Ligeti Stratos – First Open Source Microlight Aircraft (2013) | Hacker News Reader

Ligeti Stratos – First Open Source Microlight Aircraft (2013)

ligeti-stratos.com

In a tandem wing configuration like this it is unwise to make the forward wing the low one. This is because at high angles of attack (think takeoff, landing, low speed flight, climbing) the rear wing will end up in the “dirty” turbulent wake of the first wing. Wings are much more efficient in “clean” smooth air. The lower lifting efficiency in situations where you need high lift (see previous list of high angle of attack settings) makes a design dangerous.

If you’re interested in this sort of stuff, this is the best book I’ve found on the subject, it is very practical and written with the engineering minded amateur designer in mind: https://www.amazon.com/Design-Aeroplane-2e-Darrol-Stinton-dp...

This is really how all box wing configuration aircraft are laid out. Here's one from Lockheed: https://www.cbsnews.com/pictures/nasas-big-ideas-for-the-fut...

Here's a concept from Elytron: https://rotorcraft.arc.nasa.gov/Publications/files/Grima_The...

The advantage is lower induced drag during cruise. You do analysis to validate controllability during high-AoA regimes. As with every design choice in aviation, it's a tradeoff.

It's terrifying. From your comment I was thinking negative stagger like has been done for a few designs in the past, not a lower wing that sweeps the entire length of the aircraft, with the upper wing at the tail. The angle where the rear wing is exposed to interference from the lower wing ranges from 15 degrees through 90.

Losing lift on that rear wing, which there is a massive range of opportunity to do, will yank the rug out from under the back of that aircraft. How often will this design suddenly decide to fly tail-first tens of feet away from touching down on the tires?

check out Quickie and Q2

https://en.wikipedia.org/wiki/Rutan_Quickie

t tails on most business jets etc.

There was, in fact, a serious problem with T-tailed jets: Superstall, an irrecoverable stall where the turbulent wake of the stalled wing over the stabilizer and elevator prevents recovery. A crash of a BAC 111 prototype led to the development of stick shakers and pushers, as stalling presents an unacceptable risk in these cases. It is exacerbated with swept wings having a tendency to stall at the wingtips first, as this typically creates a pitching-up moment.

When first discovered it was new, but now almost all bizjets have t tails. So it definitely wasn't a fatal flaw but an engineering problem with the configuration.

Indeed - there is certainly nothing inherently risky about T-tails, as the many T-tailed sailplanes with relatively docile stall/spin characteristics attest.

There are certainly successful exceptions to this rule. In the see exceptions I’m sure that the engineering was done to mitigate the risk. For instance with T tails, the designer would hopefully either place the elevator high enough to be out of the dirty air, or do the analysis to make sure it functioned well in he dirty air.

Rutan Quickie is subject to deep stall, and it has an AOA never exceed limit because of it.

all configurations have advantages and disadvantages. Most civilian jets have tails which are mounted higher than the main wing. The position matters when you are talking about not getting into stall and then recovering from a stall. A lower tail is definitely better for an inexperienced pilot or a pilot in a bad situations. And the safety records of high wing planes like Cessna 172 speak to that.

there are a lot of exceptions. In case of Business jets it's almost a rule to have a t tail.

Nearly all aircraft have that problem with the tail.

A major exception is aircraft with an inverted V tail, like the Lazair ultralight and the MQ-1 Predator drone.

Another exception is the flying wing, like the B-2 Spirit, but that certainly isn't an aerodynamically low-risk design.

Another exception is weight-shift ultralights that look like tricycles hanging under flying wings.

Buy you also get help from the ground effect on takeoff, but year a possibility of rear wing stall puts an and to any talks about virtues of the design.

Well, in response to Sokoloff’s comment below I did some more reading (the history tab on the linked site), followed by googling. Turns out the designer of this plane died in a crash while testing it. I found a crash report here: http://aviation-safety.net/wikibase/217844

Note: the prototype is open sourced, not the production plane.

From the linked article: "This [crashed] aircraft was intended to be the production version of the "Stratos" aircraft. The prototype version had successfully flown some 340 hours. The production model incorporated significant changes made by the designer/pilot."

Good call out, thanks!

340 hours isn't really that much.

Aren’t the microlight planes about as dangerous as a wing suit?

No. But:

Test-flying unconventional prototype / experimental airplanes, or flying commercially available ultralights? With or without training?

And wingsuits – proximity flying? base jumping? Or just jumping out of a plane?

The risks of those scenarios are magnitudes apart.

Personal aircraft are about as dangerous as motorbikes. Base jumping on the other hand, whether with a wingsuit or not, is one of the most dangerous recreational activities you can do, with a fatality rate of something like 1% per participant per year.

I wonder how much the hat BASE/wing suit stat is exaggerated by the “extreme” culture associated with it. I can’t imagine hobbies airplane builders are immediately doing aerobatics at the first chance they can. Wing suit flyers are famously risk maximizing.

They're extremely overpriced, are persnickety about fuel, and certain manufacturers (Belite, i'll call em out) couldn't care less after your check clears.

If anyone wants a crashed belite in the PNW area, let me know.

Which aircraft would you recommend then for private flying?

I had a childhood dream of building my own moto-glider.

I managed to advance to an unpowered glider, but without a motor, and no chance to get a tow from anybody.

I managed to take off once, for a few meters from a slope, and that was it.

Once I hit 16, I had to leave Russia, and the unpowered glider parts still lie in my uncle's garage.

very cool design, one of the few airframes that can gain altitude without pitching up -- this is an important safety feature as it reduces chance to stall.

On the flipside, though, stalling a Ligeti Stratos is probably a major event since it's almost a flying-wing.

Remember that stalling is related to angle of attack of the wing and the speed. You can stall in a dive, even (it's hard, though :D)

In fact, all planes can gain altitude without pitching up, by increasing speed, and by decreasing speed can lower altitude. The resulting lifting force on the wings is for most important flight regimes a function of speed and angle of attack.

This is taught in basic flight school. Power is lift, not pitching up.

The things that are covered in first chapters of flight theory textbook for pilots, but which confound otherwise very knowledgeable laymen, can be summarised as close to all of them ;)

Any recommendations on the books? Because I'm definitely confounded.

I assume that AoA=0 and flat wing (not a teardrop-shaped one) would not lift off despite increasing the engine power (assuming no wind). Is that correct?

I'm very much confused by what you're saying - given that "lifting force on the wings is for most important flight regimes a function of speed and angle of attack", isn't it easier to change the AoA, by operating the control surfaces and pitching the plane, instead of adding or reducing speed?

A wing moving at its zero-lift-coefficient angle of attack will produce no lift, regardless of its speed. If it is at an AofA giving a positive lift coefficient, the lift will increase with speed.

Consider taking off in a taildragger. You start the takeoff roll with the tail on the ground, giving an angle of attack close to that of a stall, but at some point before lifting off, you typically raise the tail, because you do not want to stagger into the air on the point of stalling.

If you are flying level at a constant speed, and then raise the nose without changing the power, you will initially rise, but you have tilted the lift vector backwards, increasing its backwards component, so your thrust is no longer sufficient to maintain speed (it is the same as a car going from flat to uphill.) If you are on the back of the drag curve (where drag increases as you slow down), raising the nose can result in going into a descent.

An aircraft in a straight steady climb weighs no more than when it is flying straight and level, but there are two factors to consider. Firstly, the angle of attack is relative to your trajectory, which is now tilted. Secondly, on account of that, the lift is now tilted back, so the wing must create more lift in order that its vertical component is equal to the weight. Therefore, the angle of attack itself needs to be increased (and even more if your climbing airspeed is going to be lower that the speed you were cruising at.)

Putting it together: to go into a climb, you must increase your thrust (or 'get some back' by allowing the aircraft to slow down to reduce the drag, but that's not an option when you are going slowly.) Secondly, you must increase the angle of attack, but that happens as a consequence of adjusting the pitch of the aicraft to maintain the desired/correct speed.

>Putting it together: to go into a climb, you must increase your thrust (or 'get some back' by allowing the aircraft to slow down to reduce the drag, but that's not an option when you are going slowly.) Secondly, you must increase the angle of attack, but that happens as a consequence of adjusting the pitch of the aicraft to maintain the desired/correct speed.

You’re assuming a lot here. It basically comes down to lift coefficient of the wings when a certain power magnitude of air is forced under them. This comes from thrust. Power. You can climb with power being perfectly level. You should NOT increase your angle of attack (this will induce stall as it increases drag).

It’s weird, it’s backwards, but it’s physics. Pitching up increases drag and increases the angle of attack increasing your power requirements to stay at altitude.

This is a video I found on YouTube from my local college: https://youtu.be/_SJpXQh2KCE

In aeronautical terms, Power as P comes from lift and drag, you assume it’s from the wings, when it’s the thrust from the engine providing the compressed airflow (because we’re going fast) under and over those wings.

I’ve been flying simulators and aircraft most my life as a hobby (due to slight red green colorblindness, I couldn’t be a fighter pilot so I went into computers). One thing I always have friends try is just keep the Cessna straight and level, increase power, she’ll lift on her own when she reaches that sweet spot.

> You’re assuming a lot here.

Actually, I am not. I am assuming only that, for an unstalled wing and a fixed flap setting at a given air density, lift increases with airspeed and also with angle of attack, plus Newtonian mechanics. Nothing here is contradicted by the video you linked to.

> It basically comes down to lift coefficient of the wings when a certain power magnitude of air is forced under them.

This appears to be gibberish.

> Power as P comes from lift and drag, you assume it’s from the wings, when it’s the thrust from the engine providing the compressed airflow (because we’re going fast) under and over those wings.

Ditto.

> I’ve been flying simulators and aircraft most my life as a hobby.

Well, I fly airplanes too. This just goes to show that you don't need to understand the physics of flight in order to learn how to pilot an airplane.

Great, you know the math then. You know how an airplane flies.

One correction to my own post: when you go from level to climbing flight, the primary cause of the increase in the forces opposing thrust is that the weight vector now has a component opposing it. That's how it is for cars, and anything else moving on land, as well. Gliders maintain airspeed because their weight has a component opposing drag, which is how it is for things rolling downhill, as well.

In takeoff and landing, the most critical phases of fight, the range of useable pitch is fairly narrow... too much and you bang the tail into the ground.

Too little and you strike the prop.

The way you’re taught to land you use the controls is basically backwards - the elevators (pitch) control speed. The throttle controls rate of descent.

Given the original designer was killed by stalling it on approach to land, I’d say the safety benefit is unproven at best.

Wait, the designer of this plane died landing it?

Yes.

“During a short test flight of the new highly modified production version in September 1987 the airplane stalled during approach to landing and Charles Ligeti was killed.”

[0] http://ligeti-stratos.com/history.html

No, different plane.

“During a short test flight of the new highly modified production version"

It was a modified version of the prototype upon which the current efforts are based.

That’s not cleanly “exactly same” nor “totally different” airplane design, but the mishap airplane was clearly closer to this effort than to a 172.

Flying wings stall just fine, what makes you think otherwise?

I'm not sure "open sourcing" a major piece of hardware (like an airplane) has a lot of meaning. Kit plans for experimental aircraft are pretty easy to get for a few hundred bucks. But your still thousands of dollars and hours away from having an airplane.

It's a pretty thing for sure. And I think it's neat they want to give away plans. Just don't think it's a big impact.

Open sourcing plans allows for others to modify the plans and for the community to improve them in a way that closed-source, cheaply-sold plans do not.

Open source is about more than price.

Looks like a powered modified box kite

Looks awfully close-coupled

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