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raaaid

could you give me arguments why control surfaces in the front are unstable and in the back not?

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today my teacher touched the plane subject and explained that the control surfaces in the back makes a plane unstable as a car going backwards with the steering wheels in the back now

 

i have to present a critic to his class and i want to give solid arguments of why he got it wrong

 

any lessons guys?

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Interesting way of teaching: Present an obviously false claim and have you debunk it, explaining why it's wrong? (If I understand you correctly)

 

I honestly don't know enough physics to accurately describe the difference, but if I were you, I'd start looking into what seperates an airplane from a car:

 

1. The thrust that propels the car forward is generated by the friction between the spinning wheels and the road, unlike an aircraft, where the thrust is generated in very different ways (which are themselves very different from each other - prop vs. jet)

 

2. On an airplane there is no "other pair of wheels" that have to pivot as in a rear wheel steering car.

 

3. The forces used to steer a car are the same that are also used to propel the car (friction between tires and road). In an aircraft the force that propels the aircraft (thrust from prop or jet) is completely seperate from the force that steers the aircraft (differential pressure generated by airflow over control surfaces)

 

That's all I can come up with now. Most likely someone will now post the right explanation.

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How is a car going backwards with rear steering different to a car going forwards with front steering?

has a smaller turning radius(at slow speeds as to park) but its much more nervous and unstable

 

i dont think the teacher really made the claim knowing it was wrong, i think it worked on me unintendenly

Edited by raaaid

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if you are really interested i will ask about it tomorrow in my aerodynamics class.

 

Its a good question Raid, it got me curious.

 

:D

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yeah thank god after years with bastard teachers and bastrd pupils now its nice teachers and nice parners feels like heaven, boy what a difference

 

i think this is the first ime ever i do something like this concerning studies

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has a smaller turning radius(at slow speeds as to park) but its much more nervous and unstable

 

i dont think the teacher really made the claim knowing it was wrong, i think it worked on me unintendenly

 

It can't be, a car with rear steering going backwards is exactly the same as a car with front steering going forwards.

 

I really can't make sense of your original post but will assume you mean control surfaces on the leading edge of the flying surface, the reason it would be unstable is because the air resistance would pin it in whatever deflection made to it's maximum and the resultant force would be uncontrollable, so it's the actual control surface that is unstable in this case.

if you managed to come up with an engineering solution to a leading edge control surface that made it strong enough without unnecessary complexity and weight and controllable then there is nothing that would make the aircraft unstable.

 

Canards don't necessarily make an aircraft unstable either.

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well you cant put directive wheel on the back of the car for it would have a huge huge oversteer

 

of course the front control surface would aim backwards, thats taacit into the problem

 

in fact the wright brothers put the control surface in the front but seemed wrong, but why?

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unstable as a car going backwards with the steering wheels in the back now

 

 

This is what I can't make sense of, how is this different to a car going forward with front wheel steering.

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first you have to understand oversteer and understeer

 

basically oversteery is unstable and dangerous though actually faster(to my taste some may disagree)

 

all sims i played combat planes are oversteery thats if you pull too much the stick it spins

 

if you take a corner way overspeeding and you crash with the front you have understteer if you crash with the back you have oversteer

 

in this videos you can see both a car oversteery understeery and neutral:

 

understeer:

 

http://www.youtube.com/watch?v=wzh5dbc7qpQ

 

oversteer:

 

http://www.youtube.com/watch?v=yjgws-cqtaM

 

neutral:

 

http://www.youtube.com/watch?v=jSe1WXfhs-g

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Lets line up some facts.

Canard, or tail-first planes are not unstable. They can be stable or unstable, exactly like conventional tailed types.

To be more precise, a true canard (Wright Brothers’ style) has just control surfaces forward of the wing, but the canard surface can be tailored differently. Making it bigger, it becomes a secondary wing, carrying a sizable amount of weight. But it can be made big enough to be a true second, or even main wing, carrying the majority of the weight (as in the Mignet type). In this case, we can talk of a tandem wing, or a tailless biplane.

A certain type of canard can have a lot of fuselage forward of cg and little behind. This, in turn, can make the plane directionally unstable, here your teacher is entirely right. However, this instability can be corrected with bigger fin and rudder, or placing fins and rudders at the tips of the main wing (see the Rutan Varieze LongEz).

Hope this can help you. I can give you more details, if needed.

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Here’s the Rutan Varieze. Look the wheels to have an idea of where the cg is. As you can see, there’s a lot of fuselage in front of cg, with relative side area, and very little behind. But placing fins and rudder at the tips of main wing give them enough arm to make the plane perfectly stable.

 

CRbplyd.jpg

 

Below's the Quickie Q2. Make the canard bigger, and there you have a tandem wing, or tailless biplane. Front wing flaps act as elevator. Here, the fuselage is conventional and a very average fin and rudder are enough to guarantee stability.

 

r7POKVX.jpg

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There's nothing wrong with canards! (At least that's my incredibly ignorant understanding). I read somewhere (where? I don't know!) that with a conventional layout because the wing's centre of lift/moment of thingy/etc has to be behind the aircraft's centre of gravity, the tailane has to generate a downward thrust to stop the tail tipping up. Stick a canard on the front a' la Wright bros, and you don't have that problem - all surfaces can generate lift. And if you arrange things so the canard stalls before the main wing. The plane becomes almost unstallable. I think. Um...

 

I read recently a NASA paper (must have been drunk at the time) bemoaning the fact that postwar aircraft design has been stuck in a "B29 paradigm, unable to move forward from one basic configuration. Bloody conservative, aircraft engineers! (I want my flying wing! :P)

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A car (or a motorcycle) is set up intentionally to be stable going forwards.  The linkages in the suspension are designed to center the steering if your hands are off the wheel.  If the alignment is out, the car might tend to dart one way or the other and need constant correction, tire wear increases and shows clues about the problem.  When things are working right, although the car is steered by the wheels in the front, the suspension design generally encourages the car to track straight ahead.

 

When the car is going backwards, the suspension design doesn't help stability.  Once the car starts a turn, and the wheels pass a certain angle, there may be a tendancy to keep going until they reach a mechanical stop.

 

Aircraft aren't like that.  They get their stability from the air flowing around them.  Very generally, if the tail gets out of line with the nose, aerodynamic forces tend to push it back in line again - that is the role of the horizontal and vertical stabilizers.  A car could be designed to go backwards with a high degree of stability, but  most cars spend more time going forwards than backwards.

 

Pilots actually us this rountinely, especially on aircraft with no flaps.  If you want to descend quickly but not pick up a lot of speed you can slip. Say you are coming over the mountains, maybe from Lake Tahoe, and you want to land at Carson City, down in the valley a few thousand feet below and only a few miles away.  Getting down to pattern altitude is a good plan, but arriving there going very fast may not be.  You stomp on the rudder, often using the maximum rudder available and use the ailerons to keep the airplane from turning.  You are forcing the tail out of line with the nose, making a whole bunch of drag to slow the airplane down, and you might get a nice descent rate of 1,000 or 1,500 feet per minute without picking up any speed at all.  It takes a lot of effort to keep that much rudder in, by the way.  Slipping a Cessna 172 for 2 minutes in each direction would be a good way to build up calf muscles.

 

When you release the rudder pressure, the tail is forced back in line because pushing that huge vertical stabilizer sideways through the air is hard.  Aerodynamic pressure forces it back in line, but the tail has inertia.  Maybe it passes beyond the point where it is aligned with the airflow, an now is out of line the other way.  No worries, That is where the "Dynamic" part of aerdynamics comes in.  pressure is now on the other side of the stabilizer, pushing it back into line that way.  The less out of line it is, the less force is trying to get it back in line, and the oscillations get smaller and then stop when there is equilibrum again.

 

It is just a matter of balance.  Aircraft tend to be pretty symmetrical aerodynamically, because we want the forces that act on them to be balanced when the airplane is flying straight and level.

 

A car going backwards is not very stable, anda plane going backwards is even less so ;-)

Edited by HeavyCavalrySgt

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Here’s another interesting example. This is the Mig 8. As you can notice, designers tried to keep the side area forward of cabin (and cg) as small as possible, tapering the nose. This helped, but fins placed at mid span on main wing proved inadequate. They were then moved to the wing tips, making the plane perfectly stable.

 

 

3icffIR.jpg

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By the way, here is a decent discussion of alignment and how it affects handling in cars...

 

 

Here is a discussion of aircraft staility and aerodynamics, with comparisons to cars complete with a rally car crashing.

 

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control surfaces in the front are unstable

and

 

 

control surfaces in the back makes a plane unstable

and

 

 

any lessons guys?

Lesson 1..

 

Make sure your title and subject mater agree with each other..

 

And not 180 out from each other ;)

Edited by ACEOFACES
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No, I know what oversteer and understeer are, now can you please explain my actual question?

oh yeah with the wheels in the back the car would be extreamly oversteery, unsatble and dangerous

 

edit:

 

oh im dislexic i cant distinguish simetrical things

Edited by raaaid

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How is a car going backwards with rear steering different to a car going forwards with front steering?

Edited by DD_bongodriver

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How is a car going backwards with rear steering different to a car going forwards with front steering?

 

Probably in suspension geometery.

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How is a car going backwards with rear steering different to a car going forwards with front steering?

hell it took me so long to understand this, i dont think theres difference unless the steering wheels have negative toe  in going one way and postive the other

Edited by raaaid

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Guys, guys - can we come back to the original question, please?

 

today my teacher touched the plane subject and explained that the control surfaces in the back makes a plane unstable as a car going backwards with the steering wheels in the back

I'll make some statements first and index them so anyone with better aeronautical knowledge can make corrections.

 

First of:

1) Steering a vehicle has nothing to do with controlling an airplane.

 

That being said: Vehicles

2) A vehicles's steering contains a caster angle which stabilizes the cars steering in the forward direction but destabilizes when going backwards.

3) Vehicles with rear-axle-steering like combine harvesters also have a caster angle that stabilizes their forward motion. It is simply reversed as compared to a normal car.

 

Airplanes

4) An airplane is stabilized by - suprise - its stabilizers, so the airplane "rests" on the wings and the stabilizers. More precisely: The stabilizers make it possible to create longitudinal and directional stability.

5) Airplanes are controled by the control surfaces which are always on the rear end of the stabilizer. THIS position at the rear end could be compared to the caster angle of a vehicle.

6) It makes no difference for the principle of stability or controlability whether the stabilizers and control surfaces are located in the front or the rear of the aircraft. Both can achieve stability and controlabilty.

 

What did the teacher try to say?

7) The teacher's argument COULD work if an Airplane had control surfaces at the FRONT END of the stabilizers. Those would tend to be instable in its controlability, like a car driving backwards.

8) However the teacher mixed up the position of stabilizers with the position of control surfaces. That's like saying a combined harvester couldn't be steered in a stable manner. As pointed out in 3), combined harvesters have a stable steering. As do airplanes.

 

So in a nutshell the Teacher compared apples with oranges because he misunderstood the difference between stabilizers and control surfaces.

Edited by 1./JG42_SchwarzerPrinz
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toe-in that is not properly set leads to wheel wobble, and terrible tire wear. It won't cause over or under steer, or even directional instability - just a lot of shaking the vehicle.

 

But, it's a raaaid thread so I'm going to go back to watching TV.

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first let's quickly define our terms. Stability in this sense refers to static stability, where a control movement naturally produces a counter-force. In a conventional airplane, if you move the rudders (for example) the plane will correct itself, you have to hold the pedal down. As soon as you let go, there is less and less deflection on the control surfaces. The air over the vertical stabilizer forces the plane back into alignment with the airflow. Most planes are statically and dynamically stable. I had an instructor at my school who was fond of telling his students "just let the plane fly itself, it'll do a better job than you ever will" :P   (thankfully not my regular instructor haha)

 

Warplanes can be neutrally stable or even unstable (a bit of aileron produces a greater (and therefore still greater) roll) to make them more nimble and reactive to pilot input. Fly-by-wire controls allow a pilot to handle an unstable aircraft with the aid of a computer. Many helicopters are actually unstable and require constant control input.

 

So unstable doesn't mean "not able to control" it just means that aerodynamic forces will magnify any motion on the plane (pitch, roll, or yaw), rather than dampening them. 

 

In a conventional airplane, as you pitch up there is increased AoA over the horizontal stabilizers (and the wing). This means lift, and because the tail is aft of the CG, this force acts against the pitch-up movement. So if you pull back on the stick and then let go, the plane will try to go back to a level attitude (trim-permitting).

In a conventional canard configuration this force isn't present. As you pull back there is greater lift on the canard, which accelerates the action of pitching up.

 

This may not be ideal for training but it's not a deal-breaker. Like I mentioned earlier, many helicopters are unstable. That's the bog-standard explanation. I don't know if it's possible to make a canard design statically stable or if it's worth it.

 

http://en.wikipedia.org/wiki/Longitudinal_static_stability

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The way I see it - there is only way to test and prove it... using a custom built airplane in Kerbal Space Program :)

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yes i see stability is pretty simiar to understeer and unstability to eversteer

 

personally i prefer unstability cause you can get closer to the limit and even skid a bit extra turning

 

i think the red baron knew how to take advanatge of unstable rotative engines for gyroscopic causes

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well you cant put directive wheel on the back of the car for it would have a huge huge oversteer

 

of course the front control surface would aim backwards, thats taacit into the problem

 

in fact the wright brothers put the control surface in the front but seemed wrong, but why?

I dont have the answer but it may be for the same reason that a boat also has a rudder at the back not at the front, possibly to do with the hull or fuselage impeding the efficiancy of the rudder?

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today my teacher touched the plane subject and explained that the control surfaces in the back makes a plane unstable as a car going backwards with the steering wheels in the back now

 

 

From this description I have no idea what you mean.  As a guess at what you mean, it'd be because control surfaces at the front would guide the nose of the aircraft in its manoeuvre.  Control surfaces at the back would "push" the nose around possibly being less precise for a number of reasons including turbulence from airflow etc.

 

"Unstable" is the wrong word.

 

Apart from that, this is your education so you do the homework.

 

Hood

Edited by Hood

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Okay, I tried to simplify the thing, but being it complex, the topic probably defied me. We need a little patience and to line up a little more facts.

To begin with, we need to define a little better what we mean for “stability”.

We have static and dynamic stability, but I think they are beyond the scope of this little conversation.

We have spiral stability, or lack of it. For complex reasons regarding controllability, airplanes are spirally neutral. If you put one in a spiral, it never comes out unless you level wings first (or hit the ground). Again I think we can leave this aside.

The relevant ones are:

Stability around roll axis.

Stability around pitch axis.

Stability around yaw axis.

 

Roll stability comes from wing dihedral and/or sweepback. Both canard and conventional planes can have as much or as little as you want. Difference is zero.

 

Pitch stability comes from the correct placement of centre of lift and centre of gravity.

 

Conventional types have the centre of gravity within the wing (usually around a quarter of the chord) so that a download on the tail is required to achieve stability. Move the cg back, beyond aft limit, and you have an unstable plane.

Reversing this scheme and simply placing the tail forward – as was done with some early canard types – you end up with an unstable machine, needing artificial stability. Below is the Bleriot Canard, an exercise in instability. The nose is on the right.

DydYdfq.jpg

 

The picture changes if you enlarge a little the “forward tailplane”, making it not just a control surface, but also a true wing. Modern, and successful, canards are made this way, and have as much pitch stability as their designer wanted. The most usual mean is to give forward wing higher wing loading and a higher aspect ratio than the main wing. A particularly mild or non-existent stall can be obtained as a side benefit, with the drawback that you cannot obtain the maximum lift from the rear wing, and usually cannot use flaps (like on the VariEze, see picture in previous post) or be limited to small ones (like the Beech Starship).

 

About yaw stability, the problem is indirect. Even a modern canard will usually have more fuselage ahead of the centre of gravity and less behind. This will produce a destabilizing side area, exactly as it happens with landplanes when they mount floats, and almost invariably need additional fin area. See below these Spitfires tails.

yMZWQxW.jpg

 

Many old canards placed fin and rudder on the centreline, not too far from the cg, and this was a problem. What matters for yaw stability is “tail volume”, as engineers call it: the ratio between fin area and distance from cg. If the fin is near the cg, you’ll need a huge area, as can be seen in the Focke-Wulf F19 Ente. But, again, modern designers have found clever solution. For example, they give sweepback to the rear wing and place fins at wingtips, where they act also as useful winglets, reducing tip vortices. See below the Ente's barn-door fin.

b4oO7uj.jpg

 

 

Summary.

There is no difference in roll stability and control between canard and conventional tailed planes.

A properly designed canard can have as much pitch stability as you want, without significant added drag.

Again: a properly designed canard can have as much yaw stability as you want, without significant added drag.

Finally: a properly designed canard is perfectly safe.

However, is true that old and improperly designed canards had severe problems in stability, particularly in yaw, and bad or dangerous stalling behaviour. The ages-old debate about superior efficiency of canard versus conventional tails would be even more complex than this brief summary, and I’ll leave it apart.

Edited by Furio
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Oh. Just one last thing: as stability and control are different things, stabilizing fin (or fins) must always be placed aft of the cg, but rudder works equally well if placed forward, as is demonstrated by the Rutan Defiant, a perfectly stable canard with push-pull engines, excellent performances and safety. As you can see, the rudder isn’t ever on the centreline, but is on the left, beside nose gear.

 

hY4vMBo.jpg

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Your question does not make sense, Raaid.

 

 

Restate it please.

 

I think you are asking, "why don't we put control surfaces on the leading edge instead of the trailing edge?"

 

 

 

We have static and dynamic stability, but I think they are beyond the scope of this little conversation.

 

 

Extremely simple concepts.

 

Static stability is the initial movement of the aircraft along the axis when the controls are released.

 

Dynamic stability is the oscillation behavior over time.

 

Good post Furio.

Edited by Crump

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This raaaid guy are a good fisherman... :biggrin:

 

Sokol1

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This raaaid guy are a good fisherman... :biggrin:

 

Sokol1

 

Yeah, I always thought he was a harmless soul who was just curious, kind of reminds me of my autistic daughter in some ways.  

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