584,800 active members*
4,602 visitors online*
Register for free
Login
Page 3 of 6 12345
Results 41 to 60 of 108
  1. #41
    Join Date
    May 2003
    Posts
    550
    Diamard,

    Couple of points. Stability might not be what you think it is... The aerotreks CoG is probably around the guys backside... the engine is slung down the back, there's little significant weight up top and its stability is like a parachute, with most of the weight COG suspended under the point of lift. Slinging the lift elements in the middle will move the point of lift and cog together and you've got a stability issue. Note CoG, centre of lift and centre of effort are different things. Thats for a stable two plane solution, if you're going to move and fly and transition in three planes then you get complicated and start looking at centre of areas and aerodynamics. Bottom line is you need to work out the controls because lifting off the ground probbly isn't the biggest challenge, controlling it will be. You've got the choice of multiple thrust elements and speed controls, pitch controls like a helicopter or another control/thrust solution entirely.. The helicopter rotor head is proven and controlable and you could put a gyro stabiliser on it...

    For Prop Math there's a potentially huge amount of area to cover but look at Momentum disk theory and Blade element theory. Both together will cover most of what you want to do. Be prepared that most introductory propeller math calculates thrust from a given lift provided by the prop - but doesn't calculate the lift...

    Prop math also ignores 3d fluid effects, tip effects and inlet and exhaust factors. until it gets complicated most of the theory assumes the blade works like a wing which isn't true, there's a pressure on the underside as a result of prop slippage, the prop acting like a fan rather than a wing, which is more prevelant when the vehicle is slow or stationary.. Ducted fans are attractive because they remove the tip losses from the air spilling over the tip and so can be more efficient in certain circumstances, like Jet turbines. The same theory produced the tip fences you see on modern airliners. For Ducted fans to gain any benefit the tip has to be VERY close to the duct, the tip and the blade generally can be much broader.

    If I really had to contemplate propellors strapped anywhere close i'd use a ducted fan and LOTS of kevlar .

    For ducted fan ideas look at the hovercraft guys....

    Andrew

  2. #42
    Join Date
    Apr 2006
    Posts
    1257
    Thanks fyffe. I'll look into all of that. Im hopefully going to start working on a clay model soon. I'll post the pics for advice when its done.

    Edit: Also forgot to mention, in a very basic sense,
    Less Stability = More Manoeuvrability
    The trick is to avoid reducing stability to the point that you need true computer controlled fly by wire in order to maintain control.

  3. #43
    Join Date
    May 2004
    Posts
    600
    Here's a flying suit you'll all appreciate:
    http://www.technologie-entwicklung.d...onocopter.html
    and for you home built model turbine addicts this one has a commercially available turbine wheel/shaft/etc at 33mm (comp) diameter and 31mm turbine = 210,000 rpm (sorry for the thread hijack):
    http://www.technologie-entwicklung.d...ne__kp31_.html
    and check out the whole website while you're at it as I find it amazing.

  4. #44
    Join Date
    May 2003
    Posts
    550
    Quote Originally Posted by diarmaid
    Less Stability = More Manoeuvrability.
    nope., well maybe, More control allows more manuverability which allows less inherent stability.

    Modern fly by wire fighters are not dynamically stable. They only fly because the fly by wire system is fast enough and has enough control authority to stop the plane falling out of the sky and keep it gong in the direction required. The advantage is that when an extreme maneuver is required the control effectively removes the stabilising control as much as applying a directional control and gets a faster movement. As the planes inherently unstable it will move rapidly to the required attitude, much faster than a inherently stable aircraft could do or even to attitudes a stable aircraft could not reach.

    Exactly the opposite of what I'd look for if I wanted a personal jet pack...

    I'd think you need as stable a platform as you have a chance to engineer an as consistant and attainable control regime as possible.

  5. #45
    Join Date
    Jan 2006
    Posts
    2985
    heres a cool video. looks like its powered on compressed gas.
    http://video.google.com/videoplay?do...00568094&hl=en

    Matt

  6. #46
    Join Date
    Apr 2006
    Posts
    1257
    That's cool. It looks like one of the designs from the 60's/70's. I'd say there's a very short flight time.

  7. #47
    Join Date
    May 2003
    Posts
    550
    That's a variation of the bell rocket belt, hydrogen peroxide cylinders and one pressure cylinder. Looks like the RB-2000 scandal, which is worth a read.. That video looks wrong - check the take off and landing - close to circular path without steering and the inertia looks wrong, especially landing.. anyone see a wire?

  8. #48
    Join Date
    Jan 2006
    Posts
    2985
    it is rather questionable. The camera never turns 360 degrees and his flight path looks like a wrecking ball on a crane to me.

    Matt

  9. #49
    Join Date
    Apr 2006
    Posts
    1257
    .

  10. #50
    Join Date
    Apr 2006
    Posts
    1257
    .

  11. #51
    Join Date
    Jul 2005
    Posts
    82
    diarmaid,

    I find your project very interesting, I too, ina way, have been working on a similiar venture for some time.

    I started off with a solotrek type design, which through major component failures did not do so well on it's test run eheh. That was a major engine vibration due to a propeller getting out of alignment, the vibrations shook and destroyed the balsa frame. I since have moved to aluminum ehhe.

    Actually my main goal is to develop a computer control system that flys it for me, or at least somewhat, i'm a programmer so that's my real reason for doing it. I have a single board computer with an accelerometer i picked up at phidgets.com, some other sensors, plus a servo controller which is now connected to my 4 rotor helicopter. After my last design blew up, i figured i'd go with something that worked, 4 rotor helicopters work, so i just built one of those and want to control it with the computer. Currently i'm having speed control issues, the dumbest issue, the ESC units cut off because they don't like the servo controller. I've hesitated for months cause after buying 4 ESC's that cost $100 each, i don't really feel like tossing them.

    I have done research on ducted fans, and like you, have determined they are hard to find in large sizes. Well, I should say medium sizes, large ducted fan blades are easy to find, www.hoverhawk.com used to have them, granted i haven't checked in a while. Any hovercraft site has those. But a 12" inch one would be nice, but no where to be found. Also matching counter rotating ones are basically not out there, you'd have to make them yourself. I did see a website once of a guy who had some custom machined, but he didn't really say who/where/how he got the design.

    If yer interested in jetpacks, or whatever you want to call it, i recently noticed this website, www.skywalkerjets.com, the guy is going to kill himself, please tell me you don't plan on doing that.

    On yea, if you are looking for propellers, zingerpropeller.com is where i get mine, they have pretty much everything in clockwise and counter-clockwise setup, 2, 3, or 4 blades, 3 blades go upto 28" i think, with varying pitches. I generally only use 2, as they are more efficient with my size.

    I'd like to go gas, but never found a setup with gas to go clockwise and counter clockwise, granted, you could develop it, but i didn't want to engineer that part, just wanted to buy a gearbox that would do it for me.

    Well good luck to ya, recnetly i've decided to come up with a solution for my speed control issue, just gotta spend some money, then hopefully my thing will fly. I might do some testing of a mockup hiller flying platform with these 2 spare 1.5HP electric motors I have, set them up counter rotating, put vectoring on the bottom, see what kind of control it has.

    Also, one comment on stability, I read someone who disagreed with you, but lowering the thrust point is definatly a good thing. Solotrek hovered, no one truely knows if it was stable or not, and it definatly was fly by wire. No question putting the thrust up high will work, and with a computer might be very controllable, but by a person, not likely. Lowing your thrust output to around the center of gravity is good, it will increase stability, granted still that type of design will be very hard to control.

    Of course the big argument to that theory is helicopters. Helicopters are stable with the thrust uptop for multiple reasons, one is gyroscopic effect of the blade, another is the fact that the blade bends, producing a kind of dihedral, if you want to call it that, of the thrust output. Planes have have dihedral for stability reasons, helicopters do it just cause the blades are so long. Props and ducted fans don't bend any significant amount though, so you lose both of these stability enhancing effects, making fly by wire really nice to have.

    Anyway, i'm just rambling now, good luck to ya, keep us posted.

    Ross

  12. #52
    Join Date
    May 2003
    Posts
    550
    Quote Originally Posted by klick0 View Post
    Also, one comment on stability, I read someone who disagreed with you, but lowering the thrust point is definatly a good thing. Solotrek hovered, no one truely knows if it was stable or not, and it definatly was fly by wire. No question putting the thrust up high will work, and with a computer might be very controllable, but by a person, not likely. Lowing your thrust output to around the center of gravity is good, it will increase stability, granted still that type of design will be very hard to control.
    Interesting stuff; could you explain why you think lowering the thrust point is definitely a good thing? and what the effects of placing it around the CoG might be, on control and stability?

    I'd think about it seriously because in absence of any aero surfaces or other elements for stability or control you've got a problem. The closer the CoG, center of pressure and center of thrust get the less stable any aircraft becomes, especially if there's no airframe with aero surfaces or other contol elements with some control moment from the appropriate centre of effort.

    Quote Originally Posted by klick0 View Post
    Of course the big argument to that theory is helicopters. Helicopters are stable with the thrust uptop for multiple reasons, one is gyroscopic effect of the blade, another is the fact that the blade bends, producing a kind of dihedral, if you want to call it that, of the thrust output. Planes have have dihedral for stability reasons, helicopters do it just cause the blades are so long. Props and ducted fans don't bend any significant amount though, so you lose both of these stability enhancing effects, making fly by wire really nice to have.
    Since you're making a huge effort in this area and involving rotary wing / fan elements I'd strongly suggest you look at some helicopter aerodynamics - there are some great sites on the web that are applicable to both free rotor and ducted propulsion systems.

    Simply put a helicopter isn't stable due to the rotors gyrosopic effect. Almost the opposite. If the thing moves at all the gyroscopic and aero effect is in fact destabilising.

    As the control of a helicopter is through the rotor disk, you're appling force to a rotating mass and precession comes into effect with a bang. Precession means that an upward pressure at the rear of the disk will lift the left side of the disk, upward at the front will cause the right side of the disk to lift. Similarly any relative wind across the rotor disk will produce the same effect. For example a helicopter behaves differently when rolling into a right turn than when rolling into a left turn. During roll into a left turn, the pilot will actually apply through the controls pressure to the front of the rotor disk, lifting the right side, and then have to correct for a nose DOWN tendency in order to maintain altitude because precession from the now left banked rotor disk causes a nose down tendency and because the tilted disk produces less vertical lift to counteract gravity. Conversely, during a roll into a right turn, precession will cause a nose up tendency while the tilted disk will produce less vertical lift. Pilot input required to maintain altitude is significantly different during a right turn than during a left turn because gyroscopic precession acts in opposite directions for each. Now when a cross wind is passing over the rotor, one blade is advancing and one receding, the advancing blade is faster and so generates more lift on the advancing side than the preceding and applies an upward force to the disk perpendicular to the wind direction and an upward lift to the disk 90 degrees out due to precession. Not much difference to the pilots input earlier. Gyroscopic precession is a major challenge to helicopter control and acts to destabilise the rotor, thats why so much money is spent on rotor head design and why effective control is so critical and took so long to achieve, Some say they still haven't got it.

    Helicopters don't have dihedrel. In a perfectly stable hovering flight some 'dihedral' might be apparent but it's nothing more than the weight being carried at the centre of a beam with the deflection as you'd expect, but the cone 'dihedral' adds no stability what so ever. The more obvious 'dihedral' usually seen is usually either a product of control input or is actually a function of the helicopters rotor design for stability, that is the 'dihedral' is a product of the stability designed in, not a contributor.

    Huh? What?.

    With no list or forces a rotor disk will be flat, mostly due to centrfugal forces of the rotors rotation. In a real world the total aerodynamic force of rotor lift and drag, and including relative wind will lift the helicopter but does nothing to impart stability or control.

    To control a helicopter, one point of the disk is lifted or dropped to impart an unbalanced force and so move the rotor disk and hence the machine in the direction required. In lifting or lowering that part of the disk, the disk itself can appear somewhat coned as a result of both rotor head design and blade flex as increased forces are exerted by one part of the disk. Except in very high powered aircraft it's not that obvious.

    Most obviously, apart from exotic fixed head systems most rotors have flapping heads or the blades are hinged at the root and these can cone or appear to have significant 'dihedral'. This flapping blade allows for the advancing/retreating blade and the differences in lift across the disk, as a blade produces differing amounts of lift throughout its rotation the flapping blade will rise and reduce angle of attack under excessive lift until lift and centrifugal forces balance, with all blades doing this the rotor as a whole is stable. In a cross wind or in relative wind due to flight the disk will cone significantly but again still remain stable. Pilot input, moving the disk attitude but lifting one part of the disk is made more controllable as the flapping rotor head softens the effects of pilot input and relative wind ( in flight apparent wind).

    I've rambled on but hopefully the message is that heli aerodynamics isn't what you'd think it is. Control comes first and lift second. A rotor disk is aerodynamically unstable in anything but perfect, unloaded condition and that without control the disk can easily destabilise to destruction as a result of gyroscopic and aero forces. Try finding some of those early helicopter pioneer shows on the Wings channel.. Shows it perfectly. Those guys where both brave and clever..

    Andrew

  13. #53
    Join Date
    Jul 2005
    Posts
    82
    Ok, well I don't really want to get into a huge debate over helicopter stability, what I stated was simply stuff i've read before, interpretted over the years, was not mean to be considered the die hard facts of helicopter stability. There are thousands of enourmous books on helicopter design, it would probably take 3-4 years to come to some kind of conclusion on the reasons for their stability on this forum.

    I do see where you copied most of your gyroscopic precession explanation from: http://www.unmannedaircraft.org/page/page/1389904.htm

    I do want to say that my comment was not an attack on your comments, simply my opinion.

    I would breifly like to discuss the stability in the case where we are talking about changing the center of gravity. I truely want the most optimal position for thrust output to increase stability. So if i'm wrong I want to know before I build something.

    I believe in the case where you have a largerly vertical mass, (not flat and horizontal like a plane/helicopter) that having a large weight at the bottom creates a pendulum like effect. Not sure that is the right word, but think about what happens when you start to lean to the right, the thrust starts pushing you even more right, and if you are weighted heavily on the bottom, it resists movement, therefore making you lean more and more until yer flipping over. The same could happen in a helicopter or other vtol, but if you have a more vertical mass (like a rocket i guess, but we aren't really talking about that type of thrust/control system). that centering the thrust output more so in the middle helps when moving side to side. That way if you start to move left or right quickly, there is no swinging effect.

    I've tested this slightly in practice with mockup twin ducted fans I have by changing where the center of gravity was, now let me say none of these tests were in any way rock hard evidence of anything, just me playing around and coming to my conclusion. When it was very low, and thrust was applied, when it would start to swing back and forth it got out of control almost instanly and went crazy. Now moving it upward, not neccesarily in the middle, or above, just not down as far proved to be much more stable in my tests. It most definatly lost control as well, but did so much more slowely, i'd say 4-5 times more stable in appearance.

    At first i didn't quite understand it, and perhaps my reasons now are wrong, but i believe it to be as I explained above. I also though about devices such as the hiller flying platform, where the weight is mainly distributed vertically, and is up high. I believe it's reason is when you start to lean, the thrust projected actually attempts to correct the problem. Now granted, by no means does it mean that it will correct it completly, and control is still needed, but it was done without a computer, and it actually did fly. Also moller did make a "skycar" if you want to call it that like 30 years ago (and i'm going on memory here, don't feel like looking it up) that had 8 ducted fans, their distribution was mainly in the center of vertical gravity. It actually flew as well, and was very stable, no computer (don't think). Granted it was not weighted vertically much, so not sure how much of that applies to this.

    Really the Hiller flying platform is something that defies the basis that high is good, clearly it was produced at the bottom and was stable, at least somewhat.

    To conclude, I just want to state that I don't want to get into any huge argument about any of this. If you think i'm wrong, that's fine, perhaps I am.

    Ross

  14. #54
    Join Date
    Apr 2006
    Posts
    1257
    .

  15. #55
    Join Date
    Apr 2006
    Posts
    1257
    .

  16. #56
    Join Date
    May 2003
    Posts
    550
    Quote Originally Posted by klick0 View Post
    I do see where you copied most of your gyroscopic precession explanation from: http://www.unmannedaircraft.org/page/page/1389904.htm
    Copied? Not quite, that's just 'helicopters 101' for anyone taking aeronautical engineering or who's taken flying lessons. Regardless where it came from, it's valid.

    Quote Originally Posted by klick0 View Post
    I do want to say that my comment was not an attack on your comments.
    Not taken as such. If you hadn't said you where actually building something I wouldn't have bothered posting anything. Since you're spending the effort and money trying this, and it would be fun to see you have something that works, you will save disappointment by checking your opinions. Hence my post, there's loads of information out there that might help you and often its not what you might expect or a much wider subject.

    Quote Originally Posted by klick0 View Post
    I would breifly like to discuss the stability in the case where we are talking about changing the center of gravity.
    We're not really talking about moving centre of gravity but it's effect on the vehicle in the different attitudes the vehicle is supposed to go through. Same applies to the centre of effort, centre of pressure, centre of Lift, Aerodynamic centre and more. All of these will have a dissimilar effects on the vehicle in various attitudes. The problem is you're discussing a vehicle without aero controls that has to be controlable through take off and transition to flight. Each stage produces different challenges.

    One example is your 'pendulum effect' where you have just two elements, lift and COG at some distance below and you're seeking stability in hover. The problem isn't that the thing falls over because the COG is too low, it's partly because there's no control action, partly because as it tilts it's loosing lift vector and finally because your ducted fans are not precis and the lift isn't going through the COG, this produces a directional force and it doesn't matter where the COG is. The thing will still tilt and accelerate unless there's a control applied.

    Raising the COG shortens the moment arm on the force tipping the thing but it's slowing the problem, not removing it. Depends on your definition of stability.

    A hiller platform or rocket and a parachute are opposite ends of the COG solution. Parachute is immensly stable in a wide range of attitudes with no control input. A rocket or Hiller cannot track off vertical without significant control effort.

    For your and diamards ideas you can put the COG and lift where you like, but you have to match your choice with control. More unstable the vehicle the greater the control required...

  17. #57
    Join Date
    Apr 2006
    Posts
    1257
    .

  18. #58
    Join Date
    May 2005
    Posts
    162
    I'm sure the US Army had a turbine powered 1 man capable flying thing, sort of like a flying dustbin, guess it never really took off! ( groan ) but I have seen video footage some years ago, looked pretty scary. Here I found a picture, looks like it was called a wasp!


  19. #59
    Join Date
    Apr 2006
    Posts
    1257

    Tractor and Pusher Props ???

    .

  20. #60
    Join Date
    Apr 2006
    Posts
    1257
    .

Page 3 of 6 12345

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •