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Vortex flaps Helio mod

Started by Doug Johnson, July 08, 2011, 09:08:08 AM

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Doug Johnson

While doing a search for Helio Mods I came across this study.  Thought I'd repost it in case someone else thought it of interest. I highlighted points that caught my interest. To read the entire article on vortex flaps, lots of math, charts, and wind tunnel tests go to the link.

Excerpt from:
                                                    THE VORTEX FLAP
                                                    Brandon T. Buerge
                                                         copyright by
                                                    Brandon T. Buerge
                                              WASHINGTON UNIVERSITY
                    Department of Mechanical, Aerospace, and Structural Engineering
                                       School of Engineering and Applied Science

        The author hopes that the potential utility of the Vortex Flap in enabling STOL utility missions is self-evident. However, the existence of at least one mission currently filled by a rotary-wing aircraft which could conceivably be filled by a fixed-wing aircraft should be highlighted to demonstrate the power of the Vortex Flap. The US Navy currently employs a heavily modified Schwiezer 300 helicopter as an eyes-in-the-sky VTUAV which operates off of its destroyers (Northrup-Grumman 2008). The basic mission is to lift off, climb to 10,000 feet, cruise out 110 NM as quickly as possible, loiter for 5 hours, return and land with reserves while carrying a 200 lb. payload. If the stall speed of a fixed wing aircraft could be lowered sufficiently below the speed of the ship, then that aircraft could operate from the ship in a manner similar to that of a helicopter, with a vertical rise and steep climb-out. The author submits that the power of the Vortex Flap, particularly when employed as a part of a high-lift system, is sufficient to lower the stall speed of a number of light single-engine aircraft to below 30 knots, which is within the speed range of the Destroyer, allowing vertical operations with the fixed wing aircraft. While a clean sheet of paper design would be best, for the sake of demonstration, an existing aircraft will be used as a starting point.

        The Helio Courier, while not perfect, is a suitable starting point for this demonstration.  The Helio Courier was the brainchild of Otto Koppen and Lynn Bolinger, intended to be an 'everyman's safety-plane' to fulfill the plane-in-every-garage vision still anticipated in the late 1940's. These men took it as self-evident that very safe and very low-speed operation, as well as the capacity to operate from unusually short fields, was the primary consideration in designing a fundamentally safe airplane. Through the judicious application of basic high-lift devices, and the installation of a truly enormous propeller, the pair was able to build a prototype example capable of operating off a tennis court
(Rowe 2006).

         The eventual result of their work was the Helio Courier which became known, rather than an everyman's safety-plane, as a very expensive, very capable short-field machine with a respectable cruise speed. The Helio Courier, in all its various forms, made use of full-span automatic Handley-Page slats, and large span single-slotted Fowler flaps. The installation of a geared engine (unusual in light planes where directdrive is the standard) allowed an unusually large propeller for the horsepower, dramatically increasing thrust at low-speeds. For example, at full gross weight the Helio Courier H-395 is able to take-off and land on a 500' strip with 50' obstacles located at each end. While credit in the popular aviation media for the incredible lowspeed performance is normally given to the slats, the author's own analysis indicates that it is due also to two other factors rarely considered.

       The first is simply that the airspeed indicator errors are large (>50%) at low speeds, giving pilots the impression that the aircraft performs more impressively than it does. The actual minimum control speed is probably around 37 knots under full power rather than the often-reported 22 knots (US Air Force 2001).

         The second is that the unusually large propeller  (8' diameter 3-bladed propeller vs. 6' 8"   3-bladed propeller installed on the Cessna C-185, a similar utility aircraft (Taylor 1979) allows the aircraft to generate tremendous thrust at low speeds. In combination with the slats, that allows the aircraft to reach unusually high angles of attack, the large propeller ends up generating a very considerable vertical component thrust which significantly reduces the weight that must be supported by the wings at low speeds. According to the author's calculations, in level flight this reduction can be well over 20%. This serves to reduce the minimum control speed to below the range of more conventional aircraft.

        Eventually an entire line of aircraft was developed, including twin engine and turbine powered variants. Helios saw service in the CIA's Air America operations during the Vietnam War, and served very well as jungle delivery planes. Many of the civilian aircraft ultimately found their way into service under similar conditions for the Jungle Aircraft And Radio Service (JAARS), an evangelistic and humanitarian organization. JAARS currently operates the largest fleet of Helios.

       With over 500 of all variants built before the end of production in 1984, the Helio Courier was a modest business success (Rowe).

Modification of the Helio Courier

        The base aircraft selected is a military version of the Helio Courier Model 395, known as the  U-10B. This particular model was chosen for its combination of performance parameters, and the availability of performance information in the form of a flight manual (US Air Force 1966). The basic description follows.

          The Helio Courier H-395/U-10B is a high-wing monoplane with a cantilever wing, allflying stabilator, and fixed conventional landing gear. Capable of seating five, the aircraft is primarily aluminum with fabric-covered ailerons and a steel safety cage around the cabin. Provisions for STOL include full-span Handley-Page automatic leading edge slats, 74% span single slotted Fowler flaps, and a type of spoilers or roll control augmenters, a britisih invention, called 'interceptors' by Koppen to aid lateral control at low speeds. The plane is powered by a geared Lycoming GO-480-G1D6 good for 295 hp for takeoff and 280 maximum continuous power driving a 3-bladed constant speed Hartzell propeller of 8' diameter. The U-10B has provisions for up to 879 lb. useable fuel, and a gross weight of 3,600 lb. as compared to 3,000 lb. for the civilian version. This is probably due to the smaller margins of safety accepted for military operations rather than structural modifications.

    Performance and Specifications (True Calibrated Airspeed)
Length                          31 ft.            Top Speed               148 knots
Span                             39 ft.             Minumum speed    35 knots*
Height                          8ft.10 in.               (power on) 
Wing area                     231 sq. ft.     Minumum speed    47.4 knots*
Wing Chord                  6 ft.               Intial Climb rate    935 fpm*
Aspect Ratio                 6.6                Empty weight        2,000 lbs.
Power (5 min.)              295 hp          Gross weight         3,600 lbs.
Power (continuous)       280 hp          Best climb speed   75 knots
* Estimated from flight manual and author's calculations.

Data collected from Rowe 2006 and U-10B Flight Manual.

Performance and Specifications of  the U- 10B Aircraft

First-order estimation methods suggested in design textbooks by Raymer (Raymer
1999) and Roskam (Roskam 2001) and data from Jane's All-The-World's Aircraft
(Taylor 1961) were used to adapt the Helio Courier for this mission. The assumptions
made for this analysis follow:

• Takeoff speed must be 31 knots or less for 'vertical' operation. This is below
the top speed of modern US Navy Destroyers (ref).

• Power-on stall speed must be less than 31 knots/1.2 = 25.8 knots. This drives
the wing sizing and lift coefficient requirements.

• If it can take off, it can climb (this was later verified analytically).

• The weight build-up for the modified Helio aircraft follows:

o Original U-10B empty weight: 2,000 lbs.

o "UAV Conversion" obtained by comparison with the Schweizer 330
conversion to the Fire Scout: 688 lbs.

o Building the aircraft out of composites instead of aluminum would
save, using Raymer and Roskam's estimates: 726 lbs.

o Vortex flap weighs 10% of gross weight: 360 lbs.

o Larger wing weighs an additional 5% of gross weight over the weight
of the normally sized wing: 180 lbs.

o Total zero fuel weight: 2,500 lbs.

• The Vortex Flaps pivot during climb and cruise to align with the wind. The
details of this configuration are not specified.

• The drag of the Vortex Flaps in the cruise/climb configuration will be
estimated by comparison with the drag of pontoon floats installed on the same
aircraft. The performance data are available, and the drag of the floats can be

• The drag of the tailwheel is neglected.

• Running the Vortex Flap at an SSR = 2 will reduce the profile drag of the
cylinder by 1/3, based on experimental observation. This aids during initial

• The mission specification that the service ceiling be 20,000 ft is neglected, though this
could ultimately be met with a turbocharged engine or other similar accommodation.

• Raymer's methods were used to calculate the maximum coefficient of lift that
could be expected from the high-lift devices that were used.

• Of the options available to make an estimate of the maximum Vortex Flap lift
increment, the most conservative (constant lift increment) was used.

• It was assumed that the basic function of the Vortex Flap would not be
hindered by the presence of a single-slotted Fowler flap deflected 20°, and also
that the presence of the Vortex Flap would not change the lift increment
provided by the slat or Fowler flap. This would need verification in wind
tunnel testing.

Based on these assumptions and methods, an analysis following Roskam's Class I and some Class II methods was conducted which produced the following performance and specifications in Figure 5-10.

Performance and Specifications (True Calibrated Airspeed)

Legth                          31 ft                              Top speed               97 knots
Span                           47 ft.                              Stall Speed           
Heigth                        8 ft. 10 in.                      (power on)            25.8 knots
Wing Area                  284 sq. ft.                      Stall Speed            29 knots
Aspect Ratio              6 ft.                               Initial Climb Rate
Power (5 min.)           295 hp.                         Empty weight*       2,500 lbs.
Power (continuous)   280 hp.                          Best climb speed    64 knots
* includes the weight of the UAV conversion,  688 lbs.

Figure 5-10. Performance and Specifications of the new aircraft

With this performance, a mission analysis was conducted to determine how much fuel would be required, and how much payload could be carried on that mission.

Analysis of Original and Modified Helio Courier for Mission

The mission analysis results for the original U-10B Helio Courier and the modified
Helio follow in Figure 5-10. Note that the U-10B does not meet the requirement for
vertical takeoff and landing, but the ground roll with a 31 knot headwind was calculated and is included below. Note also that 688 lbs. will be deducted from the payload for the U-10B to account for the UAV conversion.

Phase                      Weight at end           Time (minutes)          Fuel burned (lbs.)
                                (modified/original)   (modified/original)    (modified/original)
Start, warmup          3600/3600                 10/10                         18/18
Takeoff                     3,596.5/3,598.25       1/.05                          3.5/1.75 

          Figure 5-11. Mission Analysis for U-10B and the modified Helio.

The takeoff ground run of the U-10B aircraft under these conditions is approximately 200 feet. The landing ground run is approximately 40 feet. Assuming the aircraft are fueled to 3,618 lbs. before startup, which permits liftoff at 3,600 lbs, the remaining payload of each aircraft is:

U-10B:                    3 , 6 1 8 - 4 4 2 . 5 - 6 8 8 - 2 , 0 0 0=      488 lbs.
Modified Helio:        3,618-566.3-2,500=                                 552 lbs.

Note well that if the U-10B were assumed to be made of composite instead of aluminum, the payload would be increased to 1214 lbs. The major difference in payload can be readily attributed to the vertical takeoff and landing requirement, which necessitates additional high-lift devices and a wing stretch which together at
15% of the aircraft gross weight to the empty weight. Also, this is a loiter mission, and it is apparent that payload can be exchanged for additional time on station, provided fuel tanks are sufficiently large, as shown in Figure 5-11.

Phase                      Weight at end          Time (minutes)        Fuel burned (lbs.)
                                (modified/original)  (modified/original)   (modified/original)
Start, warmup         3600/3600                10/10                           18/18
Takeoff                    3,596.5/3,598.25     1/0.5                            3.5/1.75
Climb to 10k            3,550/3571               24.5/15                       47/27
Cruise to 110NM      3,479/3,514               51.3/41.5                   70/57
Loiter 5 hrs              3,197/3,289               3 00/300                    282/225
Return 110 NM         3,098/3,220               72.5/50.4                   99.4/69
Descent                   3,086/3,208               15/15                          12/12
Maneuver/Land       3,071/3,193               5/5                              15/15
20 minute reserve   3,054/3,178               20/20                          15/15
Shutdown                3,052/3,178               5/5                              2.5/2.5
Totals                                                       484/442                     566/443
Figure 5-12. Payload vs. loiter time for modified Helio aircraft.

It should be further noted that what is required for vertical liftoff is 31 knots over the
deck, by any combination of boat speed and wind which will produce that combined velocity. At lighter weights, less is required for vertical takeoff, though not so much as to make payload reduction an efficient means of reducing wind requirements, as shown in Figure 5-12.

Gross Takeoff Weight (lbs)         Required Wind-over-deck for VTOL (kts)
                3,600                                                    31
                3,300                                                   28.8
                3,000                                                   26.4
Figure 5-13. Relationship between gross takeoff weight and wind-over-deck required for VTOL.

The purpose of this exercise is to demonstrate the hypothetical potential of the Vortex Flap, not to assert that a vehicle so modified is a better fit than the present helicopter serving in this particular mission. Nonetheless, this exercise does demonstrate that this type of mission is at least hypothetically possible with the application of the Vortex

Doug Schrage

There are a couple references to CAS in this article. My POH doesn't have a CAS table. Does anybody's? Does anybody have any experience or knowledge of how calibrating airspeed is accomplished?