votingprawn

If you want to avoid doing the maths I think you can draw the problem out (assuming the world is a flat piece of paper and the source is located on the same piece of paper with you)

From the timings given you can work out the distance from location B and C to the wavefront when it hits point A (4 x 330 and 6 x 330). If you then draw a circle centered at point B with radius 1320m, and a circle centered C with radius 1980m, then there is only one circle you can draw that intersects point A, and is tangent with the circles drawn at B and C. The center of that circle is the source (with an appropriate radius of error for timing measurement and distance measurement between A, B, and C)

https://imgur.com/a/tqV0ToU

Its a mugin if anyone is interested (take the manufacturer specs with a handfull of salt) https://www.muginuav.com/product/mugin-5-pro-5000mm-super-la...

Doing some bistromaths on the stated hover performance.

With a 231lb empty weight, 180lb rider, and 5 gallons of kerosene (ultra light version) you get

104kg + 81kg + 15 kg = 200kg

sfc of a Nike is 40 grams / (kN second) (Should have gone with Hawk ;) )

Lets say hover thrust on a nice still day will be around ~ 1.9kN -> 76.7grams per second of flight.

Assuming I've not made any mistakes that gives roughly 3.5 minutes of hovering flight on that 15kg of fuel (ignoring the fact the vehicle gets slightly lighter as the fuel burns off).

That sort of matches up with the spec, assuming the 10-22 minutes stated is for a lighter human on the experimentally licensed version of the vehicle.

Based on the spec I'm guessing it is an AMT Nike

Five engines in the render, assume 1 is redundant:

705lbs to metric: 3134N. Divide by 4 = 784N which is _exactly_ the max thrust of a Nike.

Even if you disregard motor efficiency (which I don't, as with a battery powered vehicle you treasure every Wh more than you treasure every gram of fuel!), propeller efficiency is also important. Ideally you want to match your motors power handling and efficiency with your propeller. A given propeller/motor combination will have a design point in terms of torque/airspeed/rpm, and operating significantly outside of that (e.g. having a power system designed for cruise but operating at zero airspeed) has penalties.

Unfortunately I'm traveling at the moment, but I spend a large amount of my time at work balancing propellers and motors and could provide some plots that might help explain this. One of the few things years of working in academia proved to me, is that I'm terrible at explaining things like this!

There would still be benefits to variable pitch propellers with an electric motor (motor efficiency, correct propeller pitch for the airspeed to name two).

I work in the UAV industry and would adopt variable pitch in an instant if it didn't come with complexity and weight penalties!

I was a researcher at a reasonably big UK University until about 6 months ago, and I used SciHub fairly regularly.

Sometimes I used it because we didn't have the right subscription for that publisher to get me the journal I needed, but mainly I used SciHub to access papers I already had the "right" to access. The layers of logins screens, buggy single-sign-on systems, having to be in the right IP address range, not being able to download a PDF, meant that I almost always attempted to get the paper through SciHub first, and only went through the library systems when I couldn't find what I needed on SciHub.

That comparison only really holds if you ignore the stators, which the rotors are reacting against.

In the back of the van, parked up on the side of the runway at an airfield. We were flight testing a UAV and the data logging system that should have been finished weeks before....wasn't. So we were bodging stuff together so that there would be at least some data logged from the flights.

Right in the middle of summer too, but luckily in Wales so actually quite a pleasant temperature in the back of the van.

I believe those are the homemade "star dials" / motorised planispheres mentioned in the first comment.

Fair enough, no point trying to dispute a family anecdote!

They could do it back then too.

If you dig past the internet comments and read some of the "primary source" books, the picture of the leaks is very different. I can't remember the exact book I read it in, but the author states there was a tank sealant, and it lasted around 50 hours (I think), before it needed to be replaced.

This is somewhat backed up by the Jenkins book [1] which talks about the time consuming process of replacing sealant, and the Graham book [0] that is the source for the Wikipedia claims on expansion. It talks of different sealants used, and how leaks were precisely noted and collected in _shallow_ drip trays.

[0] https://books.google.co.uk/books?id=dX5cCgAAQBAJ&pg=PA129&dq...

[1] https://books.google.co.uk/books?id=lEpWE748QUsC&q=sr71+seal...

I believe if you look for primary sources like pilots and ground handlers/mechanics you'll mostly find reference to small leaks (I think they were referred to as drips caught in cookie trays).

I had a quick search and found the KC135 chap who says the refuelling was needed due to leaks, but without being rude to him I'm not sure he's really a qualified source for that information. It sounds more to me like 2+2=5.

I'm procrastinating so lets do some napkin maths, the claim is

1) a significant amount of fuel is leaking out of the expansion gaps,

2) climbing up to 25,000 ft at 300 knots would heat the airframe enough to seal those gaps,

3) there would still be sufficient expansion room to allow for travelling at M3.2

----

Ignoring that no engineer would be happy with 1.

For 2...Total Air Temp = Static Air Temp + Ram Rise. At 25,000 ft the static air temperature is about -35 C. Ram rise for a true airspeed of 300 kts:

RamRise = V^2 / 87^2 = 300^2 / 87^2 = 12 degrees.

So skin temperature at typical refuelling altitude would be -23 C

Titanium has an expansion coefficient of 9E-69 meters per meter-kelvin. So approximating rather grossly, assuming a s tarting temperature of 20 C over the 33 m length of the plane there would be a contraction of about 1 cm.

----

And for 3:

The aircraft then accelerates up to 1900 knots. RamRise = 1900^2 / 87^2 = 470 degrees

Static air temp at over 60000 ft is roughly -55 C. So skin temperature would be 420 C. So assuming the same length and starting temp, the plane would expand by around 11 cm

----

So to summarise: According to the claims, at ground level and temperature the expansion gaps were large enough to significantly leak fuel. After take-off the aircraft needs to be refuelled immediately. Assuming this is done so (i.e. take-off, climb to 20,000, refuel) then the skin temperature is lower than ground level, and the expansion gaps should have grown ever so slightly. The aircraft then climbs up to its M3.2 cruise point and everything expands significantly "as designed" and the gaps disappear.

----

Perhaps the anecdote we'll see on the internet now is that the SR-71 had to take off and go supersonic to rapidly heat up the skin before briefly decelerating to refuel, but the refuel had to be done super fast to stop the skin cooling down too far...

The SR-71 took off with a small fuel load to reduce stress and improve engine-out performance, that is why it was refuelled immediately after take-off.

The mental image of it leaking so much fuel on the ground such that it needed immediate refuelling is a myth propagated extensively on the internet.

I cashed out 20 bitcoins many years ago...and brought myself a nice graphics card with the proceeds. (i.e. $300)

If I'd waited a few years longer I'd have been able to buy a house (nearly), but I suppose I could also have ended up with enough to buy a sandwich.

I reckon an RPi type camera would be perfectly sufficient to estimate speeds, probably to a higher accuracy than you ask.

Edit: a quick search led me to this https://gregtinkers.wordpress.com/2016/03/25/car-speed-detec...

and this https://github.com/pageauc/speed-camera

I disagree with you regarding resolution and frame rates, I don't see why you would need a high speed camera. Something like a raspberry pi camera would be perfectly sufficient for this, assuming you still just want to get someone's speed to within say +/- 1mph to shame them, as in the original comment.

Set up your RPi camera about 20m from the road, with it's FOV that'll give you about 20 m of road coverage. Rule of thumb in my world is that the items of interest should not move more than 1/4 of window width between frames. So that's 5m, to move 5m between frames at 30Hz you'd need to be doing about 300 mph. More realistically the car might be doing 25m/s giving you an inter-frame displacement of 0.8m ~130px. If that's too little for your software to register just up the inter-frame time.

I wonder if you could even use road markings or kerb stones as your calibration to save the effort of measuring things out.

I'm admittedly rather unqualified in this area (I don't even know what the ITS industry is!) but I have experience in estimating distances from video and calculating speeds from pixel displacement (particle image velocimetry) and at first pass it seems to me this would be a fairly straightforward problem to solve.

Obviously radar or lidar would be preferable but a quick search for "OpenCV speed estimation" shows people having success with simple single <$50 camera setups.

To give you a single perspective from the type of person who does occasionally sign up to "novelty" food subscriptions... I'm just not that in to candy.

I'd pay $10/25 a month for the savoury snack equivalent, or for maybe a condiment/spice selection, or for some really high quality dark chocolate, but I wouldn't sign up for something which I mentally perceive as a box of unhealthy sugars (even though it may be high quality / great tasting sugar).

Less efficient, generally speaking you want as few blades as possible. One in the ideal case but that's prohibitively difficult!

Those "power-free" landings require a collective pitch rotor system.

The volocopter appears to be direct drive fixed pitch so it wouldn't be able to autorotate.

It also presumably relies on counter rotating rotors to cancel out the torques, so loss of motors may require other motors to be shut down to prevent undesirable yaw.

I remain somewhat skeptical of these being practical beyond 5-10 minute fun flights around a field, but good luck to them.

Whilst I like and appreciate the technology in the Model S, I too would struggle to class them as luxury along side the S class.

I genuinely find the fit, finish, and comfort (cabin noise in particular - thank you double glazing!) in my 20 year old S Class to be better. The E-class feels a much better comparison to me.

Its a somewhat common joke in our office that if you ask 3 researchers "What is lift?" you'll get 5 answers (and probably a bit of an argument).

I think "It's complicated" and "Why do you need to know?" are often the only appropriate answers, as context is important. I've read some "aerodynamics for pilots" type books that from a research point of view I considered to be, to some degree, wrong. But ultimately they were _right_ in that they taught the pilot exactly what they needed to know.

In a way it reminds me of electricity. I know enough to design and make simple circuitry, but I know electrical engineers and physicists that could run rings around me at both the circuit design level and the "That's not how electricity works, you idiot!" level.

> I'd be interested to see in this thread, who here has actually studied aeronautical engineering

I studied Aerospace Engineering (PhD in Aerodynamics), and I stay out of internet conversations over "how" lift is generated. For me it is one of those topics that is just not worth debating. It seems that people get _really_ attached to their personally preferred theory of lift.

Ah, so I guess it comes down to whether you consider the inlet and exhaust nozzles to be a part of the engine, or just a part of the air-frame.

Personally I'm of the mindset the inlet and exhaust nozzles are an integral part of the installed engine, particularly in this case given the rather unique intake/engine/exhaust design.

> I've read that due to the constant leaking while on the tarmac they would have to immediately rendezvous with a tanker to refuel after takeoff.

To reduce structural loads the aircraft took off with a very light fuel load and refuelled in flight.

Rather interestingly, the modern fascination with SR-71 stories and facts has turned a rather mundane "weeping" of fuel into the much more exciting image of fuel almost gushing out of cracks in the fuselage at such a rate it had to be refuelled immediately after takeoff!

> The Lockheed SR-71 for instance, derives a fair percentage of thrust just from the shape of the airframe, so the pilots actually throttle back a little once they reach Mach 2+!

That sounds...unlikely. Do you have a source for this?

I have a suspicion you might be conflating the "turboramjet" aspect of the engine design, with the air-frame actually generating thrust.

> IMU means gyro

In aerospace the phrase IMU typically refers to the combination of accelerometers and gyroscopes.

> both were contracted by ESA.

Yes I'm aware of how ESA works and who built what. But I would have hoped the common connection would have greater instilled the need for checking for these sorts of errors, if the Schiaparelli incident is as blutack theorised.

Whilst incredibly disappointing for ESA, this is unfortunately not their first IMU-failure rodeo.

An Ariane 5 launch back in 1996 [0] suffered a catstrophic failure after the inertial reference units gave bad data, and the flight control computer accepted it as gospel.

It is sad that they might have lost another platform due to a lack of appropriate range / saturation checking, especially as there was a radar altimeter onboard telling them they weren't underground.

[0] https://en.wikipedia.org/wiki/Cluster_(spacecraft)#Launch_fa...