Month: July 2024 (Page 2 of 3)

The universally quoted reason why electric planes aren’t more prevalent, is that battery capacity is insufficient.

NOT TRUE.

How long do YOU fly for in a typical sortie?  My average is about 45 minutes.  That’s enough to get airborn, do some circuits, some stalls, and generally have a good time.  Or enough time to enjoy that early morning or after work spin, some low flying over farmland, and a circuit or two.  Or even to go on a scenic flight around the neighborhood, follow some rivers or streams at low altitude and pretend to be a WWI ace.

The OpenPPG electric power system (which costs the same as an ICE) gives you up to 80 minutes of absolutely secure never-fear-the-dreaded-engine-failure flight.  80 minutes.  That is enough for a short cross-countrty hop.  Plan your route well, and you can cover as much distance as you wish if you’re able to drop into an airfield after an hour or so and change batteries.  And that’s the secret.  Carry a spare fully charged battery.

So – let’s talk numbers.  How much does a spare battery cost, and what does it weigh?

Cost (according to OpenPPG) is $2,200 and weight is 22.5kg.  If you can afford either the 22kg weight penalty or the $2.2k price tag, get a second battery, (and use it to help arrange your CG.  Batteries don’t lose weight as they lose power…)

I’m sold on this idea of an electric Pou.  The question remains – can a 33hp-equivalent powerplant fly the Tiny Cedar Flea?

With just on 87m^2 (about 100 sq ft) of wing area, and an empty weight of 92kg (203 lbs), the OpenPPG electric motor will be MORE than sufficient to make this bird fly like a Valkierie for over 60 minutes.

I’m sold.  Are you?

in 2024, recreational aviation is in sharp decline.  Fewer and fewer younger people are joining aero clubs, signing up to Facebook aero groups, and certainly very few are spending money to buy aeroplanes.  Arguably, financial times are tough, and disposable incomes are being eroded by the rising cost of living.  Yet go along to the waterways on any given weekend, and you will see young folks whizzing by on jetskis…

Not an insignificant investment – yet this seems to be no impediment to younger enthusiasts.  And why is this?  Quite apart from the financial investment – consider the very much easier pathway into the sport.  No 40 hours of flight training, and numerous theory exams to pass.  Just about any waterway can be your playground (no trailering your new toy to one of a declining number of out-of-the-way airfields).

Foilboarding is another sport to which younger people are flocking.

Again, they’re not cheap (if one ignores those on offer at Ali Express).  But they’re exciting, there are no restrictive training or licensing requirements, and you can just slide them into the back of your car and go play.

My point is that personal aviation, once the exciting, cutting-edge passion of young men and women has given way to other more accessible sports.  And cost of entry doesn’t seem to be a significant barrier.

Closer to home, however, is the exponential growth of the Radio Controlled planes and drones.  You can buy a drone for under $100, but hundreds of new drones are sold daily around the world for $1500 plus, as people flock to the sport.
Why?  They are relatively cheap, come with all manner of flight stabilisation, onboard telemetry, cameras, navigation – you name it.  It’s exciting.

My local RC field in Brisbane is crowded to capacity every Saturday (and most evenings also) with drones and fixed wing RC planes buzzing around.  Interest in flying, per se, hasn’t diminished.  It’s alive and well.  It’s just the over-regulated, expensive and red-tape bound “recreational flying” secor which is dying on the vine.

And as interest in recreational aviation dwindles, the pace of innovation slows down, and the torch is being taken up by those sectors of aviation which currently attract high numbers.

Take the PPG world for example.  A good Powered Paraglider will set you back just short of $20k.  There are cheaper options, like the excellent Parajet for about half that price.

My point is that both the cheaper RC/drone sectors and the more expensice PPG sectors niches are booming, with hundreds, and in some cases thousands of new participants each year, even as the ranks of fixed wing rectreational aviation thin, leaving by and large old men dreaming of yesteryear.

And nowhere is this withering away more true than in the sub-genre of the Pou du Ciel.  It’s glory days are over, and increasingly, Facebook groups dedicated to this wonderful little airplane are peppered with photos of old planes, comments and photos of old designs newly built by aging builders, and a hardening of opinion ranged against anything which emerges which might not be strictly according to the “Formula”.

But this is NOT the case at rtfm-aero.

We’re not interested in strapping a parachute to our backs.  We love the quirky little Pou du Ciel (or “Flying Flea), and all our efforts are devoted on a daily basis to advancing the design.  I strongly doubt that we will ever relive the glory days of personal fixed-wing aviation, because history has moved on.  But we can aspire to attracting a whole new generation of would-be aviators to an aircraft which is:

• Cheap to build (under $15k)
• Safe and easy to fly
• Full of modern technology
• Able to spark the imagination again.

As a 2-axis plane, the Tiny Cedar Flea uses a single control stick to fly.  No ailerons, no hand/foot coordination in turns, and it is impossible to stall.  Wings which can fold in under a minute for transportation.  Easy trailerability.  Tundra tyres for rough field landings.  Very short take-off and landing.  In fact, everything which would make the TCF a delight to own.

Add in a digital screen showing all pertinent engine readouts, altimiter, airspeed, and full moving map navigation.  We’re looking at different wing configurations, different construction methods and a thorough investigation of alternative powerplants.  I’ve already written about our new construction method (drawing on the rich experience and expertise of cedar strip canoe/kayak building), and I’ve written about better wing configurations (Nanadovic, Axel Darling and Jean de la Farge have all influenced our design choices.

My friend Koen Van de Kerckhove has been busy sketching away, and his ever-inventive brain continues to come up with interesting designs.  I’ve taken one of his sketches, made some RTFM-type changes, and this is what I came up with.
It is a Flying Flea, but a canard-variant.  i.e. the front wing is smaller than the rear wing.  Axel Darling proved that this configuration is superior to the Mignet formula (which had the larger wing in front). By reducing the front wing a bit, it puts the CG roughly in the correct position (at about the pilot’s belly button).  Making the front wing smaller moves the CG rearwards.

The rudders (two of them) are mounted to the rear wings.  That is going to make a very striking package.

Engine?
Well, it needs to be light, in order to get the CG in the right place.  Possible candidates are the Eos 150 (4-stroke, 30hp, 20kg, 2.5l/hr); the Bush Dawg 4 Stroke EFI Paramotor From BlackHawk (4-stroke, 40hp, water cooled, EFI); the Vittorazi 185 EFI (the only EFI 2-stroke – 15kg, 26hp).  I’m not a great fan of 2-stroke engines, but the EFI on this one gived almost 30% greater fuel economy over a non-EFI 2-stroke, the exhaust system is extremely quiet, and there is little to no power band to speak of.  This is the first 2-stroke I’d consider flying behind.

One of the advantages of buying a paramotor is that it comes with everything – all of the above, for example, come with everything already wired up.  Just mount the digital readout, fix the battery in place, mount the throttle somewhere convenient  and that’s it.

Another interesting powerplant is the electric OpenPPG electric motor, or here:

The question is – can you settle for 60-minute flight durations?  If you can, then the electric option is a real contender.

And while we’re about it, why not opt for the automatically variable pitch prop from Scout Aviation?  30% better fuel economy isn’t something to be sneezed at.

Well, there you have it.  Lot’s to think about, and lots of fiddling, drawing, re-drawing and calculating.  But this is looking very promising.

Most Fleas have a clutter of struts to keep the front wing in place.  This is both messay, unsightly and very draggy.  It’s only benefits are that it is light, and very secure.  Here is a typical example – of a Croses design.

Interestingly. Emile Croses then went on to design a far cleaner looking airplane.
So much neater.  This is how I’ll do things on the Tiny Cedar Flea.  So the question then becomes, how does one build these struts?

And the answer is simple:  build them the way one would build the wing spars.  A box spar built from Paulownia, with a balsa airfoil shape around it, and glassed.  It will be super-rigid, very strong and look beautiful – certainly in keeping with the rest of the Cedar strip construction.

These two pillars are then bonded directly to the firewall.

I’ve been working on the wing arrangement for the reverse pou.  Mmmm  that’s not a good name.  What about Duck pou?  Sounds too much like Duck poo.  Pou Canard?  Clumsy. Axel Pou – after the aerodynamist Axel Darling, who worked out the aerodynamics of this configuration, and showed it to be significantly superior to the Mignet (et al) designs.

Quite apart from it’s rather racy lines, this configuration (in Axel’s words) IS VERY INTERESTING:
Consider the stability of such a planform… the N.P. is going to be much further back than in a standard Pou planform, even equal span, equal chord.

For instance, with 4.4M forespan and 5.8M aft the N.P. is
~1.3M as apposed to .997M for your planform. That’s
41.3% as opposed to 31.65% of total chord – all else in
form, fit and function as a real Pou, not a tandem.

In a complete analysis several things crop up but the most
obvious are foreplane loading is greater, say about 1 lb. /
sq. ft., (Better) but is still mushes at only about 44 km/h
(+20 degrees incidence.) AND it always remains in laminar flow with proper entreplans [ed: interplans = wing gaps vertical/horisontal]. In this case, the foreplane is lifting at about CL=5 in the center and like most other laminar scenarios in the Mignet/d’Escatha formulas, the total lift coefficient for the wing system exceeds CL=2, which is over twice that of a conventional Mignet planform.

An interesting aside is that as lift distribution across the spans is different because in the larger foreplane planform the aftplane is severely and almost equally suppressed from tip to tip, the smaller foreplane leaves lift bumps at about the vortex locations [Ed: at 89% wing span], which when the aeroplane yaws, allows a much greater yaw/roll force and much stronger dynamic lateral stability and much better and immediate pilotage in turns. This undoubtedly is the reason why the mad professor exclaimed that his backward Pou flew so much better than the reverse.

So, the first 25% scale model I’ll build and test will be the “Axel Flea”.

The plan is to cut a fuselage out of 30mm blue foam (of which I have a number of sheets already, and to then 3D print the two front wing supports. (just because I need an excuse to use my 3D printer for something useful).  Standard RC servos, and control linkages to mimic full-sized Fleas will be used.

I’ll CNC carve the wings (NACA 23112 airfoil), and slide in a CF rod for stiffness.

I actually can’t fly RC planes, so I’m going to try to tether the plane to a stake and fly it (with my RC controller) in circleslike the Control Line guys do.  The aim of the exercise will be to observe:
(1) the slow flight characteristics (how does it stall, and what about parachutal descent?)
(2) What effect will fast flight have on aircraft controls?
(3) Using very small capacity batteries, how long will straight and level flight last (i.e. a crude way to measure aerodynamic “clean-ness”).  Axel Darling claims laminar flow of both wings.  This should be interesting to observe with wool tufts to verify on the video camera.

And there you have it.  Let me know what you think.

After a few weeks of intensive research, I have finally narrowed down the design candidates for RC (25% scale) testing.  They are:
The jean de la Farge “Pulga” variant:

Main features:

1. Main wings of equal span
2. Wings same chord (1m)
3. Tail wing also swivels (about 1/3 area of main wings)
4. Both wing swivel (front: 20 deg, rear: 40 deg, tail 60 deg)

Expected flight characteristics:

1. Very low stall
2. 5x speed range (stall/S&L flight)

Axel Darling did pioneering aerodynamic research into the so-called “Reverse Pou” and sometimes referred to as a Canard Pou.  So called because the smaller wing was in the front.  Axel calculated (and verified) that if the Eh and Ev (horisontal and vertical wing gaps) were identical, both wings would experience laminar flow.  In addition he writes: (Pou Renew ed 41)

“the smaller foreplane leaves lift bumps at about the vortex locations, which when the aeroplane yaws,
allows a much greater yaw/roll force and much stronger
dynamic lateral stability and much better and immediate pilotage in turns. This undoubtedly is the reason why the mad professor exclaimed that his backward Pou flew so much better than the reverse”

Main features:

1. Smaller wing in front, larger in rear
2. Vertical and horisontal wing gaps equal

Expected flight characteristics:

1. Superior handling, esp recovery from disturbances
2. Very low drag
3. Slower stall/landing

Keeping to the Axel formula, I’m going to try a low-wing Pou.  Why not?

First, I’ve decided to use the well-respected NACA 23112 airfoil – as recommended by nearly all modern Flea designers.  It is a much thinner airfoil that the NACA 747a315 I had been considering, but it has a much higher max Cl (1.5 vs 1.35), which lowers the stall by 5 kts.

Second, I’ve been agonising over how to pivot wings which need to pass through the fuselage.  Why do they need to pass through the fuse?  Well, if they didn’t, I’d have to secure them to the airframe at three points (at the pivot point, and on the pivot line outboard just like other Fleas.  Three external tubes on each side.  And that would spoil the aesthetic completely.  So in order to make them clutter free, the main spar would need to pass though the fuselage like all other cantilevered wing airplanes.

If there were only one wing, I’d simply close in the D-tube, and that would provide the required stiffness and torsional rigidity.  But check out what would happen at thew rerar wing.  The D-tube would go right through the pilot’s bum.  Not a pleasant prospect.

My solution is to have a super-wide main spar (100mm in fact).  Closed in on both sides, this would form a very stiff replacement for the usual D-tube, and protect the pilot’s bum at the same time.  The main spar will be attached to a 25mm tube, which rotates in two bearings fixed to the bulkheads.  Secure, simple and effective.

The outer panels don’t require this oversized main spar, and will revert to the usual 20mmx20mm spar caps.

If you take a piece of 3mm foam (say) and bend it – it does so easily.  Now bond ordinary paper to each side, and suddenly the foam becomes extremely stiff.  Why is this?  It is because as one tries to bend the foam/paper sandwich, the paper on one side is being stretched, while of paper on the other side is being compressed.  And the force required to stretch paper is quite large.  And since the foam core will resist any buckling effect, the composite test piece becomes very stiff indeed.  Increase the thickness of the foam, and the stiffness increases exponentially.

Wings operate in exactly the same way.  A wing spar consists of two strips of material (called spar caps) separated by what is called a shear web.  This is usually (in a wooden wing) plywood.    And this web need not be massive.  The venerable Cub, for example, kept the two spar caps apart with 1.5mm plywood (1/16″).  Under load, the top spar cap is stretched, while the bottom spar cap is compressed.  If there were nothing tying the top and bottom together, the load would cause the wing to collapse.  But the shear web ties top and bottom together, and absorbs the differential forces.  At some point, the shear web’s ability to resolve the opposing forces will be exceeded and the wing will collapse.

So when it comes to designing a wing, most folks use spruce for the spar caps, and plywood for the shear web.  Plywood has a shear strength of about 2Mpa – and this has proved quite sufficient.  But it is also quite heavy (680kg/m^3).  Paulownia, on the other hand has a shear strength of 5.3Mpa, and weighs about a third that of plywood (260kg/m^3).  It turns out that Paulownia is by far the better material for shear webs.

As for the spar caps themselves, while spruce has been the wood of choice in the past, it is more than twice as heavy as Paulownia (550kg vs 260kg per cubic metre) but has almost identical shear strength.  And so it turns out that Paulownia is again far preferable to spruce for the spar caps.

In conclusion, the Tiny Cedar Flea wings will be built from Paulownia spar caps, and Paulownia shear webs.  A no-brainer, really.

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…

It’s fair to say that most if not all folks interested in sport flying (that’s you and me) are interested in what’s happening as far as electric propulsion for aeroplanes is concerned.  However, the truth is “not enough”.  Sure, there are a number of light airplanes flying all-electric, but you’d have to sell your house in order to afford them.  Slightly more promising is the option to replace your trusty gas engine with an electric powerplant – and a few of these are becoming available as turn-key installations.

For example, in conversation with Chip at Aeromarine, he tells me that he will have a plug-and-play electric alternative by the end of the year.  30Kw, 18kg, but no price.  Endurance?  No info either.  Or maybe he was just too busy filling orders to have the time to reply more comprehensively to my questions.

Meanwhile, across at Skyleader (based in the Czech Republic): they offer the following info:

Q: How long is the flight time on one charge?
A: Flight times average 1½ hours per charge at a constant cruising speed.

Q: What type of motor is used in the ElectraFlyer?
A: The motor is a custom built DC 18hp (13.5kw) permanent magnet motor.

Q: What type of batteries are used in the ElectraFlyer?
A: Each battery pack is made up of Lithium-Polymer (LiPo) cells which provide the greatest specific energy density of any battery type. The battery packs come in 3 sizes, from 2.6kwh to 5.6kwh capacities depending on the flying style you may choose.

Q: How much do the batteries weigh?
A: The largest battery pack (5.6kwh) weighs 78 lbs (35.5 kg) including all packaging and wiring.

Mmmm… the equivalent of 18hp at 35.5kg?  90 min on a clean airframe at constant speed?  That probably translates to 45 min on “ordinary” planes under “normal” flying conditions.  I’m not WILD about the numbers.  But they do provide pricing:

It looks like for the biggest battery pack, it is going to cost $13,995 USD.  When compared to either the Aeromanine or Hummel v-twin engines ($7,500) producing double the power – I think I’ll pass for now.

According to Denis Carly at Aerolite:
“The complete electric propulsion system, which includes the motor, motor mount, 2 batteries, 2 battery chargers, controller, throttle, cables, etc (everything you need to install and run the system) is $9,750. The complete system with 4 batteries and 4 chargers is $13,950.”

The motor runs off one to four lithium-ion battery packs, each weighing about 35 pounds.  So four batteries will weigh 140lbs (63kg),  Dennis states that “a typical ultralight flight profile, with a full power take off, a reasonable climb and a 40-45 mph cruise will result in about an hour of flight time”.  However, that’s in Part 103 aircraft.  Expect far less in your typical home-built creation.

So where does this leave us?  On the one hand, there is some movement on the R&D side of things, and the first of the plug-and-play electric powerplants are starting to emerge.  The downsides are: at least double the cost, and half the power.

Oh well, it’s fun to dream.