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Member postings for Megawatt

Here is a list of all the postings Megawatt has made in our forums. Click on a thread name to jump to the thread.

Thread: New Drone Laws from 30/5/2018
08/06/2018 11:45:31

I think I need to challenge the assumption that this legislation is to clear the airspace above 400' for commercial drone ops. Full size aircraft are subject to a 500' rule and it is quite legitimate for them to descend to that height for whatever reason and remain legal. IMO the 400' limit and 1KM from an airfield is to allow separation from full-size aircraft and ensure that any increase in commercial drone ops remains segregated from manned aircraft. We shall be sharing the 0-400' range with them.

Thread: A new possible scam
20/04/2017 17:57:04

If you receive an Email like this do not under any circumstance click on any of the links - these may take you to a fake login page set up just to "harvest" your login details. Always enter the URL into your web browser for any login screen. If you have followed a link then you should change your password ASAP.

Thread: Life Today and Customer "Choice".
19/10/2016 11:52:32

BEB

My problem with the Pizza shop is not the scale of choice but the limitations of what you can and cannot do (like no more that 3 toppings on a 9" )

This always makes me thing that they decide what they want to sell rather than me decide what I want to buy.

Your experience also smack a little of misrepresentation - their offer (should you really only want one 7" pizza) is really a One for the Price of Two offer !!

 

Edited By Megawatt on 19/10/2016 11:53:04

Edited By Megawatt on 19/10/2016 11:53:50

Thread: A question of bears.....
17/10/2016 09:43:47
Posted by The Wright Stuff on 17/10/2016 09:33:57:

Question. Sounds obvious, but best answer wins!

Why is there a North pole and a South pole, but no East or West poles?

Because the N & S Poles relate to the axis on which the Earth spins.

Im interested in how a mirror reverses your face left to right but not top to bottom - how does it know?

Edited By Megawatt on 17/10/2016 09:44:04

Edited By Megawatt on 17/10/2016 09:44:28

Thread: A question of physics.....
16/10/2016 20:29:57
Posted by Peter Beeney on 16/10/2016 18:31:38:

Megawatt @16/10/2016 11:11:22

Megawatt, I do apologise for the late reply, I’ve been out flying…

With respect, I’m afraid I’m still not yet convinced at all, so I guess we shall have to gently agree to disagree here.

Yes lets do that. I really have nothing further to add I have tried science, engineering and real world examples and failed to convince you. I am now certain that nothing else I can say will make any difference.

BTW I have just re read the whole thread from the start it is amazing how the "plane won't move" group have never answered a single question about how a low friction wheel has the ability to overcome the thrust of four jet engines despite being asked several times.

You are convinced that you are right and that I, Newton, Einstein and even BEB are all wrong. I will clearly never change your mind - as I said I am out of here.

16/10/2016 12:00:21
Posted by Gary Manuel on 16/10/2016 11:51:15:

A thought accurred to me while I was watching the Sunday Politics show, which was ironically about new runways at Gatwick and Heathrow.

Imagine our 747 had different sized front wheels to the rear wheels (they are actually all the same). Lets say the main tyres are 56 inches diameter and the front tyres are 40 inches diameter. If our plane accellerated to 150mph, would both sets of wheels be moving at the same speed or at different speeds? Bare in mind that they are all attached to the plane and all in contact with the belt.

This is actually the crux of the argument and both camps will give a different answer, but it will highlight where the difference of opinion comes from.

Ok I'll start the ball rolling

Both will have the same linear speeds but will have to rotate at different RPM to achieve them

16/10/2016 11:11:22

 

Posted by Peter Beeney on 16/10/2016 10:49:22:

I think I might argue with the same conviction that plane wouldn’t move forward, it would stand still. Relative to the observer and the surrounding air, that is. Belt moving backwards at 150 mph, that must now mean that the point of contact under the tyre is also moving backwards at 150 mph. The 150 mph worth of jet thrust is simply now holding the plane steady against that. The wheels are still turning at 150 mph. The belt is having to apply a similar worth of thrust backwards. Top kit, these Turnigy brushless motors!

The post implies that the plane takes off normally regardless of any sort of movement underneath the wheels. That being so, then how is our stalwart runner on the treadmill able maintain equilibrium between himself and the backwards moving belt? Why doesn’t he simply keep running into the stand in front of himself?

Eye of newt, and toe of frog, and all that… We should have a nice thick brew by Halloween…

PB

Ok One last time (this time I mean it)

Yes the bottom of the wheel is in contact with the belt and is moving back at 150mph relative to the air

No the axle is not stationary it is moving relative to the air at 150mph - and 300mph relative to the belt - it is revolving twice as fast as on a stationary runway remember

The top of the wheel is moving at 300 mph relative to the air and 450 mph relative to the belt

Draw it out if you want to - I did

Once again your man on the treadmill can only derive his thrust from the belt which reacts to his movement such as to keep him there - the 747 has no such constraint.

A final example - not that it will do any good if you do not believe your own eyes watching the mythbusters video - Consider a car, in neutral on a rolling road with the brakes off. The road is moving at 30mph and the car is held stationary by a rope attached to something off the rolling road in front of it. Now then the something it is attached to is a winch which begins to shorten the rope at 5mph - the rolling road compensates by increasing its speed to 35mph. Are you really saying that the car will not be winched off the road!

That really is it for me - I'm off back to lurking.

Edited By Megawatt on 16/10/2016 11:14:44

16/10/2016 09:38:43

Well I hope we're all still having fun because that's what this is about. Like Gary this is not about point scoring but the enjoyment of discussing logic and hopefully causing pennies to drop. Having said that I am beginning to suspect that one or two people are having a little fun at our expense so this is my last attempt - forgive the long post but I am going to try to cover it all.

I will assume that conditions are perfect throughout i.e. frictionless bearings, massless wheels and nil wind so as to show the concepts and because the real world effects are small or transitory - if you are unhappy with this approach then there are several youtube videos which deal with the real world issues and demonstrate that they do not affect the outcome - take a look if you can follow the maths.

This question all revolves (pun intended) around wheels - nothing else really.

So lets start with a car on a conveyor with a driven wheel at the rear axle and a free wheel at the front. If the Car starts moving the conveyor matches its speed and the car will not move relative to the air. This is because the car can only get its propulsion from the reaction between the rim of the wheel and the conveyor - if the conveyor moves backward then the car cannot move forward. Please note that the speed the conveyor must move at is the speed of the axle if the car is to remain stationary not the speed of the top of the wheel or the bottom but the axle which as it is rigidly attached to the car is the same speed as the car. Now think of the front free wheel it knows nothing about what is happening at the rear - it simply rotates at whatever rate is necessary for there to be no slip between the wheel and the belt. As I said before that is what wheels do - they are no good for making tea and cannot receive radio one or get the children ready for school but they are brilliant at moving something over a smooth surface with little (or in our example no) resistance.

Now think about our 747. Its propulsion is generated completely independently from the conveyor - the conveyor cannot influence its speed in any way as it is resting on frictionless bearing wheels - the belt could be moving at Mach 0.99 and not be able to influence the speed of the 747 (ignoring any small real world effects due to air disturbance). But its not even moving at Mach 0.99 - only at the same speed as the wheel axle/747. So the 747 accelerates which makes the belt accelerate to the same but opposite speed but the plane still moves through the air and takes off.

Mythbusters actually did a pretty good job of recreating this - if the plane won't move brigade were right I would have expected the plane to maybe move back and forth a little due to errors in speed but it didn't, it smoothly accelerated and took off - in the real world with the wheels rotating at twice the rpm they normally would.

Edited By Megawatt on 16/10/2016 10:05:07

15/10/2016 20:12:44

Ok what does the original question not say - it does not say that the conveyor moves in the opposite direction so as to render the plane stationary - because that would be impossible the conveyor has no means to achieve that.

What it says is that it will travel at an equal speed (note this implies that the 747 is moving, speed being distance moved over time) in the opposite direction, So the original question allows for the 747 to be moving in one direction whilst the conveyer moves in the other resulting in the rotation of the wheels at the correct rpm to result in no skidding. I really can't see why this is so difficult to grasp.

15/10/2016 19:50:24

Posted by Pete B - Moderator on 15/10/2016 18:50:17:

Posted by Megawatt on 15/10/2016 18:25:30:

Erfolg - please put me out of my misery and admit this is a wind up. I call Troll and suggest that if you want to indicate that I am wrong you reply to the simple direct closed questions I have asked without introducing yet more technobabble to obfuscate the issue.

No, Erf's his normal self - once you've read a thousand or two of his posts you'll realise that....teeth 2

Besides, we all love him and wouldn't want it any different....wink 2

Pete

Indeed -that is what makes the debate worthwhile

15/10/2016 18:25:30

 

Erfolg - please put me out of my misery and admit this is a wind up. It is so blindingly obvious that if I run alongside and push you on your skates on the travelator that you would have a linear velocity equal to mine that I can only conclude that your answer was just to keep the thread rolling. I call Troll and suggest that if you want to indicate that I am wrong you reply to the simple direct closed questions I have asked without introducing yet more technobabble to obfuscate the issue.

Sorry Erfolg I read Andy48's reply as yours on my phone please ignore the Troll call above I can see now you have yet to reply

 

Edited By Megawatt on 15/10/2016 18:50:09

15/10/2016 16:47:47

Ok lets say you are on the airport travelator on your roller skates and moving in the opposite direction at the same speed if I then come along behind you (but alongside the travelator) and give you a great big shove in the back you would move forward - the travelator would increase speed but as you are on wheels you would continue to move so long as I continued to push you. You see it does matter where the thrust comes from - if you use a driven wheel it is impossible for it to provide more traction as the conveyor will simply accelerate to compensate. If, however, you have a form of propulsion which is independent of the conveyor you will move as there is nothing to stop you.

Also think for a moment about energy - it can neither be created or destroyed - if what you say is true what happens to all of that energy being consumed by the jet engines - to paraphrase BEB if you have a problem with that take it up with Einstein not me

15/10/2016 16:06:06

Erfolg

How does propeller work - it pulls something through the air

Now how does a wheel work - it revolves to provide the minimum resistance.

Please explain to me how a wheel overcomes the pull of a propeller (or jet engine)

Did you read my post of 13:48?

 

Edited By Megawatt on 15/10/2016 16:11:22

15/10/2016 13:48:28

OK I'll have another go

This is the statement which causes the confusion -The conveyor is designed to exactly match the speed of the wheels but in the opposite direction.

The mistake is to assume that that means the wheels will remain stationary - they won't. This is because wheels are free to rotate - that is what they are good at, indeed that is what they are for ! All that will happen is that the movement of the conveyor will cause them the rotate faster. When the aircraft reaches 5mph the conveyor will be going in the opposite direction at 5 mph resulting in the wheels moving at 10mph relative to the belt so we have movement of the aircraft while the original statement is still true. So how does the aircraft move I hear you say - well it moves because there is nothing to stop it - the wheels rotate freely and the engines provide the necessary thrust. So it accelerates to take off speed and the conveyer does the same in the opposite direction and the speed of the aircraft relative to the conveyor is now 2x take off speed leading to the concerns some about design limits.

Perhaps it might help to consider the opposite case a 747 just above stalling speed being just above the conveyor going at just above stalling speed in the opposite direction. Would the plane instantaneously stop when it touched down? No of course it wouldn't what would happen is the wheels would begin to rotate at the speed necessary to not skid.

14/10/2016 22:05:43

Hi all - joined the forum after lurking for ages just to have a go at this !

If you consider the forces on the aircraft you have the thrust of the engines which will result in an acceleration (Newtons first Law). There is no way the conveyer/wheels can possibly resist this force. However the question states that the conveyer will always match the speed of the aircraft in the opposite direction which leads you to think results in zero ground speed.

To clarify the situation you need to take a different frame of reference - lets consider a point on the ground under the conveyer. The aircraft will accelerate relative to this point (and the air) and so gain flying speed - in the meantime the conveyer would accelerate in the opposite direction but would have no means to slow the aircraft. So it will fly it will just have a ground speed (relative to the conveyer) of twice the take off speed. (I think )

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