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 General Discussion
 
Design and Technical 
How Much Dihedral?
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Last Activity 2009-06-01 4:52 PM 9 replies, 947 viewings |
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| marske Member  Posts: 37 Joined: 2003-06-15 Location: marion, Ohio  User Profile |
I can actually steer the sailplane by using the rudder to a limited degree before I haveto use ailerons. If I use just ailerons they work fine giving good adverse yaw so they are still effective. If we look at beginners models they have lot of dihedral and turn with the rudder.... more advanced designs have much less dihedral and ailerons. Any thoughts on the subject? -Mat Redsell Marske Flying Wings | ||
| bildan Extreme Veteran  Posts: 374 Joined: 2003-03-15 Location: Centennial, Colorado, USA User Profile |
You are on the right track, Mat. Dihedral produces a yaw-to-roll coupling meaning that, in a skidding turn, the advancing wing has a greater angle of attack than the trailing wing so dihedral increases roll rate in the direction of the skiding turn. In a slipping turn entry, as is sometimes produced by adverse yaw, the reverse is true. Dihedral causes an adverse rolling moment that opposes the ailerons and slows the roll. This is why it is important to use enough rudder to completely overcome adverse yaw at the beginning of a turn entry. Flying wings have a big problem with adverse yaw since the rudder moment arm is small compared to the aileron moment. Reducing dihedral will help but the real problem is not that you have too much dihedral or too much adverse yaw, it's that you have too little yaw authority. Time to re-consider wing tip drag rudders? Bill Daniels | ||
| loon Veteran Posts: 166 Joined: 2003-03-15 Location: Austin, Texas User Profile |
How do yo calculate tail volume on a flying wing? | ||
| marske Member Posts: 37 Joined: 2003-06-15 Location: marion, Ohio User Profile |
Adverse yaw is not a problem of the flying wing when there is a good taper ratio on the main wing. We had proverse yaw in the rebuilt Pioneer because we had too much differential at first. The rudder actually works quite effectively, and while the tip rudders would give much more authority, for the moment I do not think they are necessary. -mat | ||
| marske Member Posts: 37 Joined: 2003-06-15 Location: marion, Ohio User Profile |
The rudder works quite well but you need more of it and with our present configuration you lead with the rudder since I beleive there is a little too much dihedral. -mat | ||
| bildan Extreme Veteran Posts: 374 Joined: 2003-03-15 Location: Centennial, Colorado, USA User Profile |
I'll have to get back to Marion and fly the PIID but I can't imagine that you had proverse yaw from ailerons mounted on a lift producing section of the wing. Excess aileron differential won't produce proverse yaw, it just reduces aileron effectiveness - which can SEEM like proverse yaw. Here's a test: Fly at a steady 45 knots and enter turns with the yaw string centered and see how much aileron you can use when full ridder is employed - i'll bet not very much. If you had more yaw authority, you could use more aileron and get a faster roll rate. Getting back to dihedral. Do you really care whether a fast roll rate comes from yaw-to-roll coupling or from ailerons? I don't. It seems that the trend in fast glass is to build wings with polyhedral which, among other things, enhances the yaw-to-roll coupling resulting in a faster roll rate. (if the pilot utilizes it correctly) The spiral stability you report is likely due to the dihedral + a slight out of turn yaw angle. (Yaw string deflected slightly toward the high wing.) You wouldn't want to eliminate that would you? For perspective, look at the opposite end of the spectrum from flying wings - my Lark with a huge rudder and heavy ailerons. Many pilots flying it for the first time will complain about the ailerons saying the glider is fighting them. (Actually, they are fighting the glider.) I demonstrate a 4 second 45 degree to 45 degree bank reversal with the yaw string center using only my index finger on the stick. My trick is that I am using the yaw-to-roll coupling that results from full use of the huge rudder and just enough aileron to keep the string centered. Bill Daniels | ||
| marske Member Posts: 37 Joined: 2003-06-15 Location: marion, Ohio User Profile |
Now on the turning and proverse yaw. In our earlier setting when one went to turn I would apply rudder and then ailerons to discover that I must use the opposite rudder which I called proverse yaw. The differential I felt was such that there was too much drag on the up aileron compared to the down aileron so I adjusted it and, lessened the differential which worked well. The turns seem normal to me now but it would be good to have you try it and make your own judgement. I seem to learn more everyday from this glider. Yesterday I flew it for about 4 hours. -mat | ||
| smjmitchell Member Posts: 17 Joined: 2003-06-29 Location: Sydney Australia User Profile |
Hi All, I don’t really have as much time as I would like to think about this issue of dihedral on the Monarch but the rock solid lateral stability and slow roll rate (i.e. lack of control power) may well be due to the pendular effects of hanging the pilot below the wing. One of the effects of dihedral is to adjust the vertical position of the CG relative to the wing and this directly effects the magnitude of the pendular restoring moment which contributes directly to lateral stability. Imagine the airplane is disturbed in roll by a gust or control input. If the CG is a long way below the wing then the airplane will tend to right itself just like the pendulum in a clock will always return to the central position – of course the oscillations of an airplane are more heavily damped than a clock pendulum but you get the idea. A conventional weight shift hang glider illustrates the power of pendular effects well …. High wing airplanes with a high proportion of their mass below the wing all tend to suffer from too much lateral stability and lack of roll control power. The Monarch is an example of this where more then half of the mass of the airplane is concentrated a considerable distance below the wing. Consider a couple of other albeit not exactly topical examples. The USAF transport aircraft such as the C-5 and C-7 together with the BAe (Avro) 146 are all high wing transport airplanes. What do they all have in common ? Wing anhedral. Why? If not the CG would be so far below the wing that the pendular effects would be so strong that you could never roll them with the available aileron/spoiler control power. Thus they use anhedral to move the CG closer to the AC of the wing. I think you need to do something similar in the Monarch. I had a look at the Marske web site and it appears from comments on there as though the Monarch suffers from sluggish roll control (correct me if I am wrong). Of course this could be for many reasons but one likely cause if excessive lateral stability. This needs a lot more thought and investigation but I suspect you need to remove some of the dihedral. Bear in mind here though that there are other issues that need to be considered. For example you still want to end up with an airplane that will pickup the lower wing in a slip if you let go of the stick (fundamental stability requirement – see JAR 22.177 for example although this states the requirement in a slightly different way). The dihedral requirements for achieving this may conflict with that for the pendular stability problem and thus a compromise may be in order. You need to think this through. Of course this issue is not relevant to conventional gliders as the mass of the fuselage and pilot and hence the CG are close to the wing and the effect of pendular stability will be weak. I would also like to comment further on the questions about tailless aircraft and tail volume that were raised in another post …. as time permits. Stephen | ||
| bildan Extreme Veteran Posts: 374 Joined: 2003-03-15 Location: Centennial, Colorado, USA User Profile |
Originally written by marske on 2003-07-18 9:54 AM .... Now on the turning and proverse yaw. In our earlier setting when one went to turn I would apply rudder and then ailerons to discover that I must use the opposite rudder which I called proverse yaw. The differential I felt was such that there was too much drag on the up aileron compared to the down aileron so I adjusted it and, lessened the differential which worked well. The turns seem normal to me now but it would be good to have you try it and make your own judgement. I seem to learn more everyday from this glider. Yesterday I flew it for about 4 hours. -mat The following applies to low speed flight as when thermalling. I once tried an experiment where only one aileron moved...up. I still had adverse yaw. The raised aileron in the first part of its movement doesn't increase drag, it reduces it - just like negative flap reduces the drag of the whole wing. So, it doesn't matter how much aileron differential you have, you'll still have adverse yaw if the ailerons are mounted on a lift producing part of the wing. ( I assume the whole wing of the Pioneer IID produces lift) Aileron diferential, at the low speeds we use for thermaling, doesn't eliminate adverse yaw (and certainly doesn't reverse it) - it just reduces making it somewhat more manageable. Bill Daniels | ||
| marske Member Posts: 37 Joined: 2003-06-15 Location: marion, Ohio User Profile |
When I first tested the Pioneer I found that in many cicumstances I was applying opposite rudder which for most pilots is confusing so I lesseend the differential and it is very comfortable now. I did have the down aileron project below the wing as well by an inch and a half. -mat |
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