Category Archives: Aerodynamics

AoA sensor first prototype

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I don’t see anybody selling an angle-of-attack sensor for FPV RC aircraft, so I’m making my own.

Here’s the first (quite crude) prototype:

AoA sensor prototype

It’s nothing more than a hall effect sensor inside a hollow tube (the black plastic spacer) with a magnet glued onto it. It’s held by the red plastic block with a hole drilled thru it.

The other end of the tube has a crude weathervane attached (the counterweight needs more work).

It seems to work reasonably well:

I’d feed the output into an ADC on a PIC.

The whole thing is too loosey-goosey for flight – this was just a prototype to see if the idea works.

Now I’m trying to figure out how to make a flightworthy version. Maybe this would be a good first 3D printer project?

About flying cars

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About three years ago I spent a lot of time thinking about this. Recently Elon Musk has spoken about flying cars.

So now seems a good time to publish this. It’s long, and it’s half-baked. I decided I had too much else on my plate, so I dropped it. Anyone who wants to pick this up and run with it – go for it!

TLDR: Flying cars must be safe and cheap. The way to do that is automation and redundancy.

Redundant systems make things cheap because (a) they don’t have to be highly reliable (as do conventional aviation components), and (b) more units means mass-production prices. Redundant brushless electric motors driving simple fixed-pitch props are the solution.

Here’s how.  (PDF version is here; 37 pages. PDF doesn’t have the crappy CSS formatting…)

1   The vision

“Liftoff”

You step into your vehicle and settle into your seat as you say “liftoff”.

Your voice is recognized as that of an authorized user – the vehicle actually belongs to your wife.  There’s a quiet click as the charging plug automatically retracts into its compartment, then 100 thrusters –  near-silent electric motors, each with a small fixed-pitch propeller – spin up all around the vehicle.  Over a few seconds, they gradually take up the weight of the vehicle with you in it.

As they do, the vehicle determines the total takeoff weight by measuring how much power it needs to send to the motors to lift you off the ground.  It checks that all the motors are supplying the expected amount of thrust and are running smoothly and in balance.

Oh no – it discovers that 2 of the 100 thrusters are producing less thrust than they should, one is producing no thrust at all, and a fourth is wobbling, probably due to a chipped prop or loose mounting bolts.  It shuts down all four, noting the problems with each one in its maintenance log.  (You meant to replace a couple of those, but things have been busy.)

96 out of 100 motors in good condition is still within the “green” zone for safe flight[1] and the batteries have enough charge, so the vehicle rises vertically toward it’s default hover height, 50 meters above the ground.  It automatically maintains its balance, rising level and straight, directing a little extra power to the motors near the 4 failed units.

You stretch out, putting your feet up and opening your magazine.  The vehicle senses the beginnings of the tilt as you lean backwards (moving the center of gravity), and shifts some of the thrust that way to compensate.

20 meters up, you rise above the treetops and the gusty wind pushes on the vehicle, but you barely notice because the vehicle’s gyro sensors and accelerometers, together with the GPS receiver, sense the movement caused by the wind.  Power is shifted to the opposite side as needed to keep you rising straight above your parking spot.

To the garden

You’re looking forward to your lunch at the rooftop restaurant (convenient free parking!).  Normally, you’d just tell the vehicle “take me to Fred’s” and let it do the flying for you[2], but it’s a beautiful sunny day and you feel like a quick look at the progress with the cactus-planting at the little garden you designed for the parks department to replace the old freeway interchange, so you lean forward and grab the control knob.

It’s a stubby rubber knob an inch high, that hardly moves at all.  You give it a gentle twist to the left against its internal spring, and the vehicle yaws the same way, slightly increasing the speed of the clockwise-turning props and decreasing the speed of the counter-clockwise props, to torque the vehicle around.   As those ugly apartment buildings across the river come into view, you let go of the knob.  It returns to its neutral position and the vehicle stops turning.  Then you press the knob forward and the vehicle tilts the same way, shifting a little power to the back and moving forward[3].  The vehicle holds it’s altitude and attitude as it moves toward the gap between the buildings.

Your phone rings.  You click the “hold” button on the knob and let go of the knob, reaching for the phone in your pocket.  The vehicle keeps going exactly on the course and speed you had it.

It’s your business partner, who is going to meet you for lunch.  She’s wrapping up the design for the playground, due to the client this afternoon.  Should the water slide empty into the duck pond or the mud bath?  The client left it up to you.  You find the alternative versions of the plans and stare at each, trying to decide – she can’t leave for lunch until this is done.

While your head is buried in the plans, the vehicle has been flying toward the apartment buildings.  You didn’t really aim it very well – you meant to go through the gap, but it’s a small gap and a ways off.  On the course you set, the vehicle would collide with the larger building in a few seconds. Continue reading →

I made a hovercraft

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I made a RC hovercraft this morning. This is something I’ve been meaning to do for about 20 years.

It took about an hour.

RC Hovercraft

It’s a green foam tray (free on request from the produce department at the supermarket) with a hole in it, a cut-off plastic cup, a ducted fan/motor from eBay, battery, and RC receiver. And a lot of hot glue.

The bolt and washer are to balance the battery – they’re hot-glued down.

Here it is running. It’s way overpowered – it works best with the throttle at about 5%.

Later I made a skirt out of tape – no pics; it looks about the same, just flies higher by the width of the tape.

I’m glad I got that out of my system – now I can move on to something else.

Where lift really comes from

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I’ve been reading Martin Simons’ wonderful Model Aircraft Aerodynamics (3rd ed.) recently. I was hoping to find out, among other things, where lift really comes from. Simons’ book has lots of fascinating detail about aircraft design (not just for models), but didn’t answer that question.

But I have finally figured it out, and thought I’d share it with you–

First, lift has nothing to do with the curvature of wings. Airplanes with completely flat wings can fly (although not as efficiently as with proper airfoils). And airplanes with curved wings can and do fly upside down.

Wing curvature and airfoil shapes are for the purpose of reducing drag – valuable, but not necessary for lift. Because most aircraft wing shapes are optimized for efficient (low-drag) right-side-up flight, inverted flight is less efficient. But possible. (Aerobatic aircraft that spend lots of time upside down often have symmetrical wings.)

Second, lift has nothing to do with “equal transit time” for molecules on the top and bottom of the wing surface – this is simply a myth (well debunked both at Wikipedia and in a nice article at Plane & Pilot magazine). While it’s true that air flows faster on top of a wing, it does not rejoin the flow on the bottom at the trailing edge of the wing.

So what is it, then?

There are two popular explanations, both of which are fully and completely correct, because they are two ways of describing the same thing. One, Bernoulli’s Principle, is easily misunderstood and leads to lots of confusion. The other, Newton’s Third Law of Motion, is simple to understand.

Wings generate lift by deflecting incoming air downwards.

Therefore, lift comes from Newton’s Third Law – the action of pushing air down results in a reaction of the wing being pushed up.

As an aircraft flies, the wing presents some angle of attack to the incoming air – a wing that is generating lift isn’t aimed straight forward; instead the leading edge is a little higher than the trailing edge; this is the angle of attack.

Fundamentally, lift is the result of angle of attack and forward motion of the aircraft relative to the surrounding air.

To just slightly oversimplify – air molecules come at the wing and whack into the bottom side, bouncing off downwards and pushing the wing up. This transfers momentum from the molecule into the wing, generating lift. This sweeping of air molecules below the wing leaves less molecules above the wing – a partial vacuum. So air rushes in to fill that vacuum, most of which comes from above (it can’t come from below because there’s a wing in the way). More lift- a transfer of momentum from forward motion of the aircraft to downward motion of air behind the aircraft. The air goes down, the aircraft goes up. Action and reaction. This is the Newtonian explanation of lift.

In Bernoulli terms (again, slightly oversimplified), because of the angle of attack, lots of air molecules are swept down under the bottom of the wing, and less go over the top. Since there are now more molecules on the bottom and less on the top, there is more pressure on the bottom and less pressure on the top. So this pressure differential pushes the wing up. This is the Bernoulli explanation of lift, and it is equally correct – Bernoulli’s equations are derived from Newton’s – it’s just another way of saying the same thing.

And, yes, air on top of the wing does move faster than on the bottom, just as Bernoulli says. In Newtonian terms, because the leading edge has swept lots of air molecules under the wing, there are fewer molecules on top. All the molecules in front of the wing (before the sweeping-away) are still pushing on those few remaining molecules on top of the wing, but there’s not much pushing back – so they accelerate. And vice-versa on the bottom of the wing.

If you find the above confusing, I can’t recommend a better introduction to the basic behavior of molecules and matter than the first chapter of Richard Feynman’s Six Easy Pieces – it should be mandatory reading for high school graduation.