If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
"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
|
|||
|
|||
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
|
|||
|
|||
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 |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Air Bus 300 crash in NY now blamed on co-pilot's improper use of rudder | Corky Scott | Piloting | 30 | October 28th 04 04:10 AM |
B2 Split Rudder | Emilio | Military Aviation | 8 | April 12th 04 10:43 AM |
P-51 Rudder Aerodynamics | Hawkeye Hughes | Home Built | 1 | March 15th 04 10:24 AM |
A lesson learned - Invisible rider with foot on right rudder | Dave Butler | Piloting | 2 | October 16th 03 09:58 PM |
Cessna 150 Price Outlook | Charles Talleyrand | Owning | 80 | October 16th 03 02:18 PM |