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Sep 26, 2018, 06:36 PM
Everything's A Compromise
Larrikin's Avatar
Thread OP
Question

Remind me what the rudder is for?


Ok, let me premise this post with that fact that I'm not in any way being critical of the pilots in these videos.

Coming from the period where 'you fly what you build' (from scratch) and brutal winch launches, the models were heavy. I'm talking double the weight and 75% of the wing area compared to these 1200g F5J models.

My flying style comes from that era where rudder is used to keep the fuselage horizontal AT ALL TIMES because speed is life.

In the first video, and in my opinion, if the model had a heavier wing loading it would have carved a hole in the ground on the first circuit. The fact that it's energetic air helped too.

The lightly loaded wing offered a much quicker recovery than I was expecting.
I'll admit that when I saw the first imminent stall and the left wing drop, I braced for impact.

Also noted is that both models in the videos are Xplorers. Is that the way Xplorers like to be flown?

My question is, is this how lightly loaded, F5J models are meant to be flown? i.e. fly as slow as you think you can, on the edge of the stall because, hey, so what if it does?

Do you fly slow and on the edge, or faster being less influenced by the turbulence.

Personally, I watch the videos and I feel like yelling at the screen, "use the crackin' rudder!"

But I'm open to your discussion.


Explorer 4m Big Flaps climbing from hand (2 min 13 sec)



Jump to 52:40

1st FAI F5J EC Dupnitsa, Bulgaria, 2018 (part 5/5) - senior flyoffs (1 hr 19 min 48 sec)
Last edited by Larrikin; Sep 26, 2018 at 11:21 PM.
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Sep 26, 2018, 07:24 PM
solastagia
kcaldwel's Avatar
Since the wing stalls at the critical AoA, which is controlled by the elevator, I'm not sure what the rudder really has to do with "speed is life"?

The Explorer seems to fly boom down/nose up in pitch a lot more than other contemporary gliders, which is only really dependent on the angle the wing is mounted on the fuselage. It may be hard to tell if the glider is just being flown at a high AoA with the boom low in pitch, or whether not enough rudder is being used to prevent a sideslip.

Even if not enough rudder is being used, what is the penalty? The fuselage may have a slightly greater angle to the airflow. Given the very small cross section of today's glider fuselages relative to the wing, the penalty for additional sideslip is quite small. Given the very long fuselages and the quite small turn radius, the fuselage can never be aligned with the local airflow along it's whole length anyway. The additional drag penalty from side slip would be quite small. The penalty from having the wing fly at small sideslip or skid angles appears to be close to zero if XFLR5 is correct. There could be fuselage wake over an area of the wing which could hurt performance. As long as the wing has good tip stall resistance (and that can be adjusted with aileron to flap mixing), the drag penalty from not using any rudder will be quite small.

When low, staying in the lift is far more important than flying coordinated and ending up outside of it.

Kevin
Sep 26, 2018, 07:41 PM
launch low, fly high
There are quite a few items at play here. Kevin covered the fuselage drag aspect of the penalty of not using the rudder. There are potential aerodynamic penalties for using the rudder to get the fuselage better aligned with the local flow. First thing to understand, is that the fin is likely to be aligned rather well with the local flow, which means that the aft part of the fuselage is aligned with the local flow in a slow thermal turn. If one puts in some "in" rudder to get the middle of the fuselage aligned with the local flow, now the aft part of the fuselage has cross-flow. In addition, there is additional drag due to the rudder deflection. The next bit is that due to the lower speed of the inboard wing as compared to the outboard wing, it needs to be operating at a higher lift coefficient. The uncoordinated flight results in most of the increase in inboard wing cl coming from the combination of yaw and wing dihedral. If the wing has zero yaw, the amount of outboard aileron required is likely to result in the outboard wing trailing edge position being at a rather inefficient upwards deflection and the inboard trailing edge having too much downward deflection for the operating cl. The result is that the lowest drag solution has some uncoordinated flight (nose out yaw), especially with the larger wingspan and low wing loading F5J planes. For most of the F3J planes (excepting the uber light specialty versions) the wing loading was high enough to result in the lowest drag solution being coordinated flight with in rudder and out aileron. The uber light F5J planes, well... less coordination results in lower drag, assuming appropriate airfoils, dihedral, etc. Kinda unusual result that!
Sep 26, 2018, 08:11 PM
E sailplane thermal hack
I THINK, what these guys are trying to say here, is that with these new Uber light planes , flying at ridiculously low wing loadings at super slow speeds, the drag penalty for flying at odd angles of attack or uncoordinated turns is low. So you can just do it. Even with my Vixen I’m finding I can do all kinds of crazy maneuvers down low with very little penulty, kinda fun really!
Did I get that right guys???
Sep 26, 2018, 09:38 PM
In F3J size does matter!
roydor's Avatar
Maybe an older design very far out of its original design point suffers more...

I don’t think all uber light models need to be flown so much out of what we consider coordinated

Ultima 2 hand launch into a thermal 2 (1 min 35 sec)
Sep 26, 2018, 09:43 PM
launch low, fly high
Quote:
Originally Posted by roydor
Maybe an older design very far out of its original design point suffers more...

I donít think all uber light models need to be flown so much out of what we consider coordinated

https://youtu.be/bu7xm8EDV0I
Quite possibly. In large part, the essential element is to fly in the lift! It is only in the margins where deltas in efficiency matter. I was a bit surprised when I did the maths on this one...
Sep 26, 2018, 09:53 PM
jjc
jjc
Registered User
On the newer versions of the Xplorer the tail boom curves down. The taillplane is lower in flight then what most of us are used to looking at. John
Last edited by jjc; Sep 26, 2018 at 09:54 PM. Reason: Est.
Sep 26, 2018, 09:54 PM
solastagia
kcaldwel's Avatar
Quote:
Originally Posted by Joe W
The next bit is that due to the lower speed of the inboard wing as compared to the outboard wing, it needs to be operating at a higher lift coefficient. The uncoordinated flight results in most of the increase in inboard wing cl coming from the combination of yaw and wing dihedral. If the wing has zero yaw, the amount of outboard aileron required is likely to result in the outboard wing trailing edge position being at a rather inefficient upwards deflection and the inboard trailing edge having too much downward deflection for the operating cl. The result is that the lowest drag solution has some uncoordinated flight (nose out yaw), especially with the larger wingspan and low wing loading F5J planes. For most of the F3J planes (excepting the uber light specialty versions) the wing loading was high enough to result in the lowest drag solution being coordinated flight with in rudder and out aileron. The uber light F5J planes, well... less coordination results in lower drag, assuming appropriate airfoils, dihedral, etc. Kinda unusual result that!
That's very interesting Joe, thanks! I'd never looked at that aspect at real low wing loading.

Kevin
Sep 26, 2018, 10:22 PM
Everything's A Compromise
Larrikin's Avatar
Thread OP
Kevin, Joe and Jerry,

Thanks for your replies and I'm glad I asked.

"The uber light F5J planes, well... less coordination results in lower drag, assuming appropriate airfoils, dihedral, etc. Kinda unusual result that!"

That pretty much answers the question. I'll be mindful of outside aileron and not so focused on a smooth turn.
Oh, and fly in the lift, too.

With that said, Roy, watching the U2 turn is thing of beauty compared to the nose-high turns in the other videos IMO. Thanks for posting.

Re Xplorer, prolly a mute point but did I read in a thread that NAN were addressing their "dragging tail" decalage and increasing the angle of the wing at the seat?

D.
Sep 26, 2018, 11:16 PM
Everything's A Compromise
Larrikin's Avatar
Thread OP
Quote:
Originally Posted by jjc
On the newer versions of the Xplorer the tail boom curves down. The taillplane is lower in flight then what most of us are used to looking at. John
Thanks John,
Wondering what NAN's logic is behind that alteration

Reduced decalage to allow for a more rearward CG?
D.
Sep 26, 2018, 11:35 PM
jjc
jjc
Registered User
I have heard that as one of the reasons, and some guys think the newer versions fly smoother. Myself personally I don’t care for the look, I am now buying older versions because I use the tail boom as a reference when I am in a sustained thermal turn. John
Sep 27, 2018, 01:32 AM
Registered User
Tuomo's Avatar
Quote:
Originally Posted by Joe W
There are quite a few items at play here. Kevin covered the fuselage drag aspect of the penalty of not using the rudder. There are potential aerodynamic penalties for using the rudder to get the fuselage better aligned with the local flow. First thing to understand, is that the fin is likely to be aligned rather well with the local flow, which means that the aft part of the fuselage is aligned with the local flow in a slow thermal turn. If one puts in some "in" rudder to get the middle of the fuselage aligned with the local flow, now the aft part of the fuselage has cross-flow. In addition, there is additional drag due to the rudder deflection. The next bit is that due to the lower speed of the inboard wing as compared to the outboard wing, it needs to be operating at a higher lift coefficient. The uncoordinated flight results in most of the increase in inboard wing cl coming from the combination of yaw and wing dihedral. If the wing has zero yaw, the amount of outboard aileron required is likely to result in the outboard wing trailing edge position being at a rather inefficient upwards deflection and the inboard trailing edge having too much downward deflection for the operating cl. The result is that the lowest drag solution has some uncoordinated flight (nose out yaw), especially with the larger wingspan and low wing loading F5J planes. For most of the F3J planes (excepting the uber light specialty versions) the wing loading was high enough to result in the lowest drag solution being coordinated flight with in rudder and out aileron. The uber light F5J planes, well... less coordination results in lower drag, assuming appropriate airfoils, dihedral, etc. Kinda unusual result that!
Regarding the first video, that plane was flown in an uncoordinated way, and also a little slow. I have also a big flap Explorer and I know that it can take that kind of abuse maybe better than any other F5J plane. The special virtue of Explorer BF is that aileron drag seems to be very small. Also the possibility to add huge amount of camber (part of it maybe via snap flap) gives some extra star protection for the moments where speed drop. But he difference between Explorer BF and some other plane is only relative. It does not mean that uncoordinated flying momentarily at the edge of stall is the most efficient and safe way of working a low level thermal with Explorer BF.

My thinking is that thermalling is very much about building up energy - altitude is energy as well as speed is energy (kinetic energy). What we want to do at the first seconds when we hook a thermal is to a) keep in raising air, b) keep altitude, and c) build up some kinetic energy (speed).

Speed is almost as good as altitude With speed it is much more easy to fly the plane efficiently and in a coordinated way. Speed improves control - this is important as the low level thermals usually come with more or less turbulence. Thirdly, there is a tactical aspect. Keeping speed moderately high gives more tactical freedom to position the plane better in the thermal. Speed also improves possibilities to cover small mistakes. Landing out is usually the outcome when one finds himself slipping out of low level thermal and having low airspeed. With higher airspeed turning to the right direction and reaching the thermal again is maybe possible.

Roy flying his Ultima is much closer to the style of flying what I want to do. Right after hooking the thermal he builds up reasonable airspeed, after which comes nice circling and following the thermal downwind. But low level F5J starts often require spit second reactions. Placing the plane in raising air with reasonable airspeed is the key to success. Style comes only second. As you say, flying in a smooth coordinated way is not so important with a light F5J plane than with heavier F3J variant.
Sep 27, 2018, 02:57 AM
Registered User
Tuomo's Avatar
Just remembered this small Youtube clip. Nice "classical style" thermalling with a model that has be come a classic

Hand launch an Xplorer and take it out. (2 min 41 sec)
Sep 27, 2018, 08:48 AM
solastagia
kcaldwel's Avatar
Quote:
Originally Posted by Larrikin
Thanks John,
Wondering what NAN's logic is behind that alteration

Reduced decalage to allow for a more rearward CG?
D.

"Decalage" (I hate that term!) has nothing to do with the allowable rear CG. CG position sets the pitch stability margin, and therefore the pitch response. Incidence angles only set the fuselage angle, and the elevator trim to fly at a chosen airspeed. Changing incidence angles does not affect the CG range.

It could be Nan has chosen to align the boom with the curved airflow and higher wing down wash angle induced by the wing circulation at high wing Cl. That is the flight condition where F5J style gliders spend most of their time, and they may have designed for least fuse drag under those conditions. F3J gliders may have the boom aligned with the airflow at near zero wing Cl, for the ballistic zoom portion of the flight, and then live with the misalignment in the rest of the flight regimes.

You usually do want the horizontal stab to remain clear of the wing wake under all flight conditions.

Kevin
Sep 27, 2018, 09:19 AM
E sailplane thermal hack
You usually do want the horizontal stab to remain clear of the wing wake under all flight conditions.
And,,,,,, hence,,,, T Tails


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