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Discussion in 'General Chat' started by HippoCrushEverything, Jan 8, 2017.
Have you ever looked at these ekip things? The lifting body + boundary layer design looks amusing.
show us some cool organic looking flying and swimming machines
-Why are thick delta wings desirable for low-observability flight at low- and transonic speeds?
-Is a thin supersonic delta (f.ex. Mirage III) more efficient than a thick delta? Even at subsonic/transonic speeds? Why?
Thin delta wings normally have a distinct fuselage, whereas thick delta wings are often flying wings are blended wing bodies. Its not so much an aerodynamic concern as it is an issue of usable internal volume.
All delta wings suffer from leading-edge separation at low speeds; this is why the angle of attack at takeoff and landing for delta wings are so high (think about Concorde, for example). This is a really high-drag mode of flight, but you make do because you have other advantages at high speeds and high pitch rates, and you have a really soft and favorable stall profile that allows super high sustained angles of attack.
Thick delta wings delay this leading-edge separation a little bit, and have lower takeoff and landing angles, but they have worse profile drag in turn and aren't really suitable for supersonic operation.
I haven't actually heard of these things until now, but it does seem like a prototypical Russian aerodynamics project. That's not an insult; they were forced to make do with a smaller, less precise manufacturing base and usually that meant specialization and trading on arcane technical knowledge that resulted in really innovative designs (like the Ekranoplan in my avatar). But I'm not sure what niche it would assume?
The Ekranoplan was my favorite mod I ad for flight simulator 2000. Only flew like 30ft off the water. But for some reason it was ridiculously fun
Heavy lift transport with STOL capability on unpaved runways or water. Something like a c-130 but with shorter take off and landing combined with jet cruising speeds. For take off and landing it reroutes the jets engines through the vents on the back, from what I understand they pull additional air over the body when in use, despite its low aspect ratio high thickness and large chord it's able to maintain airflow at speeds and attack angles that would normally cause a stall. Because it has a large surface area on the bottom like a delta wing I think when using it's boundary layer thrust and a high angle of attack it was supposed to land at very low speeds. One version had additional vanes on the bottom that sent thrust downwards theoretically allowing near VTOL. Take off still needed an open space but it got off the ground using the vertical thrust combined with ground effect then accelerated switching the vertical thrust to the boundary layer system then finally switching to all thrust being sent out the rear conventionally once flying.
They lost funding due to the USSR collapse and only that remote control model ever got flown. I'm tempted to try and build a model one with electric ducted fans. It's probably easy to keep stable with modern MCUs carefully controlling it.
cool as shit
is there a good reason all planes look alike, and not alien like this
i like to see examples of separate people attacking a certain problem (in this case flight) and coming up with different solutions based on their own ideas and restrictions and imagination
I'd imagine due to drag, lift, down force and purpose.
How would a thin delta compare to a conventional thick moderately swept wing in level flight, say @ Mach 0,85, flight level 360?
tl;dr: the delta wing would suffer from higher drag, lower lift, and just overall crap efficiency. Slender deltas are made to operate well above the speed of sound. You can scroll right to the last figure if you want to see an example of this performance loss, and save yourself the intermediate explanation.
Too long, still read:
The whole purpose of sweep at transonic speeds is to increase the critical mach number Mc. The critical mach number is smallest aircraft speed at which any part of the fluid fluid (even an infinitesimal point of fluid), somewhere around the aircraft first reaches the speed of sound, and it's usually well below a cruise speed of mach Ma=1. For straight-wings it's normally in the range of mach Ma=0.7-0.8. At this point you start to experience something called drag divergence, which is when the drag produced by your wing rapidly grows to very large values near mach Ma=1 due to the sudden appearance of shockwaves which massively disrupt the flow, and a similar drop in lift. I've stolen some figures from Anderson's Introduction of Flight to illustrate this:
Here are some actual measurements on a real airfoil that show how extreme this effect is:
If you sweep your wing back a little bit, the wing only sees, in an aerodynamic sense, the flow coming in normal to the wing. So if the inlet flow is U, a wing with a sweep angle of λ only sees an incoming flow of U*cos(λ). In that way, an aircraft using an airfoil with a typical critical mach number of Mc=0.75 can sweep its wings back 30° to cruise comfortably at mach Mc=0.75/cos(λ)=0.85 without suffering from drag divergence.
Once you start going fast enough that your nose is supersonic, all this stuff is thrown out the window, and you can invest in a very different strategy. A supersonic aircraft flying at mach Ma>1 will have a bow shock. Right at its nose, a shockwave will form behind which the flow is locally sub-sonic. This is the source of the sonic boom people talk about, and its cone angle is dependant on the mach number of flight. Because the flow behind this shockwave is subsonic, a delta wing can exploit this by tucking its entire wing behind the bow shock and ride in the more efficient subsonic flow:
This strategy is only effective up to a limit, and when you start going really fast (say, Ma>3) your cone angle is so tight that you have to start dealing with crazy supersonic / hypersonic airfoils, which isn't worth talking about here. Now, make no mistake, sweeping a wing back is bad for performance. This always, always comes with a penalty. It just so happens that at high-transonic to low-supersonic speeds, dealing with shockwaves is worse. There is a reason why the highest-performing aircraft (at least measured in terms of efficiency) like sailplanes and high-endurance drones have long, slender and straight wings. Beyond the fact that such wings are easier to build, the self-induced drag of a wing is inversely proportional to the aspect ratio - or the span of a wing for a given wing area. In fact, when it comes to sailplanes, they make the wings as long and slender as possible without risking catastrophic structural problems.
This is because, as a wing gets stubbier and stubbier, the flow becomes more and more dominated by the tip vortices. These are parasitic and increase drag dramatically. That's the whole reason for raked wings and winglets, to disperse the tip vortex so that it doesn't interfere with the flow as much. High-performance gliders can have an aspect ratio of 35 or more. Concorde had an aspect ratio of 1.6.
Keep in mind, this (and the extent of my knowledge) is all undergraduate-level aerodynamics. My research is for much slower aircraft, and so I don't deal with these compressible effects professionally.
People like myself who work in flow physics, often we don't even look at aircraft in terms of wings and bodies and real physical shapes in the real world. Instead we talk about the 'equivalent vortex' to a specific flyer. This is because, by Newton's Laws, lift has to correspond to a transfer of momentum to the fluid, that transfer of momentum has to happen through the pressure field, and that effect can often be modelled perfectly by replacing a wing moving through a fluid with a vortex in your pen-and-paper work; a wing is a really complex shape, with complicated features, but a vortex is just a singularity with a magnitude.
So if you're designing lift-based propulsion for a ship, you can use a wing-like sail as in a traditionally-rigged ship, or you can have giant spinning cylinder - they're both the same vortex. You can have a huge rectangular vortex ring like a glider, or a narrow oblique pair of vortices like in a delta wing. You can keep your vortex really close to the ground like in ground-effect aircraft, or in hovering flight you can shed continuous pairs of vortex rings like a hummingbird. The job of the aeronautical engineer is to choose the shape of those vortices and how they are produced. That everybody - Russians, Americans, and even birds and fish - have all converged on this idea of the wing implies to me that there's only so many ways to efficiently make fluid spin.
There have been a couple attempts at using active bleed to produce these vortices through the Coanda effect. These crazy Russian aircraft are one of them, the failed Avrocar is another. They produce the exact same vortex system as everything else, but it's costing them a lot more energy to do it. Maybe in the future when energy is cheap, we live in the Fallout universe and everything runs on fusion cores, we wont care about this efficiency loss and we'll be able to make really expensive manipulations to the vortex field for cool effects. But we're not in that universe yet.
Have you seen any research into electric jet engines? At the moment RC planes are limited in top speed by their ducted fans, you can only spin them so fast, it gets to a point where larger motors do nothing. You can buy model turbines but they are very basic single stage radial designs. I can easily fit a 20kw motor + controller with weight to spare in comparison to a hobby turbine. Would either an electric or hybrid jet engine work? I have seen one theoretical design where a multistage axial compressor was driven by an electric motor, as the hot air exited the compressor it reached supersonic speeds, then it went into an expansion chamber where it was rapidly heated by supersonic shockwaves and the air colliding with the chamber before finally being forced out of the engine.
Alternatively you could just have the electric motor driving the compressor and a low bypass fan, with the hot side burning fuel. With the hot side turbines and bearings eliminated it becomes something hobbyists can mostly fabricate themselves.
I have never been able to find any research to get an idea how much thrust these sort of setups with a 10-20kw motor could provide.
yeah exactly what I was thinking of !!
thanks for the details, I'll need to read that post 5-6 times
I'm neither a combustion nor a propulsion guy, so I'm ignorant of the current state of research. However, there's nothing stopping you from building a standard Brayton-cycle jet on electric power. You'll just have a backwork-ratio of zero, since all your shaft work would come from an electric motor instead of post-combustion turbines. In thermodynamics, the Brayton cycle just says that you compress fluid and add heat at a constant pressure; achieving those two effects with multi-stage axial compressors and hydrocarbon combustion is an engineering choice, not a fundamental property of the device.
Your hypothetical electric jet sounds very strange, however. Having supersonic flow after your compressor is very strange indeed. Even if your jet is operating supersonic, the very first thing any modern air-breathing jet does (including ramjets, but excluding scramjets) is use a shockwave as its first compression stage. This has two purposes: first, a shockwave is basically free. It operates on the energy in the flow, so you get an extra compression stage without burning any fuel to achieve it. Second, bladed compressors have the same problems as wings near mach 1 that I listed in the above post. They suffer from drag divergence and lift falloff, and you're going to want the inlet running subsonic if you want efficient compression. You really don't want shockwaves post-compression, because either you wasted your shock compression early on and ran your compression stages in their least-efficient configuration, or your compressor is accelerating the flow (which is the opposite of what a compressor should be doing). And you really don't want shockwaves in an expansion chamber, since they do precisely the opposite of expand the flow. They're also not a very efficient way to heat a flow, since it's a thermodynamically irreversible process. If you're letting the flow heat itself through friction and shockwaves post-compression, then when you expand it out through your nozzle you'll have less energy than you started with and there was never any point in compressing the flow in the first place (ie: your entire post-compressor stage is drag-producing, not thrust-producing). Essentially all this does is add extra elements to bleed energy off of a ducted fan, and you're better off just running the fan without the extra crap behind it.
The whole idea behind an air-breathing jet engine is that you can't accelerate a body with an impulse engine any faster than the exhaust velocity of that engine. So if you want a fast jet you might need a supersonic exhaust, but you're stuck at this paradoxical design point where you don't want any of your aerodynamic elements operating at supersonic speed. So the question is how get a high-transonic or supersonic exhaust without aerodynamic means? The solution is actually pretty straightforward: if you start with a flow at atmospheric pressure, and then compress it to a higher pressure and density, if you add heat to the flow in that compressed state then when you re-expand the flow to atmospheric pressure your outlet will be faster than your inlet. This expansion step is not a lift-dependent step, but rather just a nozzle, and so all your aerodynamic elements can operate sub-sonic.
For anyone who has lived just East of the Rockies, Chinook winds are somewhat analogous (in that it's three-quarters of a full thermodynamic cycle, just like Brayton). Moist air from the Pacific loses heat as it is expanded (via elevation change) over the Rocky Mountains. It then dumps all its moisture in the form of rain over wine-growing country in the BC Interior, leaving it with a lower heat capacity. Then when it re-compresses on the leeward side of the mountains, it gains heat with altitude faster than it was lost on the Pacific side thanks to its lower heat capacity, and the air temperature is a lot hotter than it started.
So in an electric Brayton cycle, you'd run your compression stages off of an electric motor, and then heat the fluid with electric heaters of some kind, before expanding through a nozzle just the same as its gas-combustion cousin. The issue would be the efficiency of the electric motors versus a turbine stage per unit mass, and the efficiency of electric heaters versus combustion per unit energy. The Coors Company (believe it or not, the exact same Coors of the Molson-Coors beer empire) solved this second issue by using an all-thermal nuclear heat source back in the Cold War, while the same group solved the first issue by using shockwaves for supersonic compression instead of a bladed compressor (eg, a ramjet). This whole crazy non-combustion ramjet idea was tested successfully on the real-life planet we call home, because the Cold War was a cartoon. But the project was canceled on account of the just absolute batshit lunacy of every element of the idea.
They tested these things, along with other nuclear-powered jets and rockets, at a place called Jackass Flats, which is just an incredibly appropriate name for the location of this insanity.
What sense this would make without nuclear reactors is beyond me, though. Afaik, the turboshaft engine has the best specific power-to-mass ratio of any practical powerplant that produces torque and can be attached to a generator (no, you needn't point out that a supersonic turbine attached to a hydrazine combustor would attain a better specific power output).
ok thread is officially over my head
Can't blame you, Vanilla Ice has been staying above the tropopause since 2004.
Edit: Should I make a separate thead for civilian aviation and aeromysticism?
Absolutely, I agree. The gas power cycle is here to stay. As we move slowly away from fossil fuels, I think we'll see some applications that just need the energy density of chemical fuels that will adopt synthetics. Like when people talk about the 'hydrogen economy', you hear people make the remarkably banal point that hydrogen is not an energy source. But it's not a bad energy medium, being it's easily synthesized from water, and turns into water when it's combusted, and water is in a nice three-state equilibrium on our planet.
But the question wasn't if electric jets would be practical; it was how an electric jet would work. And the answer to that is "exactly the same as a regular jet".
you dont have to but you did
i like reading it im just not getting all of it
russian sof in syria
some graphic sniping
I looked at this, there were a few nuclear turbojets but the power to weight ratio was poor, the hot side was heavily limited by the maximum temperatures the nuclear material could handle without melting.
Assuming battery energy density wasn't an issue wouldn't an electric compressor offer a large efficiency advantage? Brushless motors can reach 90% efficiency while a turbine driven compressor would be around 40%? The question is do you use conventional fuel for heat or electricity? I assume you would get more thrust from the hybrid jet due to the lack of turbines in the exhaust when compared to a similar sized turbojet.
I wasn't sure if there was a highly efficient way to convert electrical energy into a large volume of hot air, I was looking at plasma welders but I can't find much information on how much of the electricity would be converted into heat.
A fully electric jet aircraft does have some interesting capabilities. Electric motors do not weigh much, as all the weight and volume is in the energy storage you can have multiple engines for different tasks. With a small electric jet rc plane or drone you could easily have large folding pusher props for low speed then switch to the jet after take off.