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roll stability of foils?

Posted: Tue Aug 15, 2017 9:36 pm
by jaros
What makes some hydrofoils more roll stable than others?
I can imagine the cause of:
- pitch stability - bigger or smaller rear wing and longer or shorter fuselage
- jaw stability - size of vertical stabilizer and position of the strut on the fuselage, di/anhedral of wings
But roll?

Re: roll stability of foils?

Posted: Tue Aug 15, 2017 9:43 pm
by Blackrat
The angle of the front wings

Dihedral being most stable and harder to roll
Anhedral being most sensitive to roll and unstable

Flat wing having little effect on roll

Re: roll stability of foils?

Posted: Tue Aug 15, 2017 10:15 pm
by Peter_Frank
Correct.

We actually seek less roll stable foils, so they are easier to maneuver.

Thus anhedral used a lot, to increase the desired "instablity" so to speak.

But one other factor that has immense effect on roll stability IMO, is span of the wing, the higher span the more roll stable.

So both for roll livelyness and yaw livelyness, shorter span is desired - meaning a wavefoil f.ex.

8) PF

Re: roll stability of foils?

Posted: Tue Aug 15, 2017 10:38 pm
by jaros
Hm... I am comparing two of my foils, both have almost exactly the same span and projected area of the front wing. One has a wing with big anhedral and the other has flat wing. What is funny is, that the foil with anhedral has way more roll stability. I like riding the flat one more in fact, since it carves tighter and lighter (like Pater Frank mentioned above), but there are some things that are harder on the flat foil, because of less roll stability - like sitting down on the board or doing a foot change.
So, how come that a foil with anhedral has more roll stability than a flat wing, when it should be the other way around?

Re: roll stability of foils?

Posted: Tue Aug 15, 2017 11:01 pm
by Pedro Marcos
Blackrat wrote:
Tue Aug 15, 2017 9:43 pm
The angle of the front wings

Dihedral being most stable and harder to roll
Anhedral being most sensitive to roll and unstable

Flat wing having little effect on roll
Humm, lets take moses wings as an example:

The 558 is more Anhedral then the 550, but the 558 is more roll stable then the 550.

I believe the "winglets" in the stabilizers give alot to roll resistance, again with moses, the 330 stabilizer (with winglets) has is alot more roll resistant then the 325 (almost flat).

Re: roll stability of foils?

Posted: Tue Aug 15, 2017 11:03 pm
by Peter_Frank
I think it is because a flat wing is the exception because it can "sideslip" better, this gives a looser more lively feel :D

Just like a flat rear wing is much looser because of that.

Don't know, just my intuitive feel and thoughts.

8) PF

Re: roll stability of foils?

Posted: Tue Aug 15, 2017 11:04 pm
by jaros
I should add that both foils i am comparing have flat back wings....
Hm... I am comparing two of my foils, both have almost exactly the same span and projected area of the front wing. One has a wing with big anhedral and the other has flat wing. What is funny is, that the foil with anhedral has way more roll stability. I like riding the flat one more in fact, since it carves tighter and lighter (like Pater Frank mentioned above), but there are some things that are harder on the flat foil, because of less roll stability - like sitting down on the board or doing a foot change.
So, how come that a foil with anhedral has more roll stability than a flat wing, when it should be the other way around?

Re: roll stability of foils?

Posted: Wed Aug 16, 2017 3:34 am
by lovethepirk
This question interested me a lot when I was designing a kite a year ago. The answers above are correct, but I think this example may help you understand. This is only in theory though, as my Delta race wing 'looks' anhedral(downward wing angle), but a significant part of the wing could be argued to be dihedral(upward wing angle) as the wing angles upward exiting the centerline where a lot of the lift is created.

Basically, when wings are angled upwards(major passenger airliners), as the wings bank hard in one direction...the bottom wing is going to increase it's angle of attack in relationship to the prior windflow. The problem I don't completely understand is that that apparent windflow is going to change to some degree so the degree to which the angle of attack of the bottom wing increases for upward angled winds may not be as great until the plane stops yawing.

Regardless, it's hard to overcome the basic principle that upward angled wings want to self-right themselves vs downward angled wings want to continue to roll. Just my thoughts.

Image

Re: roll stability of foils?

Posted: Wed Aug 16, 2017 3:40 am
by lovethepirk
Here you can see my Delta race wing has quite a lot of upward swept wing compared to the tips which do sweep downward.
Image

Re: roll stability of foils?

Posted: Wed Aug 16, 2017 4:01 am
by alexrider
Sorry pirk, I know what you mean, but in order to avoid misunderstanding, I suggest you use proper terminology. Upward wing angle would do, as wiki defines it.
Signed, the pedantic rider.
https://en.m.wikipedia.org/wiki/Swept_wing
https://en.m.wikipedia.org/wiki/Dihedral_(aeronautics)
lovethepirk wrote: ... This is only in theory though, as my Delta race wing 'looks' anhedral, but a significant part of the wing could be argued to be dihedral as the wing sweeps upward exiting the centerline where a lot of the lift is created.
...the bottom wing is going to increase it's angle of attack in relationship to the prior windflow. The problem I don't completely understand is that that apparent windflow is going to change to some degree so the degree to which the angle of attack of the bottom wing increases for upward swept winds may not be as great until the plane stops yawing.
Regardless, it's hard to overcome the basic principle that upward swept wings want to self-right themselves vs downward swept wings want to continue to roll. Just my thoughts....