votingprawn

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!

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

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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.

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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

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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.

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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.