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1-34 Rudder



 
 
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  #11  
Old February 14th 04, 11:35 PM
Roy Bourgeois
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That's right Eric - Think of the fuselage/rudder as a lever moving the
glider about the center of the yaw axis. The farther away the rudder is
from the center of that axis the farther it must move to have the correct
effect. Only by increasing the size can we make it move laterally far and
fast (but that creates a weight problem). A longer fuselage reduces the
"force" needed - but the tail it needs to travel laterally farther faster
to have the appropriate effect. Thus, the 1-34 is very stable in yaw - but
it really doesn't have adequate yaw response when you step on the
rudder. Consider for a minute the size of the rudder on the Genisis
2: It's downright tiny (maybe 25% the size of the 1-34's) - but it is so
close to the center of yaw axis that it is more than adequate - and much,
much, more responsive than the 1-34. In general, the farther the rudder
hinge line is from the wing spar - the bigger the rudder needs to be for
proper yaw response.

Roy





  #12  
Old February 15th 04, 01:14 AM
Eric Greenwell
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Roy Bourgeois wrote:
That's right Eric - Think of the fuselage/rudder as a lever moving the
glider about the center of the yaw axis. The farther away the rudder is
from the center of that axis the farther it must move to have the correct
effect.


Being further back puts it in a slightly different airstream, giving it
a slightly lower angle of attack (less than 2 degrees, even in a tight
circle) compared to one half as far from the wing. Since it has twice
the lever arm, it only needs half the lift of the closer one to counter
the adverse yaw, so losing two degree in AOA isn't a problem; in fact,
it's probably a help because so much less force is needed.

Only by increasing the size can we make it move laterally far and
fast (but that creates a weight problem). A longer fuselage reduces the
"force" needed - but the tail it needs to travel laterally farther faster
to have the appropriate effect. Thus, the 1-34 is very stable in yaw - but
it really doesn't have adequate yaw response when you step on the
rudder. Consider for a minute the size of the rudder on the Genisis
2: It's downright tiny (maybe 25% the size of the 1-34's) - but it is so
close to the center of yaw axis that it is more than adequate - and much,
much, more responsive than the 1-34.


I think the Genesis is a poor comparison, because it's reflexed airfoil
creates less adverse yaw in the first place. My flapped glider acts the
same way at any given speed: positive flap requires more rudder in a
turn than when negative flap is used. Another reason: the rudder is
actually further from the wing that it appears, because of the sweep
forward. You can't measure to the wing root to get a meaningful
distance, and you need to account for the sweep backward in the rudder.
These factors put the rudder farther from the wing than you might guess
from looking a side view drawing.

The 1-34 poor rudder response may due things I can only guess at:
perhaps the fin/rudder airfoil is poor (note how old gliders tend to
have very wide rudders compared to newer gliders); perhaps it has a
differential aileron deflection that is poor; perhaps the airfoil at the
aileron is high drag with down aileron. I'm really skeptical that
shortening the tail boom would help matters!

In general, the farther the rudder
hinge line is from the wing spar - the bigger the rudder needs to be for
proper yaw response.


If this were true, why do gliders have such long tail booms? Why aren't
they all as short as the Genesis? If being closer meant smaller
surfaces, we'd see all gliders with the surfaces mounted close to the
wing. I'm also skeptical that this idea has been missed by best glider
designers in the world.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

  #13  
Old February 15th 04, 01:58 AM
AZSKYBUM
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I flew my 1-34R for three years and found that taping the rudder hinge on both
the inside and outside of the hinged side does very little to improve
efectiveness, unlike the ailerons where it helps alot. Make sure the fabric is
secured to the rudder and isn't loose, some people don't ribstich them. I now
have a SGS 1-23 and it has a little, ineffective rudder compared to the
1-34.Enjoy your ship its a great glider just heavy to assemble.
Curtis Clark
Scottsdale, Arizona
  #14  
Old February 15th 04, 07:16 AM
Roy Bourgeois
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Eric:

I think we are really saying the same thing - just focusing on different
effects. If you pick up the tail of a long fuselage glider and rotate it
around it's wheel 360 degrees on the ground, it will take you longer than
when you do the same with a short fuselage glider - because the radius of
the circle you must walk is different (and you need more steps with the
long fuselage glider). Or, you could change the speed of your walk (which
is what changing the rudder size does). This is why the 1-34 seems slow in
rudder response. It is about the longest fuselage ever put on a 15m ship.
Now, there are many other reasons why a designer may want a long fuselage
(particularly stability) - but the reasons selected by the 1-34's designers
(I pass on whether they were the "best in the world") were not particularly
good ones, and once they abandoned the idea of the 17m wing they should
have shortened the fuselage (as they did with the later 1-35 and 1-36).







  #15  
Old February 15th 04, 07:53 AM
Eric Greenwell
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Roy Bourgeois wrote:
Eric:

I think we are really saying the same thing - just focusing on different
effects.


I'm pretty sure you are saying shorter arm, smaller rudder, and I'm
saying shorter arm, bigger rudder!

If you pick up the tail of a long fuselage glider and rotate it
around it's wheel 360 degrees on the ground, it will take you longer than
when you do the same with a short fuselage glider - because the radius of
the circle you must walk is different (and you need more steps with the
long fuselage glider). Or, you could change the speed of your walk (which
is what changing the rudder size does).


This is not a good analogy: in a steady turn, all the pieces of the
glider rotate at the same rate (revolutions per minute). Some of the
pieces are moving _faster_ (knots) than other pieces, especially the
inner and outer wing tips. That's why it doesn't take longer to "walk
the tail of the glider around" - you walk faster if you are farther from
the center of the circle. On a percentage basis, you aren't much
farther: perhaps a foot or two out of 150 feet or so.

But it isn't in the steady turn that the 1-34 has a problem, it's during
the entry to the turn.

This is why the 1-34 seems slow in
rudder response.


I've never flown one, but I'm pretty sure it isn't _slow_ in rudder
response, it just doesn't have enough rudder force to counteract the
adverse yaw from the ailerons when they are applied to turn.

Unless I've really missed something (it happens!), the "under ruddered"
comment only applies to the transient turn entry phase. In other words:
full aileron, full rudder, but the nose moves opposite the turn. This
isn't related to the speed at which the tail moves around, because
initially, the tail movement is in the wrong direction. As it settles
into the turn, the ailerons are moved opposite the turn until you have
some "top aileron", the amount of rudder is reduced, and everything is OK.

It is about the longest fuselage ever put on a 15m ship.
Now, there are many other reasons why a designer may want a long fuselage
(particularly stability) - but the reasons selected by the 1-34's designers
(I pass on whether they were the "best in the world") were not particularly
good ones, and once they abandoned the idea of the 17m wing they should
have shortened the fuselage (as they did with the later 1-35 and 1-36).


I still don't understand how rudder power is improved by shortening the
tail boom, andI still don't understand why the best sailplane designers
in the world haven't seized on the idea of making the tail surfaces
smaller AND making the boom shorter.

This comment addresses the orginal poster's question about "improving"
the rudder: Wil Schuemann told me the way to avoid this problem is by
using less aileron, not by making a bigger rudder. Less aileron, less
drag from the ailerons (drag is what the rudder is compensating for);
less drag = good. Not as pleasant to fly, but more efficient.
--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

  #16  
Old February 15th 04, 12:24 PM
Bert Willing
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The effect of any control surface is the product of lever x surface.
Longer fuselage (lever) makes up for smaller rudder. Your line of thought
does not work.

--
Bert Willing

ASW20 "TW"


"Roy Bourgeois" a écrit dans le message de
...
That's right Eric - Think of the fuselage/rudder as a lever moving the
glider about the center of the yaw axis. The farther away the rudder is
from the center of that axis the farther it must move to have the correct
effect. Only by increasing the size can we make it move laterally far and
fast (but that creates a weight problem). A longer fuselage reduces the
"force" needed - but the tail it needs to travel laterally farther faster
to have the appropriate effect. Thus, the 1-34 is very stable in yaw - but
it really doesn't have adequate yaw response when you step on the
rudder. Consider for a minute the size of the rudder on the Genisis
2: It's downright tiny (maybe 25% the size of the 1-34's) - but it is so
close to the center of yaw axis that it is more than adequate - and much,
much, more responsive than the 1-34. In general, the farther the rudder
hinge line is from the wing spar - the bigger the rudder needs to be for
proper yaw response.

Roy







  #17  
Old February 15th 04, 01:40 PM
Doug Hoffman
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Bert Willing wrote:

The effect of any control surface is the product of lever x surface.
Longer fuselage (lever) makes up for smaller rudder. Your line of thought
does not work.


Aren't these actually competing effects? Certainly the longer fuse will
have an increased polar moment of inertia about the yaw axis. Also, the
longer fuse *will* present more air-drag resistance to yaw movement, and the
tail will have to be dragged through more air for the same yaw rotation.
Then of course the rudder has more "mechanical advantage" due to the longer
lever arm as you point out.

I think both lines of thought are basically correct. The question is
quantifying one effect vs. the other, I believe. Interesting discussion.

Regards,

-Doug

  #18  
Old February 15th 04, 03:52 PM
Michael McNulty
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"Roy Bourgeois" wrote in message
...
Eric:

I think we are really saying the same thing - just focusing on different
effects. If you pick up the tail of a long fuselage glider and rotate it
around it's wheel 360 degrees on the ground, it will take you longer than
when you do the same with a short fuselage glider - because the radius of
the circle you must walk is different (and you need more steps with the
long fuselage glider). Or, you could change the speed of your walk (which
is what changing the rudder size does). This is why the 1-34 seems slow

in
rudder response. It is about the longest fuselage ever put on a 15m ship.
Now, there are many other reasons why a designer may want a long fuselage
(particularly stability) - but the reasons selected by the 1-34's

designers
(I pass on whether they were the "best in the world") were not

particularly
good ones, and once they abandoned the idea of the 17m wing they should
have shortened the fuselage (as they did with the later 1-35 and 1-36).




Your theory goes against every thing I've ever seen in aircraft stabilty and
control text books. The effectiveness the the tail surfaces for stabilty
and control is measured by the "tail volume coefficient", which depends on
the product of the tail area and it's moment arm (tail length). More is
better for stability and control. The reason some aircraft have big
vertical tails and long tail booms is that they need alot of tail
effectiveness; one is not fighting the other. The idea that a long tail
boom lessens lateral-directional stability or control is silly.


  #19  
Old February 15th 04, 04:17 PM
Roy Bourgeois
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I think that the difference is that my original post focused on "rudder
response" and others are focusing on "yaw stability" - which are different
and somewhat opposite. The 1-34 is quite stabile in yaw but as Doug noted
in his first post, it's response (time to desired effect) to your stepping
on the pedal is pretty bad. I flew several 1-34s about 250 hours (and
still occasionally). It doesn't have a problem with adverse yaw in a
turn. But when you step on the pedal (say, to begin a slip) - nothing much
happens (compared to other gliders). I was suggesting to Doug (and still
believe) that the lack of rudder "response" is a function of the
extraordinary fuselage length and that he would need a much bigger rudder
to solve that problem.

Now - if somebody will show me how to make my Nimbus 3 do ANYTHING
responsively - I'll be eternally grateful

Roy





  #20  
Old February 15th 04, 04:28 PM
Bert Willing
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And you're still wrong.
Michael is completely right that the control surface volume is the design
parameter for a the control surface (apart the airfoil of course).
Yaw stability and responsivity both depend on the rudder volume, but yaw
stability is achieved with less rudder volume than reponsitivity - so the
latter is basically a design tradeoff between responsitivity and drag.
Now if you don't believe that, get a textbook.

--
Bert Willing

ASW20 "TW"


"Roy Bourgeois" a écrit dans le message de
...
I think that the difference is that my original post focused on "rudder
response" and others are focusing on "yaw stability" - which are different
and somewhat opposite. The 1-34 is quite stabile in yaw but as Doug noted
in his first post, it's response (time to desired effect) to your stepping
on the pedal is pretty bad. I flew several 1-34s about 250 hours (and
still occasionally). It doesn't have a problem with adverse yaw in a
turn. But when you step on the pedal (say, to begin a slip) - nothing

much
happens (compared to other gliders). I was suggesting to Doug (and still
believe) that the lack of rudder "response" is a function of the
extraordinary fuselage length and that he would need a much bigger rudder
to solve that problem.

Now - if somebody will show me how to make my Nimbus 3 do ANYTHING
responsively - I'll be eternally grateful

Roy







 




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