Jump to content

k1234

Members
  • Posts

    28
  • Joined

  • Last visited

Everything posted by k1234

  1. Thanks for the explanation - I had never thought of having a twin-IC hybrid. One question - do engines last well on full power, or is it just a matter of tuning and designing them so that they can run on full power indefinitely?
  2. Hybrid as in 'electric'? I'm fascinated, as I anticipated it would be a decade before this sort of technology made sense for light aircraft. Can you tell use more?
  3. I saw this debated a few years back, and someone posted the regulations regarding position lights. They were quite stringent regarding brightness, beam direction and colour. It would be easy enough for someone who knows basic electronics to make lights, but you might need specialist equipment or knowledge to verify that they met the regulatory requirements. Another issue that came up is that the average current draw might be lower for flashing LEDs, but the peak current draw is actually much larger for LEDs than Xenon strobes unless you do some clever stuff. A traditional strobe light works by charging up a large capacitor to hundreds of volts, which then discharges near instantaneously. An LED flasher, like for a bicycle, typically just switches the LEDs on full power for a short duration. This means that you are drawing high power for a short period of time, rather than low power all the time. As the previous poster said, strobes are more difficult than steady lights. I think heating was also an issue. It's a fallacy that LEDs don't make waste-heat. They do, and for some of the new high power types, getting rid of it can be a major consideration. Unless there is a get-out clause for homebuilt aircraft, which I haven't heard of, I don't think it's as simple as it might first appear.
  4. Sorry, I just read over the thread again and I don't know where I got that from.
  5. You say you're a student. Are you seriously entertaining designing a motor, or is this part of some class exercise to find out what sort of motor you should build if you were building one for real? If the former, then I'm personally with Mark 100% of the way. If the latter, then I think people might be more willing to offer help if you simply came out and said so. Presumably you have access to an academic library - go look up market-research techniques. You probably can get some valid qualitative information out of asking questions on the internet - but there are good and bad ways of going about it. I don't know much about it myself, but I used to live with someone who studied these things. I learned to appreciate that it's not all nonsense. Lastly, even a non-engineer such as myself can see that half the innovations proposed on this forum would kill someone if they were taken to completion. In serious fields like engineering or medicine, we need straight talkers like Marc. Learn to welcome negative feedback and take it with good grace, even if you don't agree with it.
  6. One thing would be to ask whether you should scale the weight by volume or by surface area. Surface area should be squared, and volume cubed (to see why this should be so, imagine a 1cm cube, and a 2cm cube. The first will have an area of 6cm^2 and a volume of 1cm^3. The second will have an area of 24cm^2 and 8cm^3). Some components of the aircraft will scale with volume - such as the foam cores - but others will scale with the surface area. This is likely to be the case for fiberglass because you won't generally be able to reduce the thickness of (e.g.) 3 ply by 10%, and the same will probably be true for foam sheets and plywood used to make bulkheads. Likewise, the instruments and some other components (seatbelts) etc. will probably weigh the same as for the full-size aircraft. What this means for you is that although you are reducing the volume by 25% you are only reducing the area by 18% and you may end up weighing considerably more than your calculations would suggest. You might be able to ameliorate this with different materials - thinner fiberglass etc.
  7. My understanding is that the Junqua Ibis uses flaps to get a stall speed of 52 knots. I think the canard elevator acts as a flap too. I haven't seen plans, so I don't know precisely how this works or whether the flaps are large enough to make a substantial difference.
  8. This is a problem in the UK - it's almost impossible to get non A4/series paper sizes. I solved the problem for the printed text by simply shrinking to fit A4 (this is simple on Linux with the pdf2ps and psresize commands). I will print the templates onto A3 if/when I need them - wasteful but there aren't that many. I reckoned the paper was 'legal' but I cant confirm this. The good thing about printing pdfs is that they don't normally rescale unless you ask them to. They just chop bits off of the edges. But I second the question about rescaling issues - is there any good way to check the metal templates for size?
  9. Would the micro bond to the unsealed foam be stronger than the bond to hotwired foam? There might be another reason aside from weight.
  10. Well, guess who bought that particular CD? Mr Tait was a pleasure to do business with.
  11. It isn't the same situation at all - if a single engine quits, your decision is made for you. If you have a twin you need to decide for yourself what to do. And it's very difficult to decide to crash. If you decide to continue under power, a conventional twin leaves you with problems such as asymmetric thrust and yaw. You can easily get yourself into situations such as flipping inverted whilst still above the stall speed. As a general subsonic rule, narrow wings (high-aspect ratio) produce lift more efficiently than delta wings, so a traditional long-ez canard will almost certainly be more efficient than one with a delta-canard like the one you showed. That much is basic aerodynamics. But designing a new canard is the sort of thing I wouldn't personally mess with unless I had a degree in the subject - and I don't and I gather you don't. Look at the history of the GU / Roncz canards and how long it took to get it right. I've nothing against innovation (quite the opposite - we're all reading this forum because we're willing to accept the unconventional), but I've learned through experience that true innovation is very hard and takes a long time. As a scientist, whenever I start a new experiment, something I never considered emerges to make my life complicated. Whenever I program a computer, bugs appear when the program is asked to do something I didn't consider. Both these spheres of life are relatively safe, but you seem to have decided to risk your life with a design that is really very different from anything that has gone before. Twin engines. A delta canard. A larger wing. Other innovations that you're keeping secret. You say you believe in progressive 'baby steps', but this certainly doesn't seem to be the approach you're taking. Are you working alone? If I were you, at the very least I would pay a qualified competent person to check over my design. There's currently a thread just as long as this one about the design of blended winglets. The changes you're proposing really aren't trivial. Seeing as we like to be radical, my 2c would be that within 15 years, electric - or at least hybrid - aircraft will become quite feasible. My radio controlled helicopter uses batteries that give 4 horsepower / kg in a tiny motor that weighs a few hundred grams and is over 80% efficient. So 25kg of batteries and 25kg of motor and electronics could very feasibly give you the same power output as a 125kg Lycoming 0-235, albeit only for a few minutes. I figure that an electric Long-EZ might save 100kg over an O-235 engine, so you could carry a quarter-ton of batteries instead of the usual fuel load. With today's technology that would give you a flight time of an hour or so. But if some of the rumours I've heard about Lithium-air batteries are true, then we might reasonably expect this to increase by a factor of up to 10 over the coming years. This would put an electric and a IC long-ez nearly on a par in terms of range, except I'd wager the electric aircraft will remain more expensive for some time. What does this have to do with twins? Electric motors are so much smaller and lighter than IC engines that it would be far easier to integrate them into a wing or strake. They also require far less cooling, which would be good for aerodynamics. But the real experience from radio-control modelling is that it's now far simpler to build twin/multi-engined aircraft than it ever was with the internal-combustion variety. With a hybrid approach you could carry 25kg of today's batteries at a cost of about $3000. If your main engine failed, you might get half an hour at 80 knots before they ran flat. You could have five 4-kg motors spread along the wing, each producing 25hp flat out. Loss of any single motor would be a relative non-event. Loss of 2 or more motors would only be significant if they were on the same wing. Lose the generator... And unless you are over the Pacific you will very probably have time to find a suitable airport and set up a good approach. I admit I'm being more than a little speculative here, but at the same time I don't think that this is pure fantasy. In your position I would be significantly worried that by the time I'd got my complex new design worked out and built, it would be nearly obsolete because buying a few electric motors off the shelf was so clearly the right way to go.
  12. I read the cheese-eating comment as humour. There's a current thread on this very topic on the yahoo canard group for anyone who can stomach some genuine jingoistic prejudice. I'm with Marc all the way on that one.
  13. Some more non-building noobie comments... It's something I've wondered, and like the previous poster I figured that there are things I clearly don't understand well enough that I would modify them unless it was well established that it was safe to do so. Having said that, I believe the Berkut has a lengthened fuselage, not to mention a bigger engine to counterbalance it. Also, I probably weigh 30kg less than some builders. I see a lot about people building longer noses in order to move the battery forwards... Perhaps I could afford to build a slightly longer fuselage with more room for the passenger, even if many other people couldn't? (I'm seconding the question).
  14. I do pre- my pushbike before I ride it. Brakes and tyres. And I have sometimes found problems. I once found a problem with my hang-glider that would almost certainly have resulted in loss of control. Your choice.
  15. First of all, delta V is the change in velocity, which is not the same as acceleration. Acceleration is dV/dT. If you are travelling at 20mph and reduce speed to 19mph your delta V is the same as if you decrease velocity from 10 to 9mph. The time taken is the same - assuming constant deacceleration - but the distance is doubled. If you limit the distance (as in a crash situation) then the time is not constant. Perhaps my braking example was confusing. I was referring to emergency stopping distances, where the acceleration is (approximately) equal at all speeds and limited to a fixed proportion of the acceleration due to gravity. If you have a coefficient of friction of 1, the largest deacceleration you can maintain without skidding is 10m/s/s. Typically cars seem to manage 0.5-0.8 of this value. There are caveats (sports cars can produce downthrust at high speed) but at lower speeds it's basically correct. I obviously wasn't claiming that a car couldn't brake gently, or that it's harmful to land an aircraft at high speed even though it can dissipate the energy in the brakes. We're talking about crashes and accidents here.
  16. It is largely semantics. Acceleration is what does the damage, but in a crash situation the accelerations are likely to be approximately proportional to the kinetic energy. To see why this is true without doing the maths, consider car braking distances. Drivers should know that braking power is limited to a fixed deacceleration. If you double your speed your braking distance quadruples. To stop within the same distance, you have to quadruple your deacceleration also. I can see an argument that if you're moving faster you're likely to have a longer distance to deaccelerate, provided you don't hit a cliff. For example, if you fire two bullets into water, a high velocity round will penetrate deeper. If you land and hit a hedge, perhaps you'll penetrate further through and the acceleration will not be quite proportional to the square of the velocity. On the other hand, if you land with a high sink rate or nosedive into the ground, most of the deacceleration will be provided by the landing-gear and aircraft structure, rather than the ground. There's a fixed amount of structure, whatever the change in velocity. It's also absolutely clear that if you have a given amount of kinetic energy and you reduce this to zero by stopping, all that energy has to go somewhere. Conservation of energy is a fundemental law of nature. Ideally it will all be dissipated by the airframe, hedge and by digging up earth or heating water during a water landing. In the worst possible scenario it all gets dissipated by mashing the pilot to a pulp, leaving the rest of the world intact. This isn't an entirely theoretical problem - early protective paragliding harnesses were rigid carbon fiber, and could survive just about any paragliding accident unscathed. They actually increased pilot injuries though. Thanks for the references Mark. There's some interesting reading in there.
  17. force, mass, energy... KE versus acceleration are really two sides of the same coin, surely? If you double your speed then fly into a cliff*, you quadruple your deacceleration (delta V). *to ensure a fixed distance of deacceleration.
  18. No, really it means you should build a light pilot. Small, light people are significantly more likely to survive car crashes (automobile wrecks) than obese 6'6" people. What you would want out of your aircraft is for it to limit the maximum deacceleration in an accident to a level that your body can withstand. Cars (automobiles) do this by using a space-frame that crushes gradually, absorbing energy as it goes. But there are some small cars that will withstand ridiculous accidents and still keep the passenger compartment intact, but through failing to limit the deacceleration the occupants may still be killed. To some extent adding weight to the aircraft will also increase the structure available to absorb it's energy, and assuming the pilot mass remains constant the ratio of sacrificeable mass to non-sacrificeable mass will increase. How effectively this happens is a matter of design. To use an extreme example, paraglider harnesses typically only weigh a few kilograms but provide relatively little protection for the pilot, in comparison with an aircraft airframe. For any given design, lightening the aircraft will give you a lower stall speed which will give you a lower landing speed, which is obviously good. But as Drew pointed out lowering the landing speed, even by a little bit, is far more significant than lowering the mass by a similar proportion.
  19. I'm not sure that's really relevant. I gather Rutan used to start the maneuver at 120 knots. Hardly normal landing speed. If you normally make a landing approach with the engine on, as I see it any engine failure will cause you to land short unless you can reduce the drag configuration of the aircraft in response to the engine cutting. Or unless you were landing faster than optimal. Or unless the engine was producing no appreciable power whatsoever - perhaps this is possible; I don't know? So do you normally land the LongEz with the airbrake on or whilst sideslipping?
  20. I agree... To a point. I'm a scientist and I know the literature is crammed with papers showing statistically significant effects that I don't believe in. The real questions are 'how large is the effect' and 'is it larger than the unknown error terms'. As the unknown error terms are, by definition, unknown there's always a degree of subjectivity in their assessment. So I agree about being cautious, but I still think statistics can provide an useful starting block. If one aircraft has 25% more fatal accidents than another then I don't see anything in it - even if it's statistically significant. If one type has 5 times more fatalities than another then I think you have to work hard to think of reasons why that should be. When I picked on forced landings, I was trying to count out pilot attitude issues. Obviously if you're assiduous about engine maintainence you can reduce your chances of having to make a landing, but once the prop stops you're committed to a hazardous landing - however careful you are the rest of the time. It may well be that Kitfox pilots are more used to landing in rough airstrips and therefore make a better job of forced landings. On the other hand, a high-flying Long-EZ should perhaps have a better choice of landing sites. What I would bet is that every pilot in a forced landing situation makes the best job he or she can. When I have time, I'll look at some more slow-flying types and see if the pattern still seems to hold. All aircraft are equally safe..? No way. There's a dictum that flying is safe, but very intolerant of mistakes. This is nonsense. We're all human. We all make mistakes. And I think it's inarguable that some aircraft reprimand mistakes more harshly than others. When I was learning to glide, I had a very near miss when a thermal picked up my wing and swung me towards the hill. I was flying an old training glider that had a relatively sharp stall and not enough control authority at low speeds. After I landed (and was unfairly reprimanded for having chosen to make a turn towards the hill rather than away from it) several other pilots told me they'd had trouble with that glider, and one had had a similar incident. The experience persuaded me to invest in a considerably more expensive glider, that had much better handling. Safer? No statistics, but I'm convinced. The other moral of the story was that had I died (and I came extremely close) the accident report would undoubtedly have said 'inexperienced pilot chose to turn towards rather than away from the hill: pilot error' when the real issue was a glider that was structurally sound, whose flight characteristics were fully known (albeit too exacting for a student pilot) but which was basically not fit for purpose - at least by modern standards.
  21. There were a fair number of aircraft who crashed due to an engine-out on final approach. But I too wish you luck. Mostly these incidents didn't end in fatalities (I may be wrong, but I don't recall that any of them did). There were an awful lot of the taildraggers being damaged by ground-loops. But the damage was more to the airframes than to the person, so these types of accidents don't personally concern me greatly. On a personal level, unlike WT Johnson I don't have a plane built, so I'm trying to come to a conclusion about the relative risks. Certainly not decided against... These aircraft have a mix of frugality and performance that is certainly compelling.
  22. I just had a good read through the NTSB database, including all the fatal/non-fatal long-ez crashes and all the fatal Kitfox crashes. I realise that the Kitfox is - in many ways - a very silly aircraft to compare with the Long-EZ. The reason I chose it is that there are a lot of them around - 1500 in the FAA database - and because most versions have a stalling speed under 40mph compared to the Long-EZ's 65mph. What I was wondering was basically whether the slow stall speed would lead to better chances of surviving a forced landing (engine out, propellor out, fuel supply issues etc...). Of 97 long-ez crashes, 41 were due to forced landings, and eight of these were fatal, from a total of 27 total accidents causing fatalities. Of 154 Kitfox crashes, 47 were due to forced landings and two of these were fatal, from a total of 20 accidents causing fatalities. I haven't been through the figures for the Vari-Eze properly, but losing your engine in this aircraft also often seems to precede tragedy. Anyway, you do seem to have a better chance of surviving an engine-out scenario in a Kitfox than a Long-EZ. This is statistically significant, if you use a 1-tailed test (which seems reasonable to me). I'd also wager that the statistics are biased in the Long-EZ's favour because it seems feasible to have a forced landing in a Kitfox that you can fix then fly away from without notifying anybody, whereas I doubt this often happens in a Long-EZ. In terms of Kitfox fatalities, there were a lot of CFITs - perhaps it's more likely to be flown in the mountains - and there were a lot of low-level stalls and spins. A general impression was that kitfox pilots were a lot more creative in terms of how they crashed. No Long-EZs ground looped, had problems with their floats or hit deer on the landing strip. In terms of the Long-EZ crashes - a few were certainly due to low flying and similar stunts but my impression was that only a minority of people died through being clearly daft. One thing that surprised me was the large number of long-EZs either recorded as 'stalling' on final approach or 'being caught in downdrafts' and flung to the ground. I had to wonder whether some of the latter problems were actually unrecognised stalls. They were fairly frequently fatal, though I didn't keep a tally. I might go through the figures again. I'm curious about exactly what 'stalling' an 'unstallable' aircraft means. I assume it just means 'going a bit nose-down' and hitting the ground with too high a rate of descent. There were also an alarming number of unexplained accidents where the plane veered off to one side for no apparent reason, then crashed the ground whilst on its side or inverted. I don't know all the caveats of searching these databases - so feel free to point out flaws in my reasoning or search technique. In some ways I'm not even that concerned about whether or not my quantitative findings are correct. It's fairly clear that many of these accidents are due to the same old issues over and over. I'm sure it's been a good exercise for me to look through them and like Mark I'd recommend it to anyone who hasn't. I like the pirate graph, btw.
  23. I was using the NTSB database. I've searched for 'long-ez', 'longez', 'longeze' but I still only got 13 accidents until I thought to search for 'long ez'. Now I get the same figures as you for the Long-EZ - thanks. I get slightly higher figures (50) for the Vari-EZ. If you put your figures of 29/47 + 600k/700k through the number crunchers, it isn't statistically significant - p=0.17. Although more likely than not the truth is still thereabouts. I think you misunderstood my intent. I didn't mean to argue that Vari-Ezes have twice the number of hours, but that the statement that Vari-EZs have a worse safety record than Long-EZs was robust to our assumptions being as wrong as they could feasibly be. Obviously as my facts were wrong, the rest is moot.
  24. I'm afraid I see red whenever anybody comes out with that one. Personally I like to limit my exposure to somewhere between the third and fourth rows - 1/100 - 1/1000. Which is probably within the 5th percentile when it comes to sports. Probably higher. So don't call me a chicken for considering risk. I've known a few people who've killed themselves through one sport or another. Some were competent enough to know they were pushing the limits. Some were incompetent enough to exceed them without realising. I also know a man who killed five other people through sheer arrogance. I have some respect for the first group. Some sympathy for the second. But what really riles me is when people refuse to acknowledge the risks they are taking. In the gliding community many people put up all kind of mental barriers so that they don't have to deal with it... They all know cognitively that what they do is dangerous, but never truly stop to think about it. To me this is fundementally dishonest... but it's a form of dishonesty that is very easy to slip into, and of which I'm sure we are all guilty of, from time to time. 'Driving to the hill is the most dangerous part of the day' or 'you could get killed crossing the road'... These are nothing less than lies we tell ourselves. My attitude is to do whatever seems fun - which normally implies an element of danger - as safely as possible. And to be honest with myself. I see nothing shameful about that. I don't mean this in a personal attack. But it is something I feel strongly about.
×
×
  • Create New...

Important Information