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Taxiing, brakes, rudder - how does it work (technically)?


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I have no real problem with the way the 21's bake system in conjunction with the rudder pedals work in oder to get around on the airfield.

 

But I wonder how the system actually works. The braking logic is clear to me, I think. You steer with the rudder pedals and if you apply brake pressure, differential braking "happens".

 

But I was a bit surprised, that I can also steer without differential brakes - just with the rudder pedals alone - given that you move at least at a certain speed. The turn diameter is then ofc much larger, but it works really well to align on the taxiway or the runway, etc.

 

But how does that exactly work? Afaik the nose wheel is not steerable? So is the rudder alone, aerodynamically, responsible for that? If so, how can it be so effective - at such low speeds?

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It can't possibly be aerodynamics, that would be utterly unphysical.

I was also surprised at what you're talking about. I assume that giving rudder input actually differentially brakes the undercarriage, but I'm not sure that it's working as intended. Was, like you, expecting having to brake with the lever and simultaneously give rudder to get ANY turning at all.

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The brake pads are activated by air pressure (pneumatic). Maybe there is a control valve that is link to rudder movement that directs the air pressure to either wheel and when the rudder neutral the air pressure is allowed to reach both sets of brake pads. I'm intrigued as well...Hopefully someone smarter comes a long...:helpsmilie:

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Very good question, I have wondered about it aswell. It seems that above about 65% RPM the nosewheel become steerable a little bit.

 

I too was under impression that at lower speeds you could be only able to steer with brakes. Then I read this:

 

On the MiG-21 F/MiG-21F-13, the nose unit is castoring; steering on the ground is by differential braking. Late versions have a hydraulically steerable nose gear unit.

 

Source: http://www.kamov.net/general-aviation/mig-21-landing-gear/

 

As it stands now, the plane with nose breaking disabled will turn even with rudder movement only at speeds lower than 5 km/h.

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Very good question, I have wondered about it aswell. It seems that above about 65% RPM the nosewheel become steerable a little bit.

 

That's been my experience, too. I was steering during taxiing without using the brakes unless I need to do a really tight turn. And I noticed that at lower engine RPM steering doesn't work as well or at all.

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There's only one logical explanation. A pnuematic valve is linked to the rudder and applying the brake as you move the rudder. The maximum force that you can apply with rudder alone is limited, that's why you need to apply brake in order to turn tight.

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You are mixing multiple things.

 

DIFFERENTIAL BRAKES are linked to the rudder, and require use of the brake handle, period.

 

NOSE WHEEL STEERING LINKED TO THE RUDDER is something else entirely, and the aircraft behaves like this only connects at speeds > 40 kph (to me it seems more like 25 kph).

 

Thus, when taxiing VERY SLOWLY, the ONLY way to turn is with DIFFERENTIAL BRAKING.

 

At high speed (let's say 100 kph) then NOSE WHEEL STEERING is available.

 

I'm very intrigued as to why it doesn't have full-time nose wheel steering, and how the system works.

 

Best regards,

Tango.


Edited by Tango
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There's only one logical explanation. A pnuematic valve is linked to the rudder and applying the brake as you move the rudder. The maximum force that you can apply with rudder alone is limited, that's why you need to apply brake in order to turn tight.

 

Ok that does it. I'm going to watch some porn.

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The MiG-21 fighter maneuverability in today's terms document (or whatever it's called) in another thread claims that the MiG had extremely good low-speed rudder authority, down to as little as 30 kts (~50kmh). It could just be aerodynamics at fast taxis. (And presumably that's why the manual/procedures say to taxi at 50-60kmh, so you don't have to spend your compressed air on keeping centered on the taxiway.)

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Thus, when taxiing VERY SLOWLY, the ONLY way to turn is with DIFFERENTIAL BRAKING.

 

At high speed (let's say 100 kph) then NOSE WHEEL STEERING is available.

 

I'm very intrigued as to why it doesn't have full-time nose wheel steering, and how the system works.

 

Not as it is currently simulated. In training taxi mission I can turn without brakes with or without nose wheel brake with as little as 10 km/h and power between 70-80%.

 

If this is the correct behavior then obviously there is some kind of a pneumatic device that engages only after power is advanced to 70-80% and is linked to the rudder movement (i.e. nose wheel steering).

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FROM THE MANUAL

 

"When the aircraft starts to move, press brakes again to check if braking

is even and then release the brakes to continue your taxi. If you taxi straight from your parking place,

allow the aircraft to gather about 40-60km/h (20-30nmi/h) then decrease the power to maintain that

speed (~65%). At this speed you will be able to use the rudder to maintain direction, so you’ll avoid

wasting compressed air when braking to maintain direction. However, if you are making turns, you

need to decelerate to 15-20km/h (9-11nmi/h) and use rudder controls and brakes to turn the aircraft."

 

Doesn't really explain much.

 

I only wonder if the BLC system is somehow effecting this

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You need to apply partial brakes (not reaching 50% even) and keep speed below 50 km/h (65% or less power) to have sufficient nosewheel steering. I don't have problems taxiing this bird but I takeoff without holding the brakes but gradually till reaching full afterburner.

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Ff4life4, yep it does, back and down along the top of blown surface, like every blown flaps system, or even normal slotted or Fowler flap, just harder. In either case, the air ends up under the elevators and clearly nowhere near the rudder.

 

WildBill - nobody has problems taxying this bird, we're just curious what makes the plane turn at low speeds even WITHOUT using brakes, when: a) nose wheel is supposedly free-castoring; b) rudder ain't working, because there's clearly not enough airstream hitting it.


Edited by Art-J

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Maybe, when LN started to make their first flight test model they started with a simple propeller driven aircraft.

 

And since then they forgot to delete the prop wash and that's what we are seeing here, it makes perfect sense!

 

Case closed, move along guys.

 

Clearly I'm the annoying person.

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WildBill - nobody has problems taxying this bird, we're just curious what makes the plane turn at low speeds even WITHOUT using brakes, when: a) nose wheel is supposedly free-castoring; b) elevator ain't working, because there's clearly not enough airstream hitting it.

 

That!

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WWII bomber formations | DCS P-51D: [TEST] TO distance / gross weight / temperature

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WildBill - nobody has problems taxying this bird, we're just curious what makes the plane turn at low speeds even WITHOUT using brakes, when: a) nose wheel is supposedly free-castoring; b) rudder ain't working, because there's clearly not enough airstream hitting it.

 

Fixed! :P

 

Best regards,

Tango.

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