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Archive for the ‘airplane’ Category

Sukhoi T-50 Video

Showing closeups of the new Russian stealth fighter on the ground, testing thrust vectoring, showing the drooping forward wing gloves, and then in flight. It’s a treat!

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Gah, got tired of monkeying around with various software (XFLR5 is never provided as an executable on ANY platform despite countless requests, just some packets) so loaded up a profile in Matlab and plotted it in multiple sizes and printed those. Took only a few minutes. It’s still the best software ever.

It went something like this:

load MH78.txt %text file is copy-pasted from MH-aerotools.de
whos %check out if it worked
MH78 %check out what we just loaded
figure
plot(MH78(:,1)/100,MH78(:,2)/100)
grid
axis equal
legend MH78
%perhaps print here if it fits on one paper

%this is three views for a larger print:
xlim([0 0.4])
%print here manually
xlim([0.3 0.7])
%print
xlim([0.6 1])
%print
%Etc you get the idea...

Had to read some documentations for this, but the gist is that Matlab actually has some proper documentation. I just typed help gca, help set, help axis, help axes, help xlim etc and they contained some quick examples that I then tried and got the hang of everything. Matlab should be the gold standard for all similar software developers. I’ve tried coding in Python and Scilab and both have large stumbling blocks in quick and efficient numeric work.

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But is not a real RLV program. It’s just a narrow test for one technology. Hence I think naming it Reusable Booster System Pathfinder is misleading.

Overspecification

They overspecify the problem by requiring a glide landing. Why is it superior to powered landing? At the moment, there’s no clear reason to believe it is! Both need to be developed further to understand their advantages and drawbacks. To my knowledge, there have been only six liquid rocket VTVL prototype manufacturers so far: McDonnell Douglas, JAXA (who was the contractor?), Armadillo Aerospace, Blue Origin, Masten Space Systems and Unreasonable Rocket. Only a few of those have flown to higher than a few hundred meters. The design and operations space is mostly totally unexplored.

Nevermind the large number of other alternatives to boostback. Jon Goff had a recent “lecture series” about these.

I understand that this is just one program, but this should not gain the status of the reusables approach of the air force – stuff like that easily happens.

Master Design Fallacy

They also discard evolution and competition – instead just requiring a single masterfully designed prototype before something operational. Sure, this is much better than starting a multi-billion dollar program without a first lower cost prototype, but nevertheless, it sucks. Somebody brief them on newspace! Rand Simberg, Monte Davis, Jonathan Goff, Clark Lindsey, or one of the numerous people who get it. Or one of the prominent company leaders: John Carmack, Jeff Greason, David Masten.

An Ideal Program

Just specify some boost delta vee points and let companies demonstrate progress towards that. A popup tailflame lander would perhaps give more vertical velocity while some good glider or even a booster that has engines for cruising back could boost far down range to give lots of horizontal velocity. There ain’t a clear winner – there might not even be and multiple approaches would have their uses.

(more…)

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This is something we’re after.

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Here’s the technical information. It uses a piston engine to spin two smallish propellers. At first look it seems inefficient: the propellers are so small (diameter 1.7 ft so radius is 0.26 m) so the air mass flow is low and hence the velocity of the jet must be high, meaning high power needs for the thrust. (Mind you, it’s efficient compared to a peroxide rocket or a low bypass turbofan engine that have been used for jetpacks in the past.)

But let’s look closer. Engine specs. It’s a two stroke 2.0 liter V4 that produces about 150 HP at 6000 RPM and it weighs 60 kg. The quite high RPM means it produces quite high power.

If the engine spun at a lower speed, you could use bigger props, but the engine would produce less power.

If it spun at a higher speed, you could have higher power for little mass growth in the engine and then use a gearbox to still use the same size or even bigger props (if you geared it down further). But gearboxes are heavy, expensive and often unreliable.

6000 rpm is 100 Hz (rpm is a totally weird unit for spin rate anyway, why is it always used?). Speed of sound is 320 m/s. Hence at 100 Hz the supersonic radius would be 0.5 meters (100 1/s * 2 * 3.14 * 0.5 m  = 307 m/s). At the Martin Jetpack’s 0.25 m blade radius it’s about half the speed of sound. At the 7058 max RPM it’s 180 m/s or about 60% of Mach 1 – the transonic region should be easily avoided. Maybe they could be even slightly bigger.

It’s a compromise design. With the small props you can use relatively high engine speeds so your engine stays light – and you avoid a complex expensive failure-prone gearbox. The machine also stays safe as there is no free flailing propeller. With a larger propeller (or two) you would have to give up the shroud(s) since their weight would be prohibitive. On the other hand, fuel consumption would go down and hence the range could increase. It’s a fascinating design space.

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Karoliina has some thoughts on plane design, looking at High Altitude Long Endurance (HALE) UAV:s as inspiration for high L/D craft, to ultimately cruise at fast speed with little power. I disagree somewhat, and I’m sketching out why, below.
Probably the drones, like sailplanes, want low sinkrate and high L/D is secondary, because they need to just stay aloft and not go anywhere.
Power needed is P = v*D (we assume). Since D = CD*v*v, P = v^3*CD.
Lift is L = CL*v*v so v = (L/CL)^(1/2). (Note this CL is different from the L = 0.5*rho*A*cl*v^2, so CL = 0.5*rho*A*cl. It’s more practical here.)
Power thus is P = v^3 * CD = L^3/2 * CD * CL^-3/2.
The lift equals mass, so the power needed is
  1. proportional to mass^1.5,
  2. proportional to the drag coefficient and
  3. inversely proportional to the lift coefficient^1.5.
This means the lift coefficient CL needs to be large for the craft to be able to loiter for a long time. So long wings and somewhat cambered profiles. A little drag doesn’t hurt as much as low lift so struts are a possibility.
Instead, for a piston cruiser, the L/D needs to be maximized for a certain minimum trip fuel consumption, not per time. Basically, you want to minimize delta_E = P*delta_t = P * delta_x/v so the cost function J = v^2 * CD = L * CD / CL which minimizes at maximum L/D. The CL term is less important compared to the loiterer. As a first guess this should optimize to a less cambered airfoil and smaller or shorter wings. And no struts.

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“Rockets are special” is an interchangeable meme with “rockets are expensive”. Well, hopefully they get less special. Armadillo’s been making progress, from the latest update:

For the very first time, a complete system was operated without the presence of any of the manufacturer’s representation on site. This may seem like a small thing in light of the fact we all knew it would eventually come, but getting there is always a good thing.

Some day launch vehicles will be Refuel And Go Again.

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