Flitzer Sport Flying Association
Engine Cowlings
From a September 11, 2001 post to the Flitzer-Builders group:
Dear Flitzer Builders,
Patrick Rose' comments regarding the cowling design on the Z-series, show an appreciation for the aesthetics of the way that the engine cowl harmonises with the airframe. This extends to the cowling break-line flowing aft, and continuing where the decking ply overlaps the upper longeron, forming a visible 'cheat line' just below the top longeron, continuing all the way to the tail plane leading edge.
But more than this, the design of the cowling provides all the streamlining to the forward fuselage, which is essentially a parallel sided box with a domed top. Thus, the Flitzer fuselage is very easy to build. with everything being straightforward immediately behind the firewall, almost to the rear of the cockpit, where the airframe starts to pull in. It is also important to keep the tapering rear fuselage side-walls running as straight as possible to the sternpost, at about 8 degrees taper, to facilitate the easy fit of the curved rear decking ply (ply is reluctant to bend as a compound curve!). This can be achieved by clamping substantial timber sections to the four rear longerons when setting up, and fitting the cross members.
The 'bullet nose' of the cowling, behind the propeller hub also provides improved propeller efficiency, since fluid follows a curve, so, dependent on the design of the actual propeller, thrust should be greater. Tests carried out on Austrian-built Albatros fighters in WW1, indicated that their 135 and 235 series Albatros DIII developments, without the streamlined spinner (which sometimes would detach in flight anyway, causing propeller damage) with the same design of 'bull nose' cowling as the Flitzer, were 9 - 11 mph faster in straight and level flight, as a result of improved propeller efficiency.
The Flitzer, for a small 'period' biplane, given that it has spoked wheels and a cross-axle undercarriage, is quite a 'clean' aeroplane.
The 'I' interplane struts, 'A' frame cabane, and the low 'sit' of the pilot all contribute to this. The prototype has a 1834cc motor, which delivers about 60 hp., and the present propeller lacks finesse, restricting max. straight-and-level speed to about 91 mph. at 3000 rpm. With an earlier 'climb' propeller and about 1/4 tank of fuel I recorded a rate of climb of 923 fpm. on one occasion, from an established straight-and-level maximum throttle setting at stabilised speed (ie. no 'zoom' component, nor any thermal activity).
I believe that blade efficiency can be much improved, and Tony Morris' Z-21A is climbing at a regular 800 fpm., despite his airframe being about 25 lbs. overweight. With the fairings on the landing gear legs, as shown on the plan, as soon as one eases forward on the stick (prototype a/c, D 692), the speed rapidly builds to around 121 mph in the shallow dive, indicating a maximum of 125 mph with the stick forward at 2/3 throttle, or even throttle closed at around 55 degrees angle of dive.
At these speeds the controls firm up nicely, so that there is no tendency to overcontrol, and I would put the manoeuvre speed at about 100 mph. The a/c is limited to 120 mph. under its UK flight limitation schedule, but in any case, terminal velocity dives create so much din, especially with a 'climb propeller' fitted, with the rate of descent driving the prop tips to near sonic speed, you'd think a flight of T-6's had just arrived overhead!
Watching the the accredited CAA test pilots attempting TV dives with various tailplanes fitted, reminded me of all those B-movie sound effects of screaming fighters in full throttle combat, and it was far more adrenalin pumping and heart thumping to watch as he neared the ground at what seemed meteoric speed, than to be actually in the cockpit, where you are more insulated from the propeller tip noise, and where you can feel the control pressures.
Changes to the engine cowling could change these characteristics, as I have made clear on the drawings.
Every change in aviation has a sometimes unexpected 'knock-on' effect.
Regards to everyone,
Lynn Williams