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You may notice that most airships' propellors are poorly placed: either below the envelope thus wasting energy fighting an offset drag, or off to the sides which requires heavy reinforcement. Front and rear placement are contraindicated because then you'd be blowing air onto the envelope creating even
more drag/waste of energy. And as nifty as the concept of a ductfan running through the entire envelope is, it would probably be too heavy and draggy compared to other existing solutions.
But... if we give an electric (ie: very lightweight) engine/propeller its very own little dirigible, we can hook it to the front of the big one's envelope at the end of a cable or rope and be towed around like a towplane and glider. Pitch/yaw steering is accomplished via offsetting the towrope attachment point to the towship (like reins on a horse except not).
"Electric" because you can have the battery(/fuel cell/generator) in a more weight convenient spot in the main ship.
For simplicity I'm envisioning something like a stick with a motor on one end, a cable on the other and a mini-dirigible envelope on a spike poking up from the main stick... but you might even be able to make the towship a working duct-type due to the extremely low weight of the "cargo".
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Would it not be easier to have a couple of helicopters and lines, rather than the extra remote derigibles? Either that or have these remote propellers simply on a framework protruding from the ends of the derigible? |
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The alternative would be to run a 'hot air derigible' by having a small jet engine providing the thrust, with the fuel piped to the pod. |
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[Sl] helicopters make horrible traction engines and protruding supporting framework is *heavy*: fulcrums and all that... |
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Hmm... gotta rename it I think, it gives the impression of tugboats and ocean liners whereas I'm trying for "outboard motor on a rope" sortof. ah, got it. [post edit] title changed from "Tow Dirigibles" |
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Fair enough, although does the whole splitting up of the machine bode well for efficiency... adding an entire derigible extra of surface area (drag), that has to have it's (additional) weight supported compared to having things on board. |
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For the duct theory, your explanation makes me think of a H turned on it's side, with the outboard derigible the top horizontal and the duct the bottom, rather than having the ducting through the baloon. Maybe it's just too late at night for me! |
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You'd also get a bit of backwash from the motor module... but I don't think the sum areal surface increase would negate the advantage of less weight and almost no offset loss. I think the main advantage here relies on a very lightweight engine, ie: electric, powered from the main: that minimizes the outboard dirigible's size. The main blimp wouldn't require stabilizers/rudders either. |
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I only mentioned duct-type because the mini will suffer from offset wasteage just like the big one does (except much less). |
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This is rather clever. [+] |
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If the tug (engine+prop) is suspended below its private blimp, but above the main blimp, it will have clean air fore and aft. The suspending cable would form a forward-pointing, lopsided V, with the tug at the vertex pulling the V forward, mini-blimp at the end of a short top arm, and macro-blimp at the end of a longer, lower arm. |
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Alternatively, is giving the tug wings feasible? |
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The towship's cT isn't the same as its cD but that's compensated for by the rope attachment point being vertically offset from the thrustline, proportional to thrust (and a bit horizontally to compensate for torque). Steering the towship consists of moving the att.point around via an arm on a motorized universal joint. I think this would be much more efficient than using aero-control-surfaces. |
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[note: that's not what I originally wrote in the post but that's what I meant, post corrected :) ] |
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[BH] picture the tugboat as a small dirigible rather than a blimp: the motor/propeller is fixed underneath and an articulated arm comes out the back of the motor mount which holds the rope that goes to the bigship. The arm is controlled from the ship and automatically compensates for cD vs. cT differences at various power levels. It also does the steering: if you pull the arm up while under power, the tug points upwards, down: down, left/right:left/right. |
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Just to be clear, by "offset drag" do you mean that when the centre of thrust is not in line with the centre of drag most means of compensation, such as altering the angle of thrust, using the control surfaces, or simply allowing the craft to fly somewhat nose up, have associated losses? Or is there a more direct effect? I assume that dirigibles with gondola mounted thrusters are designed to be nose down when not powered but approximately level when cruising; transferring ballast fore-aft could also help. |
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you can't put an engine on a dirigible or blimp at a thrustline-convenient point without doing something odd like as you say tilting the prop downwards or having the fins on the ship compensate, or even design the envelope to do that. All of that costs power in the form of extra drag along the direction you want to move in. You can put an engine on each side of the envelope, but then you have to have scaffolding that can take not only the engine weight, but the engine power, and the closer to the envelope the more the propwash will spend itself on the skin. |
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"offset drag": my own terminology for extra drag caused by the dragline/thrustline not being the same. |
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Thanks for the explanation. Another configuration would be a thruster hanging by a cable from the nose or tail. From the airship's POV, the downward component of force would be constant and accounted for, and the horizontal component (the thrust) would be through the centreline. It would tend to act like a pendulum, but that could be reduced by some fancy power control to simulate critical damping. |
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//hanging from the nose or tail//
since the aerodynamic dragline is going to be from the nose to the tail, how are you going to get thrust along that line from something that's dangling from the nose or tail ? |
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Is there a problem here to solve? |
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What is the extent of losses due to 'offset drag' and how much extra weight and complexity could be justified to overcome those losses? |
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//something that's dangling from the nose or tail ?//
Presumably in the nose/rope/engine case, as the thrust increases the angle of dangle (love it) reduces until you reach a point where the motor is (nearly) straight out in front of the dirigible? The critical bit is that the engine assembly is built such that the engine thrust is always applied horizontally. Then you have 2 vectors acting on the rope, Gravity (verically) and Thrust (horizontally). Does that make sense? The original dirigible idea is just removing the gravity vector so [+] |
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[sm] neither yours (helicopterish) or [BH]'s(wings) work well or at all at low speeds and both require vertical bracing at the nose of the bigship to support their weight while at rest. |
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There may be a question as to whether the parasitic drag of the (mine) towship is greater than the lifting drag of [BH]'s or the angle offset wasteage of yours, at cruising speed, though. |
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[RB] I imagine finding the center of drag of the airship as well as the amount, combined with the center of thrust (middle of the propeller) and thrust amount, could all be plugged into a Pythagorean equation to calculate useful vs. wasted power, though if the propeller is too close to the skin there's wasted propwash to consider as well. |
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Sorry, I didn't explain that very well. I wasn't suggesting something helicopterish at all; [RattyBunyip]'s interpretation is exactly what I had in mind. At low thrusts the cable would hang nearly vertically. At all times the thrust vector would remain horizontal and not be used (wastefully) to lift the thruster, which would instead simply swing of its own accord to the angle at which the force vectors are balanced. |
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"helicopterish" only in the sense that the engine being tilted a bit upwards is what's holding it up, otherwise you'd have to compensate for a nice long moment arm with the engine on the end prepending off the nose of the blimp. |
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I was thinking that the moment caused by the engine's position and weight would be compensated for by weight distribution in the rest of the craft - having the gondola slightly further aft, for example. The thrust could indeed remain horizontal - or rather at the best angle to direct the craft. |
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I like your original idea, though. It's nicely self similar, and places low stress on the airframe. |
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now my head hurts trying to figure out dangle angle. |
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Taking 0° as vertical and 90° as horizontal, and thrust and weight as the other two sides of a right angled triangle whose hypotenuse is the resultant force (the force in line with the cable), dangle angle = arctan (opposite / adjacent) = arctan (thrust / weight). For example, when thrust = weight, dangle angle = 45° from vertical. I have ignored the weight of the cable. |
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