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Jon Matcho

Reducing landing speed using both rudders

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From the full narrative in the NTSB report on the John Denver crash, I noticed this:

 

Concerning the yaw flight controls, the representative from Scaled Composite, Inc. reported that the rudders move and serve as vertical spoilers.

In addition to the speed brake and slipping the aircraft, apparently extending both rudders would be another device to reduce landing speeds in EZ types. However, I haven't heard much about this practice and wonder whether it's a viable and safe approach to slowing the aircraft on landing.

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apparently extending both rudders would be another device to reduce landing speeds in EZ types.

Extending both rudders is a drag increasing move. It does not slow the plane - it just increases the descent rate. You don't land slower, you just come down faster. I use the Landing Brake, both rudders, and slipping to modulate descent rate as needed. The approach speeds remain the same.

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It helps, and people do it, although many Velocitys now have the "toe-brake" upgrade, inteconnecting the rudder pedals, and so cannot use this technique any more.  In fact Velocity will tell you the rudder is really only a drag device, slowing one wing down and causing the airplane to yaw that way, and in training point out that when slow, as in the pattern, it's easier to just use the rudder for turns and keep the stick basically level, and to even use the opposite rudder to raise a dropping wing on final as the ailerons are very sluggish (on the Velocity) at those speeds.

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In fact Velocity will tell you the rudder is really only a drag device, slowing one wing down and causing the airplane to yaw that way...

Which is incorrect. The rudder is a primarily a flap (like the elevator or aileron), and at reasonable deflections creates far more lift increment than drag increment, by about a factor of 10X. The yawing is caused by this lift increment being aft of the CG. The drag does assist in the yawing, but is creating only about 10% of the yawing moment. The fact that the rudders are off the centerline doesn't mean that they act (aerodynamically) any different than rudders that are on the centerline, which, if they were primarily drag devices, wouldn't create any yawing moment.

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I think the addition of vortex generators to the canard surface would/should allow for a slower landing speed. But I am not an aero engineer and I did not stay at a Holiday Inn Express either !

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Extending both rudders is a drag increasing move. It does not slow the plane - it just increases the descent rate.

How this does not also reduce speed is beyond my understanding of physics, which I thought was rather good. Perhaps I need to get my understander checked... I thought more drag = less speed, and less drag = more speed?

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Jon,

I'm usually high on final and use belly board plus both rudders all the way to the numbers to get down.  Could have better technique I suppose but would rather make the runway than come up short.  Just the way I was trained.  I like slipping but it has never felt right in the EZ.

 

Randy,

Jim Price set an altitude record using VGs on both canard and main wing.  He developed the right installation, I believe, in a wind tunnel with a college project backing the research.  They allowed him to fly very slow which happens at the upper end of the service ceiling.  No Holiday Inn necessary, you nailed it!

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I don't see much drag increase with both rudders out.  I think a full-rudder slip works better.  It puts the fuselage, winglets and MLG at an angle to the relative wind and seems to make more drag.

 

When I have checked out guys, they are sometimes confused about which rudder to use and how much.  I use full top rudder (outside the turn) and as much aileron as required to make the turn.  I guess I never use full bottom rudder.  It seems counter-intuitive but I am not sure why.  Bottom rudder seems to align the fuselage more with the turn direction which would seem to produce less drag.  I think it tends to increase the sink rate and speed while the top aileron required to balance the rudder rolls out the turn and keeps you from turning.  It is the lift-vector that turns the airplane.

 

Anyway, bottom rudder feels very strange.

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How this does not also reduce speed is beyond my understanding of physics, which I thought was rather good. Perhaps I need to get my understander checked... I thought more drag = less speed, and less drag = more speed?

The issue isn't that increasing drag reduces speed - it does, if you attempt to maintain the same altitude or descent rate. The issue is that adding drag does not change your stall speed, so if you're trying to approach at 1.2 - 1.3 Vso, then you'll need to increase your descent rate when drag increases in order to maintain speed. You cannot just arbitrarily slow down in an airplane - there is a lower limit.

 

So you maintain speed, increase descent rate (which was the whole point of deflecting both rudders at the same time anyway) and continue the approach.

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I use full top rudder ... Anyway, bottom rudder feels very strange.

I've found that the landing brake will increase my descent rate by about 400 fpm on approach (COZY MKIV), adding both rudders will add another 200 - 250 fpm, and slipping can get me 1000 - 1500 fpm, with a slipping turn getting up to 2K fpm. Also, I think by definition you use top rudder for a slip - you bank away from the yaw direction (being the definition of a slip, no?) Bottom rudder would get you a skid, not a slip.

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I think the addition of vortex generators to the canard surface would/should allow for a slower landing speed.

There are two types of VG installations - one, like "flyingriki" mentioned, that puts VG's on both the canard and main wing - this increases the maximum Cl for BOTH flying surfaces and allows you to fly a bit slower. There is a very specific installation procedure for this, and the VG's MUST be mounted in the correct place on both surfaces.

 

There is also an installation for VG's on only the canard, but this is used not for stall speed reduction but for eliminating trim change when wet or contaminated, and ONLY for the GU canard.

 

You would NOT add VG's to the canard ONLY for stall speed reduction.

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