Technology is changing – moving on

In the last few years (2011 to 2015)  RC control technology has gotten better, smaller and cheaper; and multi-rotors came along. This has changed my thinking slightly. I have experimented with multi-rotors, and are now heading back towards FPV planes, but this time smaller and with much less payload capacity. There are two parts to this: choosing a a platform and fitting it out.

The Platform

I am still in favour of a flying wing, pretty much for the same reasons; see below.

Fitting Out

Technology changes have made some big differences to the fit-out. This warrants another page, as it’s another complete project; HERE.

The Updated Plan

As of mid 2015, to build a simple fpv flying wing to carry a small equipment payload. The wing must be simple, robust, easy to work on and easily repaired or replaced. It must be waterproof enough to fly in real rain. The target is for efficient stable flight. It’s got to be relatively quick and have a flight time of 60 minutes or more.

Concept so far

A flying wing because it’s easier to construct, make waterproof. transport and has good payload capacity. In-line puller is simpler, quieter and more efficient than a pusher or something with a traditional fuselage and wing.

Likely construction of a 0.8 to 1.0 meter wing would be of foam with the strength coming from a vacuum bagged glass outer skin. The wing contains everything except the motor (attached to the front), LEDs, servos and control surfaces and cavities for the control system. Smaller vertical fins will likely be attached to the wing tips,

Small payload modules will attach under or on top of the centre of the wing

One payload module could contain a motorized moving weight to adjust CG in flight, just to determine best CG and trim. A small parachute would be interesting to try for vertical landings. Being relatively small, light and robust, the parachute could be small and just need to “take the edge off” the descent speed. It will fall flat.

A basic stability controller assists with flight.

Details – to be revised to suit updated plan

• 1 meter max. span – flying wing – puller (front motor)
• Wing: profile = unsure, probably semi symmetrical (MH60 – MH64)
• Wing: 300 mm chord at root, 200 mm chord at tip, minimal reflex (1°-2°)
• Washout root to tip (tips angled down for less AOA, therefore less lift)
• Motor/prop: 470KV, 300 Watt, 12×6 prop, 30A esc
• Power: two 3-cell 2200 LiPo, series connected for 6-cell
• Flying weight: less payload : <1Kg
• Payload : various options up to 200g
• 5.8GHz FPV system with OSD
• Safety: autopilot stability with RTL, parachute landing/fall-arrest

Original details – left for reference only

• 2.1 meter span of two 1 meter wings and a central 100mm section/joiner
• Wing: profile = Midnight (thick for longer version of Clark-Y)
• Wing: 300 mm chord at root, 200 mm chord at tip, minimal reflex (1°-2°)
• Washout root to tip (tips angled down for less AOA, therefore less lift)
• Winglets: leading edge not swept, 500 mm ea, 200 mm to 100 mm chord
• Winglet: 100 mm base chord, 100 mm U/D, reflex (2°)
• Fuselage: 500 mm 60×50 or 100×50 rectangular plastic down-pipe
• Motor/prop: 1000KV, 500 Watt, 12×6 folding prop, 40A esc
• Power: 3-cell LiPo, 2200 to 8800 mAh
• Flying weight: unknown, likely 2Kg min.
• Safety: autopilot RTL, parachute landing/fall-arrest

Notes:

While originally looking at RC model flying wing design I found lots of information that was relevant but padded out with stuff that was not.

The following is just a few notes most relevant to this project:

To fly slower or carry a load and not for aerobatics; choose a flat-bottom or under-cambered airfoil.

Leading edge radius has biggest effect on stall characteristics. A small radius makes for a shallow stall angle (will stall sooner). Tip stalls occur when wing tip stalls before wing root.

1. Make leading edge more blunt towards tip.
2. Avoid wings with high aspect ratio and high taper ratio.

Washout is when built with a twist so that the wing tips have a lower angle of incidence than the wing root. Not for aerobatic models.

Taper ratio is tip chord divided by the root chord. Aspect ratio is wing span divided by average wing chord. High aspect ratio (like gliders) with high taper ratio is more prone to tip stall than say a delta.

Airfoil thickness: eg. a wing of 300mm chord that has 10% thickness will be 30mm at the thickest point.

Thickness compromises speed and lift. Thicker wing has more drag, more lift and can fly slower. Thinner wings fly faster and more smoothly as they are less affected by the air.

Wing loading is a compromise : flight speed, predictable landings, rate of climb from wing lift, control response, and stability in flight.

One of the most important decisions : wing aspect ratio effects : lift-to-drag ratio, roll rate and pitch sensitivity. High aspect ratio is the most aerodynamically efficient; long and thin. Too high and roll rate will be sluggish. Too low and the plane may be twitchy and loose too much speed in turns. Low aspect ratio = larger drag as angle of attack increases.

The Taper Ratio is the tip chord divided by root chord. For a highly tapered wing,keep the aspect ratio down. For a high aspect ratio then keep the taper ratio closer to 1 (root = tip).

Pitch stability: Because a wing has no horizontal tail for pitch stability another method is needed to move the aerodynamic centre back behind the CG to make it stable. Sweeping the leading edge back and angling the tips down (washout) relative to the root has this effect.

Reflex is where the wing trailing edge curves back up so that when the bulk of the wing has positive angle of attack and provides lift, the trailing edge is horizontal, providing no lift and acting a bit like a traditional tail-plane. You can also think of it as pushing the back of the wing down to cause a positive angle of attack and therefore lift.

Symmetrical wings need less reflex than flat-bottom wings to produce the same effect; therefore less drag.

Wing sweep (swept back) has the same effect as dihedral (5° of sweep = approx. 1° of dihedral). Sweep also improves stability as it causes the aircraft to pitch down in a stall. Sweep increases the CG range and moves the CG back.

Dihedral has several advantages:

• Increased stability
• Allows an aircraft to be steered with the rudder alone (no ailerons)

Washout is a twist built into the wing putting the tips at a lower angle of attack than the wing root to delay or prevent tip stall. Washout is beneficial for high aspect ratio wings : gliders, drones etc. Trim level flight so that the tips don’t provide lift.

Strip ailerons are easier to build and have a good roll rate. Tapered strip ailerons are better and less prone to flutter. Strip aileron area is typically 10% to 20% of wing cord. Tapering to wider on the outer end is preferred, reducing drag and increasing roll response.

Nose length forward of the wing is simply to balance the plane without adding weight. Probably necessary if the motor was at the back, pushing.

Not directly relevant for a flying wing

A combined fixed stabilizer/elevator area of about 20% of wing area is safe but can be reduced for better efficiency.

The vertical stabilizer (including rudder) of 12% of wing area.

Pitch Stability: The balance point of a wing is at about 25% of the chord back; the 1/4 chord point. To begin with, knowing nothing else about the model, put the CG at about this point or slightly forward, 10% max.

Construction and fit-out

The details have not been finalized yet.  So this section is mostly just a series of notes.

Remember the primary goals: as light-weight as possible, waterproof and simple.  Construction and assembly must be as simple and quick as possible, so that I can make a few and won’t be too concerned if I loose one.  It must be easily repeatable, even if that means making a few jigs early on.

Construction

The plan at the moment is to wire-cut foam, either gold-foam or standard polystyrene, then cover it with a very thin layer of fibre glass, vacuum bagged.  This makes it light and strong, and solves the problem of complex leading and trailing edges.

The most difficult or fiddly part will be installing servos, fpv cameras, LEDs and wiring inside the wing.

I’m thinking of cutting the cavities out of the foam and putting in temporary filler blocks for support during glassing/vacuum-bagging.  The glass outer can be cut, the blocks removed then the gear installed and a cover attached.

Some strengthening, wiring and possibly LEDs can be installed in the foam before glassing. 

Details for Fit-out

The only often accessed hatch is a large central hatch/panel on the top surface provides access to batteries and the control system.  The cover can be taped to keep it waterproof.

Wing servo linkages will be on top to protect from landing damage.  Covers may be best vacuum formed and stuck on.  There are smart ways to hide control linkages inside the wing profile, but I want to avoid the complexity any extra weight.

The two wing mounted cameras must be waterproof and aligned with each other; allowing switching without a changed view.  I’m not sure yet how to handle mount or cover the camera lens to cope with rain without needing a wiper. Hopefully air-flow will keep it clear enough.  Possibly setting the camera back slightly and having a curved window that follows the wing leading edge profile. Will it distort the view ??

Some LEDs can be just under the surface of the fibre glass.

Trimming a Flying Wing

This assumes your flight controller is turned off or set to manual pass-through.

• We need to get the CG as far back as possible while maintaining stable pitch control.
• Little tail moment makes it very pitch sensitive.
• Exponential control is required on elevator – pitch axis.
• Differential aileron mixing will result in pitch while rolling.
• The distance between the elevon and CG is the tail moment. Short, so elevons have less effect -why CG is so critical.
• If nose heavy, will require more upward deflection of the elevons to fly level – adding drag and a nose up attitude. It also improves pitch stability, making nose heavy and elevon reflex (up) a good starting point.
• Start by setting correct balance at the ideal (theoretical) CG point. then add a bit of weight to the nose.
• Add a click or two of up trim. Run/Bike with the wing to get some wind over it and see if it’s about right.
• Try a test launch, holding a little up elevator for safety 🙂
• You should have to start adding down trim and launching harder.
• When flying level with correct trim, begin moving the nose weight back a bit at a time adjust trim and repeat.
• Keep going until pitch control just starts to become a bit twitchy – the move the weight forward until it feels right.
• This is a good CG balance point.
• Now dial in the best glide-ratio and speed. Adjust trim using the standard shallow dive method while watching it pull up or tuck under. If it pulls up from the shallow dive by itself, add a click of down trim and repeat. Keep going until it is neutral, stays in the shallow dive on its own.
• A slight almost unnoticeable pitch oscillation is level flight may be OK.
• The wing will have a preferred speed in level trim – probably quite fast. This is the best glide-ratio speed.

Links – more Information

• Airfoils for flying wings – here.
• Flying Wing Set-Up – Wash-Out and Reflex – here. – FliteTest good general description.
• Flying Wing – here.
• Flying Wing – here.
• Flying Wing – here.

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