Miles Heiner
Active Member
Why do the Long-EZ and Varieze have anhedral in the main wings?
Why do the Long-EZ and Varieze have anhedral in the main wings?
- Thread starter Miles Heiner
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I'll take a stab though I doubt my answer will be entirely accurate or complete. By sweeping the wings you add stability to an aircraft similar to what happens when adding dihedral. Since too much stability could reduce control effectiveness or have other negative consequences a little anhedral was added to conteract the effects of sweeping the wings.
Aircar
Banned
In essence that's correct and the dihedral effect of sweep is exacerbated by the winglets at the tips --the tendency to Dutch roll created by the balance between restoring forces due to skidding initially is one of the most significant problems for swept wing aircraft in general and for one preceded by a lifting canard it is worse --directional 'stiffness' and roll coupling are interelated --you can understand it better if you look at a yawed model from the point of view of the incoming airstream --an air molecule entering at the tip of the leading wing will exit at a higher point than it would have if the wing was not anhedralled and on the other wing the opposite is true . The relative drag of the two wings is a function of their lift and 'frontal' area and the fin(s) add their forces in the yaw plane as well --the yaw creates speed differential (small) and alpha change (large) setting up a sort of dance called Dutch roll . You can see that many Russian swept wing aircraft -even low wing ,have anhedral and need long landing gear while most Western swept wings don't -- the C5 galaxy,C17 etc and most high wings do have anhedral but low wing swepts (Boeing 7xx series etc) don't and most need electronic yaw dampers . High wing aircraft have a natural increased dihedral effect anyway to counter and don't have to worry about wing tip clearence .
Monty
Well-Known Member
This is actually an interesting historical thing.
The original Vari-EZ prototype did not have anhedral. The prototype had elevons on the canard and no ailerons on the main wing. Roll control was lacking, so anhedral was added to reduce roll stability. This helped, but there was an additional problem: loss of elevator authority with combined pitch and roll input. So ailerons were added to the main wing. My memory may be incorrect, but I think a lot of wing attach fittings had already been ordered, so the anhedral was left in the design. Several aircraft were build without it and flew just fine.
The Long EZ has a straight wing.
The original Vari-EZ prototype did not have anhedral. The prototype had elevons on the canard and no ailerons on the main wing. Roll control was lacking, so anhedral was added to reduce roll stability. This helped, but there was an additional problem: loss of elevator authority with combined pitch and roll input. So ailerons were added to the main wing. My memory may be incorrect, but I think a lot of wing attach fittings had already been ordered, so the anhedral was left in the design. Several aircraft were build without it and flew just fine.
The Long EZ has a straight wing.
Norman
Well-Known Member
Actually the Long-EZ does not have anhedral but the VarieEZ does. The Long EZ deals with the excessive dihedral effect in a different, and really cool, manner. The dihedral effect due to sweep is not quite the same as the effect of actual dihedral and directional stability of swept wings is very speed sensitive because of it. The swept wing develops two speed (actually Cd and Cl) dependent moments. the skid/yaw moment and the skid/roll moment.
The skid/yaw moment is the result of the drag differential between the two wing halves. Neglecting parasite drag that means that the skid/yaw moment is proportional to induced drag or CL^2.
The skid/roll moment is due to the lift differential between the two wing halves.
When those two moments are balanced just right you will have an airplane that tracks through its maneuvers nicely without hunting or dutch roll. Unfortunately a swept wing only develops these two moments in coordination at one speed. At high speed the skid/yaw moment is too small, which means poor yaw damping, and at low speed the skid/yaw moment is too big which leads to dutch roll. The anhedral of the VarieZE is there to reduce the skid/roll so the two moments are coordinated over a wider speed range. The Long-EZ is a much bigger plane and anhedral would have created a ground handling problem so they found a different, and possibly better, way.
ThadBeier
Well-Known Member
I've built only small toy canards, but it seemed to me that there was substantial dihedral-like effect caused by the downwash of the smaller front wing over the larger back wing.
If you yaw the plane to the right, then the right wing gets more downwash from the canard than the left wing, causing the plane to roll right; much like dihedral would. For the model airplanes I was building, even straight wings with no dihedral were remarkably stable.
If you yaw the plane to the right, then the right wing gets more downwash from the canard than the left wing, causing the plane to roll right; much like dihedral would. For the model airplanes I was building, even straight wings with no dihedral were remarkably stable.
Aircar
Banned
Thanks for the link to that other thread Norman _ I thought I was being a bit too detailed and technical for the query as it was --when you get to books that explain these sorts of things and have a "list of symbols' that goes for eight pages (eg "aircraft stability for pilots and engineers" -Dickinson --better titled ( '' '' for calculus and alegraic geometry masters' it becomes neccesary to simplify .
--another book called "Meteorolgy for glider pilots" (CE Wallington) was always called "gliding for meteoroligists" such was the level of technical detail in it --anyway the points you make are true enough -- the case of skid in an Eze introduces the upwash outboard of the canard and the downwash inboard (offset on the wing now ) plus the lift centre shift due to yaw on the wing as the geometric twist varies with yaw as well --the 'ailerators' that he used on the first VariEze had the effect of creating an opposing roll from the wake of the elevator on the wing --reverse of tailerons on jets where the close coupling and flap effect comes in (the F 111 in full sweep for example)
Canting of dual fins to reduce effective roll moment goes back decades before Rutan (look at the SR 71 for one example --the idea is traceable to at least 1915 ) -- I designed built and flew a 'mini varieze' Stiletto in 1983 and experimented with models adjustable for dihedral/anhedral and tip cant and it is possible to delete the anhedral or even dihedral the canard to get better reaction to skid or yaw -- a joined wing on Wolkovitch lines is exactly that - dihedraled canard plus anhedral wing or two joined V tails even --anyway they can be made dead beat stable even if thrown 90degress sideways with no trace of dutch roll --on launch is another matter . I once and only once got a spin along the span axis (falling leaf sort of manouver) and that is front of a Boeing engineer whilst showing just how 'bulletproof' such a configuration can be .
Swept and twisted wings are usually on transonic aircraft or flying wings rather than in the realm of homebuilts so there is very little actual experience to work from --air density also affects the roll/yaw behaviour --the progenator of the Vari Eze (Dale Reeds' super snooper/mini sniffer)became unstable at high altitude as Re Nos and inertials altered.
--another book called "Meteorolgy for glider pilots" (CE Wallington) was always called "gliding for meteoroligists" such was the level of technical detail in it --anyway the points you make are true enough -- the case of skid in an Eze introduces the upwash outboard of the canard and the downwash inboard (offset on the wing now ) plus the lift centre shift due to yaw on the wing as the geometric twist varies with yaw as well --the 'ailerators' that he used on the first VariEze had the effect of creating an opposing roll from the wake of the elevator on the wing --reverse of tailerons on jets where the close coupling and flap effect comes in (the F 111 in full sweep for example)
Canting of dual fins to reduce effective roll moment goes back decades before Rutan (look at the SR 71 for one example --the idea is traceable to at least 1915 ) -- I designed built and flew a 'mini varieze' Stiletto in 1983 and experimented with models adjustable for dihedral/anhedral and tip cant and it is possible to delete the anhedral or even dihedral the canard to get better reaction to skid or yaw -- a joined wing on Wolkovitch lines is exactly that - dihedraled canard plus anhedral wing or two joined V tails even --anyway they can be made dead beat stable even if thrown 90degress sideways with no trace of dutch roll --on launch is another matter . I once and only once got a spin along the span axis (falling leaf sort of manouver) and that is front of a Boeing engineer whilst showing just how 'bulletproof' such a configuration can be .
Swept and twisted wings are usually on transonic aircraft or flying wings rather than in the realm of homebuilts so there is very little actual experience to work from --air density also affects the roll/yaw behaviour --the progenator of the Vari Eze (Dale Reeds' super snooper/mini sniffer)became unstable at high altitude as Re Nos and inertials altered.
Has anyone in the group or anyone for that matter changed the airfoil on the VE/LE design. A thinner airfoil at the expense of higher landing speeds but with a trade for higher cruise speeds seems ideal for what I want. Thoughts?
StarJar
Well-Known Member
Are you sure you want to add takeoff and landing distance to an Eze?
kent Ashton
Well-Known Member
Klaus Savier has used a thinner canard. I do not think anyone has changed the main wing airfoils. Any main wing airfoil that results in a more abrupt stall or higher stall speed could be hazardous. You never want wing-stall in a canard airplane.
Turd Ferguson
Well-Known Member
Or increase the speeds for takeoff and touchdown?
I am very very comfortable with a 130knts or more approach speeds. Vrot speeds of 120knots are also okay. I doubt a thinner wing on an EZ will come to that.
Lets assume landing and takeoff distances aren't of concern.
Higher take off and landing speeds may require mods to the wheels and landing gear.
As far as anhedral, only a guess, but maybe the wing is designed with enough flexibility that it hangs a bit while on the ground. Can anyone confirm or deny this?
As far as anhedral, only a guess, but maybe the wing is designed with enough flexibility that it hangs a bit while on the ground. Can anyone confirm or deny this?
Kyle Boatright
Well-Known Member
To elaborate: Spin up loads on landing gear and heat capacity of brake discs are not trivial factors.
Turd Ferguson
Well-Known Member
especially since most VE's used go-kart tires
TFF
Well-Known Member
A big plane landing at 120Kts is ok because it is designed to be strong enough. Something like a VEZ/LEZ is asking for the gear broken off down the road. They are not designed for that. Skill does not matter.
". . . because there's no possibility I'll ever have to set this single-engine plane down anywhere but on a runway" . . .(?)
And on how many occasions do off-field landings result in putting the aircraft on a long stretch of road wide enough and without traffic? Your statement assumes that an aircraft requiring a shorter takeoff and landing requirement will always find that ideal size road or field to land in an emergency. Is that how it really works?
Turd Ferguson
Well-Known Member
add 20-30 kts to the TO/Ldg speed and you're expecting what kind of change on the top end? 10-15?? I'd just choose another plane that goes faster.
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