Mechanics of inverted flight

[kentuckyglider]

Hello all,

I own a Carl Goldberg Gentle Lady, a R/E sailplane with significant polyhedral. I've been practicing flying inverted, and noticed that although the elevator's effects are flipped as I expected (push forward on stick to go up), the rudder stil behaves the same as when upright (push left on stick, glider turns left if viewed from rear).
I suspect that this has something to do with the interaction of the rudder and the polyhedral of the wing being different when inverted. I understand that when the aircraft is upright, the polyhedral causes the plane to bank in the same direction as rudder input.

Could someone explain how this works when inverted?
Thanks in advance.

KG

[Ollie]

The rudder makes the plane with yaw and no turn by itself. With no dihedral and no ailerons the plane it flies straight on a straight line yawing or not. With a positive dihedral or polyhedral wing the rudder yaws and the wing banks the plane. It is the bank of the wing that allows the plane to turn.

[kentuckyglider]

Right, Ollie, it just seems that even though the plane is upside-down, that it should bank the same as it does upright, meaning if you tried to turn left, the plane would turn right. Sorry if this is a confusing discussion...

KG

[JetPlaneFlyer]

KG,
To understand why this is the case you first need to understand the way dihedral works. This article in Wikipedia has a couple of good diagrams to explain it: http://en.wikipedia.org/wiki/Dihedral

You should see from this that, in an aircraft with dihedral (or polyhedral), the rudder yaw causes the aircraft to roll 'in-synch' with the rudder input. The sideways force generated by the banked wings is what causes the aircraft to turn.

When inverted the aircraft effectively has anhedral, which causes a roll and a resultant turn opposite to the yaw induced by the rudder.

Hope this helps
Steve

PS... I thought when i saw the title of the thread this was going to be another debate on if/how a non-symmetrical airfoil can work when inverted... This is a much more controversial subject :rolleyes:

[Sparky Paul]

That is neat, innit!
You have to fight to keep the GL inverted, but having the rudder automatically "reverse" helps a lot there.
When I tried a Goldberg Mirage with the wing on upside down and backwards, rudder, no ailerons, I had to have two ruddder controls.. the normal left stick moved the rudder properly for taxiing and takeoff, the right stick had the command reversed for flying.
Worked OK.

[vintage1]

Glad to see the reputation of California is being upheld, even since the 60's ;)

[kentuckyglider]

Thanks for the link, Steve. That helps a lot. :)

KG

[yoyoML]

KG,
To understand why this is the case you first need to understand the way dihedral works. This article in Wikipedia has a couple of good diagrams to explain it: http://en.wikipedia.org/wiki/Dihedral

You should see from this that, in an aircraft with dihedral (or polyhedral), the rudder yaw causes the aircraft to roll 'in-synch' with the rudder input. The sideways force generated by the banked wings is what causes the aircraft to turn.



I think the yaw induced roll somewhat lags behind the rudder input, not quite in synch. It's that the change in yaw AoA is in synch with the change in the AoA difference across the main wing:

AoA difference = sin(yaw AoA)*2*(dihedral angle)

And this difference corresponds to the difference you get when you deflect ailerons. The LAG part is the time from deflecting rudder to actually getting a yaw AoA (and hence main wing AoA difference).

Of course with a low yaw moment of inertia and matched yaw damping, yaw and roll may be quite in synch, just not exactly.


Hope this helps
Steve

PS... I thought when i saw the title of the thread this was going to be another debate on if/how a non-symmetrical airfoil can work when inverted... This is a much more controversial subject :rolleyes:

Exactly.

[BMatthews]

It's all done with mirrors......

[markdrela]

That is neat, innit!
You have to fight to keep the GL inverted, but having the rudder automatically "reverse" helps a lot there.
No it doesn't.
When flying a rudder/elevator airplane inverted,
the rudder-to-yaw control is reversed, and
the yaw-to-roll control direction is also reversed because of the negative dihedral.
So with the net combined rudder-to-roll control direction is the same:
Right stick to roll right, and left stick to roll left, as usual.

[JetPlaneFlyer]

No it doesn't.
When flying a rudder/elevator airplane inverted,
the rudder-to-yaw control is reversed, and
the yaw-to-roll control direction is also reversed because of the negative dihedral.
So with the net combined rudder-to-roll control direction is the same:
Right stick to roll right, and left stick to roll left, as usual.

Errr... Isn't that what he meant :rolleyes:

[RCAV8R13]

It's as simple as as a double negative. Inverted dihedral (anhedral) and inverted rudder. Two wrongs make a right (and a left).
RCA

[Sparky Paul]

Errr... Isn't that what he meant :rolleyes:
.
Yes. That's what I meant with the "automatically"... :)

[RCAV8R13]

I can stir things up here. The Wickpedia explanation is antiquated and wrong. That is not how dihedral works. That explanation works only if the vertical stabilizer goes on vacation in the Bahamas or some place. But since it doesn't, then how come the plane doesn't weathervane and end up turning into the low wing panel and into a spiral dive? The answer is simple: that's not how dihedral works.
Here's how it works. Dihedral, and all other dihedral effects, cause yaw to roll coupling. When a plane dips a wing, un-commanded by the pilot, it will also turn. As the plane turns, air will travel around the fuse in a curved shape and applies pressure to the out side face of the vertical stabilizer causing the plane to yaw in the opposite direction of the turn (this is the reason why full scale pilots have to hold rudder through out a turn). It is this opposite yawing that is often confused with the phrase "side slip". As the plane yaws in the opposite direction of the bank, the yaw to roll coupling of the dihedral causes the plane roll in the opposite direction of the bank. This has been proven by experiments with the length of the tail. The longer the tail, the more effective the dihedral.
There, that otta stir things up.
RCA

[Sparky Paul]

When I first encountered this inverted thing with my Dynaflite Skeeter, I was patting myself on the back for "knowing" to apply opposite rudder when turning inverted.
And then I fingered out what was going on.. I wasn't applying "opposite rudder" out of prescience ; When turning inverted with lots of polyhedral, the stick went to the right, the plane went to the right, and the rudder, inverted, was going to the left.
Then I did the upside down wing thing, and that indicated a rudder reversal was mandatory for the plane to be flyable, with the rudder pointing up, and all that anhedral.

[JetPlaneFlyer]

I can stir things up here. The Wickpedia explanation is antiquated and wrong. That is not how dihedral works. That explanation works only if the vertical stabilizer goes on vacation in the Bahamas or some place. But since it doesn't, then how come the plane doesn't weathervane and end up turning into the low wing panel and into a spiral dive? The answer is simple: that's not how dihedral works.
Here's how it works. Dihedral, and all other dihedral effects, cause yaw to roll coupling. When a plane dips a wing, un-commanded by the pilot, it will also turn. As the plane turns, air will travel around the fuse in a curved shape and applies pressure to the out side face of the vertical stabilizer causing the plane to yaw in the opposite direction of the turn (this is the reason why full scale pilots have to hold rudder through out a turn). It is this opposite yawing that is often confused with the phrase "side slip". As the plane yaws in the opposite direction of the bank, the yaw to roll coupling of the dihedral causes the plane roll in the opposite direction of the bank. This has been proven by experiments with the length of the tail. The longer the tail, the more effective the dihedral.
There, that otta stir things up.
RCA

I know Wikipidia has to be taken with a pinch of salt in many things but in this one I think they have it pretty much spot-on... Dihedral works due to side slip. The AoA of the 'leading' wing in a side slipping aircraft that has dihedral is higher therefore the aircraft tends to roll away from the side slip, producing a stable aircraft.

The fin does play a part, if the fin area is too large, or the tail too long, and this is not balanced by sufficient dihedral, then the aircraft will, as you suggest 'weathervane' into side slip too quickly and be spirally divergent. I have seen with my own eyes that reducing fin area improves spiral stability.

But you don’t have to take my word on this... NASA says the same thing: http://history.nasa.gov/SP-367/chapt9.htm. I guess you also would claim NASA to be "antiquated and wrong" :rolleyes:

[HELModels]

Here is what it looks like. If it is wrong, say so, if you know so. :) I'll be glad to edit.

[JetPlaneFlyer]

Here is what it looks like. If it is wrong, say so, if you know so. :) I'll be glad to edit.

Sorry but I really don’t understand the diagrams...
The aerodynamic principal behind the phenomenon of rudder reversal (or 'lack of' rudder reversal depending on how you look at it) when inverted is down to the reaction of Dihedral/Anhedral to yaw... I can’t see how your diagram addresses this, maybe it's just me?

Steve

[kentuckyglider]

:eek: I know you guys said controversial, but I didn't know you meant CONTROVERSIAL! :eek:


Just teasing. :D This is a much more interesting discussion than I thought it would be.

KG

[Chrono]

You kidding me? It became interesting at the very moment you said a gentle lady could go inverted. Never tried that with mine. Now that you people said that is difficult but doable, I'm gonna clean the dust of my old GL and give the maneuver a try.

Now the real point of my reply is to ask you guys to explain again the dihedral - rudder thing. I didn't quite understood the explanations, nor the article in wikipedia.

Thanx a lot!!!

[kentuckyglider]

A word of caution, Chrono...
The GL has a horrible glide ratio inverted, and you will lose altitude very quickly; in fact, I heard of this maneuver as a way to get out of really big thermals. I can't say I've gotten into any of those, but I just had to try it when I heard of it. :D
I recently put my GL into a tree when I tried this too low. I pulled the split-S required to return to normal flight and misjudged my height. I had to use a bow and arrow to get the wing out of that tree... Punched some pretty good holes in it. :(

KG

[Sparky Paul]

The GL (or any polyhedral plane) really doesn't like inverted flight. :)
It'll fight you all the way, but you can get it there, and many times even get some decent turns.. but do this pretty high.

[Chrono]

A word of caution, Chrono...
The GL has a horrible glide ratio inverted, and you will lose altitude very quickly;
KG

Yeah that's what I suppose... gotta try it anyway :p