Who was the early designer of the 'inherently stable biplane'?
When the test pilot tried to bank it in order to turn, it righted itself. And went on righting itself, the story went, until it wrote itself off.
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>>>>It would be difficult to have a more direct analogy.
Eurofighter - unstable because the centre of lift is ahead of the c of g
An oversteering car - unstable because the neutral steer point lies ahead of the c of g.>>>>
NC, the oversteering car is inherently stable in normal "flight", i.e. a straight line and it only oversteers when (subjectively) excessive steering cornering speeds are called for.
However a Eurofighter is inherently unstable even at level flight, stability is controlled by computer and if it that (and its backup) failed it would literally fall out of the sky.
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>>Bikes stay upright because the rider instinctively steers "underneath" the bike just before it falls over.>>
Yes that is the main reason, however ............
Experiments reported in New Scientist with a special bike with counter-rotating masses to offset the effect show that the contribution is negligible. >>
In motorcycle racing it is recognised that the giroscopic effects of the wheels slows the bike's ability to steer quickly, hence smaller 16" front wheels are still commonplace on race bikes and Michelin have developed seom new tyre compounds in 16" (where as 17" has been the normal on the road for some years). In this respect the Yamaha M1 MotoGP bike's engine rotates "backwards", this helps to counter the giro effect and also counters to a degree the bike's natural tendancy to rotate around the axis of the driven wheel under power, i.e. to wheely.
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I had just the same thought (about Rossi's bike) - amazing no-one thought of it before really! IIRC, Honda geared the alternator on their CX500 to run backwards to offset the torque reaction of the motor, which was mounted along (rather than across) the frame. Not sure how much difference it made - the torque effect of my BMW never bothered me, and even the daft/fabulous 2.3 litre Triumph Rocket 3 doesn't seem to suffer in that regard. No mention of it here, anyway...
tinyurl.com/yvqh69
I notice that Dr Hunt's piece for the NS says that "the gyroscopic effect helps but the trail is the more important factor", so not quite as reported. At motorcycle speeds and wheel masses, I suspect it's quite significant.
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>>NC, the oversteering car is inherently stable in normal "flight"
Cheddar, there is a bit more to it - really! Have a read of a good vehicle dynamics text like Gillespie, or Miliken & Miliken. There's a critical speed beyond which a car with a fundamental oversteering nature* becomes unstable.
* not the Clarksonesque limit oversteer in extreme corners you mention, but under ordinary conditions where the rear slip angles are greater than the front - or, put anther way, where the neutral steer point is ahead of the c of g.
Virtually no-one on here will have experienced this, because all cars have a fundamentally understeer characteristic - more of us have experienced the gross skidding of limit oversteer can be provoked in some cars by doing something ugly with either the throttle or the brake to unstick the rear wheels.
>>However a Eurofighter is inherently unstable even at level flight
Yes, and a car with built in oversteer tendencies can be unstable on a straight road! There's not much difference!
Number_Cruncher
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Cheddar, here's a snippet from the wiki on the subject of the instability which you get with a car set up to oversteer. (Accepting, of course, that the wiki isn't an authority on anything, and No!, I didn't write the article!)
-----8<-----
Oversteering cars have an associated instability mode, called the critical speed. As this speed is approached the steering becomes progressively more sensitive. At the critical speed the yaw velocity gain becomes infinite, that is, the car will continue to turn with the wheel held straight ahead. Above the critical speed a simple analysis shows that the steer angle must be reversed (counter steering), but this may be an oversimplification, as the model used is linearised in many important ways. (see Gillespie: "Fundamentals of Vehicle Dynamics", or any basic vehicle dynamics text). Understeering cars do not suffer from this, which is one of the reasons why high speed cars tend to be set up to understeer.
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Number_Cruncher
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Cheddar there is a bit more to it - really! ................... Yes and a car with built in oversteer tendencies can be unstable on a straight road! There's not much difference! Number_Cruncher
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OK NC I take the point however unlike the inherrant instability of a Euro fighter enabling it to turn quickly I dont see that the car with a fundemantal over steering nature held in check by electronics would offer any benefit over a well set up conventional/active/ESP approach. After all the objective in cornering is for the chassis to enable the tyres to maintain max grip on the road and to offer feel and feedback to the driver so he/she can maintain a consistent line through the corner, apex to apex etc, all factors that would not apply to the fundemental over steerer are are simply not applicable to the Eurofighter.
Intersting debate.
Regards.
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Yes, I don't know how much - if any - agility benefit there might be in doing this. It's an operating regime that, until recently, has been impossible to consider, but, as mentioned above by Cheddar and Garethj, the required sensors, computers, and perhaps most of the required actuators probably already exist.
You might think that the Eurofighter would be a handful to fly - but, according to one of the test pilots who flew the first test flights who addressed us at Loughborough, the plane is really easy to fly. There's no reason why a computer stabilised car could not also be easy to steer.
In the current situation, we have cars which are inherently understeering by various margins, and are cadjoled by various fancy footwork to be more neutral steering. The margin is there to prevent a member of the public experiencing an inherently oversteering car. With a stabilised car, you could do away with the margin, and be closer to a neutral steering car, and hence get maximum grip, without being in fear of catastrophe.
All pure conjecture of course - I doubt it will ever be specified for road cars.
Number_Cruncher
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but as mentioned above by Cheddar and Garethj .........
Just to say NC twas me that you were replying to.
The motorcycle analogy discussed above is most relevant I feel, a motorcycle is inherrantly unstable, i.e. without rider intervention it will fall over, as has been said at low speeds it is kept upright by the rider subconciously steering "under" the direction in wants to fall. At high speeds the giro effect of the wheels effects its ability to turn quickly, a different dynamic though a similar effect to way that airflow over the control surfaces of an aircraft maintains its stability, the latter is of course rather like water around the keel of a boat. So on a motorcycle the steering can be quickened by making the forks nearer vertical, this makes the bike a bit more difficult to ride at low speeds, to steer "under" as per above though it allows it to turn more quickly at higher speeds, to "fall" into a corner, it also makes it less stable in a straight line, a problem countered by a steering damper, these can be friction, hydraulic or speed variable electronic.
Regards.
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Yes, all the cases we are discussing are those of an unstable equilibrium.
If you imagine the motorcycle instead of riding along on top of tarmac riding along a tightrope to which its wheels had some magic grip, the motorcycle would have two positions of equilibrium - the unstable one above the rope, and a stable one, hanging from beneath the rope.
If it could fly backwards, Eurofighter would then be in a stable configuration with regard to its pitch dynamics.
If an oversteering car could roll backwards - well it would be stable from and under/over steer point of view, but probably unstable because of rear wheel steering, as mentioned by TR7V8 above.
Number_Cruncher
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