RTFM Aircraft ~ Home of the Tiny Cedar Flea

All airfoils tend to nose over – that’s just a fact of life.  It’s part of the aerodynamic mystery that surrounds flight.  So “normal” single-wing aeroplanes have an elevator – a tailplane which can compensate.  The wings lift, the nose wants to dip, and the elevator applies an opposing force.  Result?  The plane flies level.

Essentially, Fleas (like flying wings) need airfoils which have as little pitching moment as possible, because they have no tail-plane.  The way to achieve an airfoil with a low pitching moment is to curve the trailing edge of the airfoil upwards slightly.  The airflow off a wing is directed (initially, anyway) downwards, by about half the wing’s angle of attack.  So if the wing is set at 12 deg, then the downwash immediately behind the wing is about 6 deg.  Why not 12 deg you ask?  Because the airflow UNDER the wing meets the air coming off the top of the wing, and they balance each other out, more or less.  This is why most Flying Fleas set the rear wing incidence to 6 deg – basically resulting in the rear wing being aligned with the airflow.

But this presents a problem, because when the front wing is settled back to (say) 3 degrees in straight and level flight, the rear wing is still sitting back there ar 6 deg – no longer aligned with the oncoming airflow, but now at positive 3 deg angle of attack, and it is lifting “too much”.  This results in the Flea’s tail lifting, and the pilot once again has to pull back on the stick to maintail level flight.

But that’s another issue.  What I’d like to talk about now is the choice of airfoil.

When dear old Henry started out, he got a bit carried away and not only designed a new plane, with a new control system, but he also created his own airfoil.  Letc we be accused of unkindness, let’s just say it wasn’t a great airfoil.

Soon after the initial teething problems with the control system, he switched to a recognised airfoil – the NACA 23012.  Later designers moved to the NACA 23112 which is the same as the 23012, but with more reflex to lessen the nose-down pitching moment.  And until recently, that’s what has been recommended.

Then along came Richard Fraser.  He created a new airfoil for the Flea.  And noone used it.  Actually, not true.  Someone did use it, and it failed miserably to live up to its hype.\

So, are we stuck with the NACA23112?  Not exactly.  The NACA 747a315 is an excellent candidate for Flying Fleas.  It has extremely low drag, extremely low pitching moment, but it also has quite low lift.  So the question is – higher lift?  Or lower drag and pitching moment?

I’ve taken the step to put the 747 into the design spreadsheet.  With 8m^2 of wing area, the TCF will have a stall of 40kts.  Can I live with that?  I plugged in the lift for the 23112, and suddenly I get a 35kts stall.

Mmmm  Time to rethink my airfoil choice, I think.  Maybe those old buggers knew a thing or two…