -- Retract to 1/3 DOWN after reaching 87 Knots
10) Landing Gear -- Retract after reaching 87 Knots
11) Accelerate to Climb Speed -- 95 Knots to 105 Knots
12) Wing Flaps -- RETRACT to UP


1) Airspeed -- 96 Knots
2) Power -- Full Throttle & 2800 RPM
3) Mixtures -- Adjust to 140 PPH


1) Wing Flaps -- 1/3 below 143 Knots - 1/3 to full below 94 Knots
2) Final Approach Speed -- 57 Knots
3) Touchdown Speed -- 44 Knots
4) Flare to Touch Down -- Elevator and Power to arrest sink
5) Wing Flaps -- Retract immediately after touch down
6) Landing Roll -- Apply heaving braking

STOL approaches utilizing ROBERTSON equipment are accomplished after practice. A constanty decelerating approach is utilized, requiring power to arrest rate-of-sink in the final portion from 50' to touch down.

The Roberston STOL equipment consists of a recontoured drooped leading edge cuff, two chordwise stall fences forward of the flap-aileron junction, interconnected drooped ailerons that deflect symmetrically and automatically with the flaps, automatic trim from a mechanically actuated spring operating in tension as part of the revised elevator control system as the flaps are dropped, two sets of vortex generators -- one set at the aft-end of the belly and another set aft of main wheel wells, and dorsal fairings on the upper and lower portions of the tail fins. And on our '77 P-model, the outboard fuel tanks are replaced with a slightly smaller tank, making useable fuel 71.5 gallons per side (143 total). This is necessary to accomodate the mechanical linkage to the alierons. On some of the earlier models with different tanking arrangements, yours may be different.

Typical sea-level ISA takeoff performance using the "NORMAL" Robertson method and 2/3 flaps results in a ground run of 650', or 1088 with a 50' obstacle in zero wind conditions at full gross weight of 4700#, and at 4200# a ground run of 495'.

Utilizing the "STOL" method with 4465# results in a takeoff run of 550'.