Date: Fri, 13 Aug 1999 14:49:03 +0200 From: Johan Barnard Reply-To: aeronca@westmont.edu To: "'aeronca@westmont.edu'" Subject: f-AA: Drag reduction and performance gains. Hi Aeroncateers, Some time ago John and Mark asked whether someone can look into the drag reduction of airknockers and do a simulation. I eventually got round to do it so here it goes. Note that one must not do any of the suggested changes to an aircraft because it may or will cause people to get killed. I do not claim that my drag estimates and simulation are correct, but I got good correlation with numerous flight tests with my 7AC. Following are estimated percentage cruise drag reductions: 5% Fixing badly fitted leaking wing root fairings and windscreen top. 5% Aileron hinge gap seals. 5% Landing gear axle fairings. 3% Stabiliser root seals. 2% Elevator hinge gap seals. 2% Streamline tail brace wires. 2% Wheel spats. 2% Three to four degrees washout. 2% Remove cowl lip and drop cowl bottom 1 inch and increase outlet area. 0.5% Fixing badly fitted leaking door. Note that letting the ailerons float higher to get washout causes discontinuities in the spanwise lift distribution that produce vortexes at the aileron edges that increase drag. I feel that streamlining the undercarriage is not all that practical because one often cause more discontinuities at the fairing edges, and spats must cover most of the wheel and seal at the bottom against the tyre. If one does put in the effort to cover all struts and attachments and wheels with continuous flexible fairings the drag reduction of the undercarriage alone is about 15%.(Like the old aircraft with trousers.) Carl you mentioned that the "other" tail dragger is slower than a Champ. In my opinion this is mainly due to the following: Longer span ailerons with a gap that cannot be sealed due to hinge moment changes. Aileron rigging and horns in the breeze. U/C bungee boots. Larger wheels. Engine un-cowled. So what does the simulation give us? I worked at 7500ft da and assumed that one can reduce the cruise drag by 20% and kept the throttle position or sort of the manifold pressure the same. (This means rpm goes up at the new cruise speed causing the engine to deliver slightly more power on the "low drag" aircraft with the same prop at the same throttle position). The cruise speed goes up by 10 mph. Climb rate goes up by 110ft/min. Payload at the old climb rate goes up by 170 pounds. (This is a good reason for reducing drag for us who fly at high density altitudes. - Changes the aircraft from a single to a two seater in summer) Range at the new speed goes up 7%. Range at the old speed goes up 20% (Throttled Back). On ABG I fixed badly fitted wing root fairings and sealed the aileron hinges, elevator hinges and the stab root. This gave me 7mph, 75 ft/min, brought a few places just within safe range and made two-up flying safe in summer - 8000ft dens. alt. Another secondary advantage is a cooler engine at the same speed which reduces piston drag on a hot day which can rob you from up to 6hp. ('Ever felt the friction force at the propellor tip to turn the engine after landing on a hot day? 5 Pounds is about 6hp loss at 2300 rpm). This friction means an increase in fuel consumption of 12% at cruise rpm or a loss of half the excess or climb power at the altitudes where I fly.(IOW one loses half of one's ROC). So one can make an aircraft faster or reduce fuel consumption and climb better by fitting the engine baffles properly. One must also keep this in mind when test flying. Later climbs in a series of saw tooth climbs usually produce lower climb rates due to higher piston drag in the hotter engine. Well this is a mouth full, I hope it is found useful. Excuse my English - it is also not my first language. Take care, Johan Barnard 7AC's ZS-BBS and ZU-ABG South Africa.