Slips in turns and landing with winglets

On Thursday, February 11, 2016 at 4:09:52 AM UTC+3, wrote:
On Wednesday, February 10, 2016 at 8:03:15 PM UTC-5, Bob Caldwell (BC) wrote:
On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?

Sorry for the questions, but would appreciate any help in this.

There are two other considerations:

A. The side of the fuselage meeting the airflow in a slight slip produces some lifting force.
And:
B: A bit of top rudder reduces the back pressure on the stick needed to hold your turn. That reduces the induced drag at the horizontal.

So what do you think?

Bob

I think both are a crock of stuff. Lift is better generated by a wing and top rudder only serves to make the turn rate lower than it should be for the bank angle.

The best LD ratio of the side of a fuselage is not very good, but if it's heading directly into the airflow then the lift is zero but there is still drag, which makes the LD ratio zero. Having just a little bit of yaw giving just a little bit of lift does not significantly affect the drag, so that first little bit of extra lift is for free.

On Thursday, February 11, 2016 at 4:14:58 AM UTC-5, Bruce Hoult wrote:
On Thursday, February 11, 2016 at 4:09:52 AM UTC+3, wrote:
On Wednesday, February 10, 2016 at 8:03:15 PM UTC-5, Bob Caldwell (BC) wrote:
On Sunday, February 7, 2016 at 3:09:42 PM UTC-7, wrote:
I know this is a silly question, but I was reading an article by Dick Johnson that talked about holding a slight slip while thermally. This article was written before winglets and I was wondering if the same idea applied to gliders with winglets. Also, as for slipping to lose altitude for landing in a glider with winglets, does this place a lot of side load on the winglets. Should you not slip in landing with winglets?

Sorry for the questions, but would appreciate any help in this.

There are two other considerations:

A. The side of the fuselage meeting the airflow in a slight slip produces some lifting force.
And:
B: A bit of top rudder reduces the back pressure on the stick needed to hold your turn. That reduces the induced drag at the horizontal.

So what do you think?

Bob

I think both are a crock of stuff. Lift is better generated by a wing and top rudder only serves to make the turn rate lower than it should be for the bank angle.

The best LD ratio of the side of a fuselage is not very good, but if it's heading directly into the airflow then the lift is zero but there is still drag, which makes the LD ratio zero. Having just a little bit of yaw giving just a little bit of lift does not significantly affect the drag, so that first little bit of extra lift is for free.

I suspect that the gliders that seem to benefit for slight slip do so for 2 reasons.
First, as mentioned above, is that the slight slip in a glider with a good bit of dihedral will have the force opposing over banking provided without much or any control deflection. Profile drag and spanwise lift distribution benefit a bit.
Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
One guys opinion.
UH

On Thursday, February 11, 2016 at 9:14:08 AM UTC-5, wrote:
Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
One guys opinion.
UH

That would tend to correlate with early mentions of holding a slight slip in turns from George Moffat in regards to the Standard Cirrus and Nimbus II, both of which, IIRC, suffered from poor airflow around the wing roots while thermaling.

Chip Bearden

I think that all of this reasoning is pure speculation.

So for me, I try to keep it simple.

Even after 3000 h in saileplanes, I can't keep the string straight.

So, rather than skidding out of the thermal, I prefer to slip towards the core.

And it works ;-)

Bert
Ventus cM "TW"

On Thursday, February 11, 2016 at 8:14:08 AM UTC-6, wrote:
I suspect that the gliders that seem to benefit for slight slip do so for 2 reasons.
First, as mentioned above, is that the slight slip in a glider with a good bit of dihedral will have the force opposing over banking provided without much or any control deflection. Profile drag and spanwise lift distribution benefit a bit.
Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
One guys opinion.
UH

I think there is a little more to it. It would be interesting to have wind tunnel data comparing the lift and drag of a complete glider in a pure coordinated 40 degree banked turn at CL max (ball centered, forward mounted yaw string slightly to the outside due to geometry), vs the same glider with enough slip to remove the overbanking force (ailerons neutral, top rudder). My guess is that some gliders will show a small but significant amount of drag due to deflected ailerons (which may be more than the drag of the yawed fuselage and deflected rudder), while others may be less affected.

All I know, it works good on all gliders I've tried it on - from K-21s to my LS6. It may be less of an issue with more modern gliders with smaller wings and less control deflections.

But hey, if you don't believe it, just keep on thermalling with that front mounted yaw string perfectly centered.... ;^)

Kirk
66

On Thursday, February 11, 2016 at 2:44:49 PM UTC-5, kirk.stant wrote:
On Thursday, February 11, 2016 at 8:14:08 AM UTC-6, wrote:
I suspect that the gliders that seem to benefit for slight slip do so for 2 reasons.
First, as mentioned above, is that the slight slip in a glider with a good bit of dihedral will have the force opposing over banking provided without much or any control deflection. Profile drag and spanwise lift distribution benefit a bit.
Second is likely related to wing root separation issues which vary a lot from ship to ship. From my simple observation the gliders with larger well developed root fillets seem to be the ones that are not in the group described as benefiting from notable slipping.
One guys opinion.
UH

I think there is a little more to it. It would be interesting to have wind tunnel data comparing the lift and drag of a complete glider in a pure coordinated 40 degree banked turn at CL max (ball centered, forward mounted yaw string slightly to the outside due to geometry), vs the same glider with enough slip to remove the overbanking force (ailerons neutral, top rudder). My guess is that some gliders will show a small but significant amount of drag due to deflected ailerons (which may be more than the drag of the yawed fuselage and deflected rudder), while others may be less affected.

All I know, it works good on all gliders I've tried it on - from K-21s to my LS6. It may be less of an issue with more modern gliders with smaller wings and less control deflections.

But hey, if you don't believe it, just keep on thermalling with that front mounted yaw string perfectly centered.... ;^)

Kirk
66

What more? I was agreeing.
I provided a couple reasonable explanations for why it is/may be true for some ships. I know it doesn't help my '29 and does not seem to help my '28.
UH

Yes, I see that now (on RAS with too little sleep last night).

I wonder if winglets have any impact on it? I would think newer ships with "draglets" would prefer as little slip as possible.

Cheers,

Kirk

Yes Kirk winglets help it! Wanna race?

I was training a student in a 2-33. If you don't learn to slip that thing, you are going to land it all over the place. Let's just say I was fully practiced up on slips to land. So here I am at New Castle in my LS-6, I'm high on final, spoilers and flaps are not going to get it done, that's how high I was. So I put it in a big honking slip, didn't think about it just did it. Landing and roll out went fine, but something wasn't right, can't explain it but I never did it again, and I was shaking when I got out of the cockpit.
1. Read the POH
2. Practice first at altitude
3. remember what plane you are flying

SF

On Sunday, February 7, 2016 at 4:09:42 PM UTC-6, wrote:
Should you not slip in landing with winglets?

For the LS7 fitted with winglets (LS7-WL), the flight manual states:

WARNING: Sideslip with winglets prohibited, because during speed reduction stalling of the leading wing occurs (example: during left hand slip, stalling to the left!)

Jordan