2 Design concepts

F1-X1 on the runway2
RTFM-aero aircraft are designed to meet a specific need.  Well, four specific needs, to be precise.

  1. They have to be QUICK to build.
    Decide in Spring, and fly before the end of Summer.  That was the plan.  I wanted a plane which could be built in two or three months, not two or three years.
  2. They have to be affordable.
    And by affordable, I mean the sort of money a normal 9-5 working guy with a couple of kids can afford.  I decided it should cost no more than a reasonably priced family car.  While aircraft aren’t cheap, most people can afford a car or two in the family.  So why not a plane if they so wish?
  3. They have to be exciting to fly.
    I wasn’t much interested in a low-and-slow plane.  I wanted to climb like a WWII fighter, and fly fast enough to get nods of approval from my mates at the airfield.  But I also wanted to join the rest of the crowd at the weekend fly-in’s on Joe Blogg’s farm strip.
  4. They have to look like a million dollars.
    So no rag-and-tube, no ugly ducklings with ‘nice personalities’.  It had to look like it just won a makeover contest.  I want heads to turn when I’m lining up…

So… Quick build, cheap, fast and sexy.  A pretty tall order.  This is how I met these challenges…

First, the F2-X is as small as possible.
Small aircraft are quicker and cheaper to build than big ones.  That’s just a fact.  And if money is in short supply, then the smaller the plane is, the sooner you’ll be able to save the money to get it built.

Second, the plane is generously powered.
Generously powered – not over-powered.  The X-wing design is extremely efficient, so the F2-X will be powered by 85hp. And if the design calculations are to be believed, it will go like a rocket, so that you can give those sardine-tin RV’s a go for their money…  And talking of money – I was also not interested in fitting a Rotax or something similar.  Mere mortals can’t afford engines like that…

Third, the F2-X has an extremely low parts-count.
i.e.  As few pieces as possible.  So – the fuselage is a single composite piece (including integral seat, control panel, firewall and all bulkheads).  Add to this two sets of one-piece wings, a single piece stabiliser and a rudder, and in total, five (rather large) pieces make up most of the aircraft.  For the other bits, I’m placing a heavy reliance on off-the-shelf items from Aircraft Spruce, since it is easier to buy parts than make them – besides, why reinvent the wheel?  So, engine, undercarriage, wheels/brakes, and controls, are just a phone call to Aircraft Spruce away.

Some of the more distinctive design features of the plane are:

  1. Fully digital controls
    Drawing from the sophisticated world of Radio Controlled aircraft, plans are underway to implement computer controlled, servo activated control surfaces.  What this means is that for the first time ever in a production light sport aircraft, we can take advantage of the wide range of sophisticated digital enhancements which the RC world have developed and have tested over many thousands of flight hours.  For example:

      • Differential aileron deflections
      • Aileron/elevator/rudder/throttle coupling of whatever type you wish. Typically (as we know), a wing with dihedral in a turn requires (1) additional throttle, (2) some back pressure (3) some rudder. All this can be provided by a simple controller mix (and can be adjusted infinitely if you don’t get it right first time). How cool to go into a beautifully co-ordinated turn simply by deflecting the ailerons?
      • Audible feedback (ie a sexy female voice) on altitude/speed/battery levels
      • “Exponential” – i.e. movement of the stick close to the centre produce less control surface movement than the same stick deflections further from the centre. This has the effect of “damping” small control inputs, making the ride smoother
      • Feedback from the telemetry of the receiver can auto trigger the degree of exponential – so, for example, at slow speeds, the amount of actual flying surface deflection can be at its maximum, but as speed increases, so the maximum control surface deflection decreases.
      • Fitted with one of the gyro-enhanced stability modules, the plane can be flown (1) “naked” – i.e. with no stability enhancement, (2) in “2D mode” which is basically wing leveling mode or (3) “3D mode” which fixes the orientation of the plane according to stick input – automatically compensating for wind gusts, turbulence etc. So, for example, you get into a spin. Let go the stick (good idea anyway) and hit the “2D” switch. The onboard gyros instantly apply rudder to stop the spin, and have you flying S&L in seconds. Want to do it yourself? Be my guest. Just don’t hit the “Panic” button… and be sure to stomp on the rudder yourself.
      • Fitted with a GPS module (about $100) if you get lost, you can hit the Return to Base button, and your plane will fly you unerringly back to where you took off from and circle the field once it gets there.The list goes on and on…
  2. Full physical linkage backup
    What happens of the flight computer fails?  Or goes haywire?  Or the battery decides to quit mid-flight?  No worries.  For this reason, the entire system will be fitted with a fully physical control linkage (like “normal” light planes have) which works in parallel.  In the event of a computer failure, simply switch the power off, grab the stick and fly the plane home as usual.  No dramas.  It’s a non-event.
  3. Speed brakes
    A plane like the X-wing is going to be extremely difficult to slow down.  So it will be fitted with speed brakes.  Deploying them will instantly add over .25G of drag.  Tests have shown that one can go from 125kt to 75kt in under three seconds.
  4. Flaps
    One of the major compromises which must be made when designing an aircraft is stall speed vs cruise speed.  Generally, a slow stall requires a large wing, and a high cruise speed requires a small wing (available power being equal).  Rather than go for the middle ground, I have decided to go with the smaller wing, but implement the biggest flaps possible.  In aircraft, there are basically three flap “families”.
    (1) Plain flaps, which simply hinge the rearward 25% or so of the wing’s trailing edge.
    (2) Fowler flaps, like the ones fitted to commercial airliners, these extend rearwards as they are deployed, increasing the effective wing area, thus producing more lift in their own right, even before deflection.
    (3) Split flaps.  Basically, the bottom surface of the wing (from about 25% chord) tilts down, leaving the upper surface of the wing unchanged.

Plain flaps are widely used in private aircraft.  Fowlers are usually found on commercial airliners.  And split flaps seem hardly used at all nowadays.  The X-wing will employ a combination Fowler/Split flap.

How does this work?
When initially deployed, the bottom part of the wing skin moves rearwards by almost 40% of the wing’s chord and angling downwards slightly, greatly increasing the effective wing area, producing significantly greater lift but almost no additional drag.  This is the setting for takeoff and for low-and-slow flight.  However, when the flap is extended further, it angles downwards sharply, producing both high lift and high drag, slowing the plane down and keeping it in the air at the same time.There are three flap settings.  (1) no flap (cruise condition) (2) extended flap (takeoff and low and slow flight) and (3) deflected flap (landing configuration only).The flaps will be electrically controlled.

Construction materials:
The wing spar is an all carbon fibre affair, producing an incredibly strong yet remarkably light structure, capable of full aerobatic loading.  The fuselage shell will be a 10mm foam/glass fibre sandwich, with seat, and control panel both acting as integral bulkheads.  The wing will be a molded composite skin.

The limiting factor is weight – in order to keep the CG where it is supposed to be.  The F2-X is designed around one of the VW conversions, although any engine provided it weighs between 150lbs  and 180lbs should suffice.

With a constant eye on keeping the build simple and the cost low, I’ve designed the windshield to be a single wrap-around piece of Lexan.  No costly or tricky blowing of a complex “bubble” canopy.  Just a sheet of flat Lexan, bent and fastened to the airframe, and yet it “looks” like its been blown.  It’s all in the underlying contour lines.

The left side of the windshield hinges outward from the side brace to permit extremely easy access to the cockpit.

The X-wing (tandem) wings, joined at the tips, provides for an extremely rigid yet light construction.  The F2-X is essentially a biplane.  The fore wing (swept back) is the main load bearing surface, responsible for about 65% of the lifting capacity of the plane.  The rear wing (swept forward) is responsible for the remaining 35%.  What this does is:

  1. Makes the plane extremely spin-resistant.  The worst case stall scenario is a gentle mushing forwards as the front wing loses lift.  The rear wing never stalls.
  2. Provides exceptional forward and downward pilot visibility.
  3. Allows for decreased span – making the plane easier to hangar.

Control panel:
Rather than the flat 2-D panel so common in light aircraft, the X-wing will have a sports car-like sculpted dashboard housing the required instrumentation.  This panel will be built from carbon fibre, and besides looking sexy, also acts as an integral structural bulkhead.  Everything depends on my skill as a sculptor.  I will try to find a sports car dash on which to model the CP, and then customise it from there.  That is gong to take a few iterations, I suspect, before I get it right.