FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1960
1960 - 1085.PDF
fUGHT, 15 M- I960 -.1 - 105 Full-scale results were obtained by towing the Otter on wheeled pylon .. structures. Note the area of the double-slotted flaps STOL Otter SINCE 1956 the de Havilland Aircraft Co of Canada has, withthe help of the Canadian Defence Research Board, investi-gated some of the problems of STOL (short take-off and landing) aircraft. From the outset it was decided to adhere to theconventional fixed-wing and propeller formula, and it was early appreciated that the DHC.3 Otter was an excellent basis for aSTOL modification. Not only was it a proven vehicle, but it seemed clear that excellent results would be forthcoming from amachine of simple conception which would show marked economic advantages over all VTOL devices. Studies showed that it was possible to employ propeller slip-stream as a source of high-pressure air for boundary-layer control. Direct effect of slipstream was to pass high-velocity air over theflaps, generating lift and drag even at zero forward speed. Indirect effect was to spread slipstream spanwise and thus cause high-velocity air to flow through the flap slots over a considerable portion of the span. Rig tests revealed that rough flow over the flaps gave rise tounstable flow round the tail and reduced lift. Flexible extensions were added to the rear of the fixed part of the wing, to direct slotflow down over the flaps, and the inboard part of each flap was arranged to be depressed to a large angle to provide thedrag needed for landing. Thus a system offering adequate lift, drag and stability was achieved after only 11 test runs. A further24 runs gave the required coefficients needed for the flight vehicle. Analysis of aileron and rudder effectiveness indicated that someform of BLC (boundary-layer control) would be needed, and a model mounted on a truck driven at various speeds sufficed to givethe information necessary. Control investigation on the actual aircraft indicated that additional tailplane power would berequired. A tailplane of revised section was fitted, with vortex generators to increase elevator effectiveness. This was found tobe inadequate, and an entirely new tailplane, using Caribou data, was designed and accepted without further testing (it appears tohave an upside-down section and reversed slot ahead of the elevator). Widi flaps down rudder buffet was experienced, and,while it was cured with a modified rudder used for BLC experi- ments, the effectiveness of the blown rudder was found to beseverely dependent on elevator angle. Accordingly the tailplane was mounted at a lower point on the fin and the desired increasein height achieved by dihedral. Standard Otter ailerons were investigated and replaced by BLC surfaces. In all this work an IBM computer proved of great advantage;but the sheer magnitude of the research effort made it prudent to carry out flow-visualization tests. A smoke producer wasdevised, discharging through a 3in hose. This at once confirmed the BLC effect of slipstream over the flaps, showed how misleadingtufts can be in many instances, demonstrated the strength and location of trailing vortices and showed that the ailerons wereclose to a disturbed region and that the huge flap lifts caused a strong upflow and separation at the leading edge. The smoke alsorendered visible the strong downwash even close to the ground, minimizing ground effect and allowing manual elevators. While testing of models, full-scale rigs, towed low and hightrestles and tunnel runs all helped to gather the data needed, the actual aircraft was built and flown. At first it was held at lowaltitudes along the Downsview runway, but by March of this year it was regularly airborne at several thousand feet for over an hourat a time. Already "very interesting" information on the dynamic stability and control parameters has been obtained and work iscurrently in hand on lift, drag and static stability. Dynamic characteristics will also be measured; and the STOL Otter hasalready demonstrated outstanding stability. Very few data have so far been divulged, but in a paper sub- The four-wheel undercarriage was adopted in order to maintain good airflow over the tail even when the aircraft is on the ground mined to the Canadian Aeronautical Institute, Dr D. H. Henshaw,of D. H. Canada, states that with 60° flap (full design deflection is 90°) the aircraft can fly comfortably at 35kt. Based on thelarge area, the C_ is 4 at this speed. Landing, at 7,5001b in zero wind, takes under 500ft from 50ft, with a ground roll of less than200ft. Future testing, with greater flap angles and drooped ailerons, will yield even better values. MASEFIELD CHIPMUNK (Continued jrom page 98) a position error of — lkt into account this seems to agree quite wellwith the 118kt at 1,000ft and 120kt at 2,000ft true airspeed recorded by Mr Masefield over a measured course.The Chipmunk gives very little indication that it is flying at a speed greater than its designed level maximum. At these speeds,the coarse-pitch propeller limits r.p.m. to 2,320 and noise and vibration level are slightly lower than on the standard model.Close fitting of the canopy and careful sealing of the cockpit is partly responsible, and draught exclusion must certainly add to thecomfort of flying an unheated aeroplane. A joy of this essentially touring conversion is that, provided it iskept within weight limits, the Chipmunk remains fully aerobatic; to my pleasure I found that it can be looped without difficultyfrom straight and level cruise without losing height, and the crisp ailerons and well-harmonized controls of the standard aircraftremain unimpaired. The normal 120kt for a loop comes up in a very short dive and there is no danger of building up excessiver.p.m. From a high-altitude cruise, full throttle, high-speed descents are a normal time-saver. I liked the aerobatics so muchthat I spent some enjoyable minutes forgetful of my intention to compare the handling of this Chipmunk with a standard one;but I was sharply reminded of Tango Mike's essentially executive nature in a rather slow roll which brought a veritable shower ofmaps and paper down on to the roof. I was nearly spiked by the two famous "house" flags. At the owner's suggestion and with respect to increased weightI made the first approach 5kt faster than is normal for the standard aircraft. But while at 650ft/min the heavier Chipmunk indicateda rate of descent 50 per cent greater at 65kt power-off than the standard version, the elevator remains effective in checking descentvery nearly down to the stall, and I decided after the first attempt that a 60kt approach resulted in a more reasonable arrival speed.A flash of the Masefield slide-rule confirmed this as 1.3 V SL.There are no limitations on sideslipping the modified Chip- munk, but here I noticed one of the few aerodynamic effects ofthe modifications other than higher speed. I could just detect in a sideslip the increased area forward of the e.g. provided by themodified exhaust manifold. A bigger bootful of rudder was needed than in the standard aircraft, but adequate control wasavailable and the Chipmunk slipped with the stick neutral. After one or two landings on grass I returned to Gatwick fora landing in a 12kt wind 90° across the runway. If any control deficiencies were there, I thought, this is where they would showup. But the Chipmunk wheeled on smoothly and was quite easy to hold although some weathercocking developed towards the endof the landing run, possibly due to fading of the shrouded brake. The Masefield modifications are undoubtedly successful. Stan-ing on a silk-purse trainer, the conversion has made the Chipmunk —if not more comfortable—much more practical for touring.Aerodynamic differences are barely detectable except on the a.s.i. and the modified aircraft remains just as easy to fly. Similar con-versions, including many of the features described, are now obtainable at a cost of about £600. An alteration not yet incor-porated on the Masefield Chipmunk is two additional fuel tanks in the leading edge giving an endurance of more than five hours. Mr Masefield's next idea, now under discussion with TiltmanLangley, is to sheathe the complete wing in ultra-thin gauge light alloy sheet to see what effect can be gained by cleaning up thesurface. Another one is to fit a Hartzell or McCauley constant- speed propeller. The ultimate target is 130kt in level flight. Byany standards that is a really useful piece of light-aircraft research.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
