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Old Jan 25, 2009, 10:02 PM
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United States, CA, Midway City
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I was able to get some time on mine today and think I got it about 95% dialed in.

I am using no differential, but of course need to add lots of rudder. I have it on a mix, but need to add more with left thumb. I'll have to measure my throws, but my ailerons only need a very small amount of deflection for normal flying.

As a matter of fact I have it dialed in at 60% of normal throw ( normal max is about 22 degrees) and an additional 30% expo so it's not as twitchy. My CG is somewhere between 106 and 108. I had it at 106 but found it was too stable there and added some aft weight and slid it down the boom some. I will measure and post later. It will not pull out from a reflex dive at about 30 degrees but maintains very straight line fast cruise.

I believe my hook is right around 109-110 back from wing LE. I was going to move it back another "click" to try but ClayH said to leave it alone as it rotates very well already and pulls to point of stalling winch. Yet, it's almost a hands free launch. I say almost, because you do need a blip of down at about 60-70 ft?

I tried a few launches varying camber 10mm-20mm and it's solid throughout. You will need to vary this depending on wind condition to optimize your launch so don't be afraid to experiment.

Hopefully Machild and ClayH can chime in some more as they got some time on it. I am very happy with this plane.

My landing mode does have crawl setting on it. My crawl setting is 1/3 down throttle stick and flaps matching ail just like in launch setting with a hair more down elevator. 100% differential and max ail throws and rudder mixed in. It seems to float forever there. At 2/3 down throttle stick, flaps are down 3/4, but ail moves back to neutral and no diff and no rudder coupling, you will need to use left thumb here. At full down throttle stick, flaps down about 85 degrees and ail in crow (up) about 5mm at tips. Diff is reverse but still have some up about 2:1 down/up and rudder coupling remains off.

That's a lot written there, but I maintain solid roll control throughout the landing process no matter where the throttle stick is.

Have I mention this plane to me is a landing machine? Well, it is a landing machine.
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Old Jan 25, 2009, 10:36 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Quote:
Originally Posted by fnnwizard
I was able to get some time on mine today and think I got it about 95% dialed in.

I am using no differential, but of course need to add lots of rudder. I have it on a mix, but need to add more with left thumb.

Hi, Tuan-

Glad you're getting some flying in!

I wonder why not just introduce a little diff in the ailerons so that the rudder doesn't have such a job to do to keep the nose straight??
I'm confused, but it could be my newness to TD flying.

Thanks, Target
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Old Jan 25, 2009, 10:43 PM
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fnnwizard's Avatar
United States, CA, Midway City
Joined Dec 2003
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Quote:
Originally Posted by target
Hi, Tuan-

Glad you're getting some flying in!

I wonder why not just introduce a little diff in the ailerons so that the rudder doesn't have such a job to do to keep the nose straight??
I'm confused, but it could be my newness to TD flying.

Thanks, Target
Hey Target, from all my reading on here, using rudder instead of diff is a little more efficient.

Of course, some planes just don't do well with zero diff.

I have found that when I am flying thermal circle, I am not looking directly at plane but more like looking at the circular path the plane is flying. This visualiztion of the flight path makes it easier for me to see if fuse lines up with that path and know how much rudder to add to keep fuse lined up.

In other words, my focus is not on the plane itself but more open to the space the plane is flying in.

Anyone else fly like this? It has helped me.

Lastly, one thing I noticed is that I also need to work on my vision. Especially my depth perception.


tuan
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Old Jan 26, 2009, 12:18 AM
RIP MC
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United States, CA, Midway City
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I just actually measured hook and cg here are my numbers.

CG 106mm 1 inch out from side of fuse. From root + another .5mm?

I took skeg off and measure from tip of nose to LE at center and it was 445mm. Turn plane upside down, measured to 445m and mark. From there to back edge of tow hook is 108mm, and like previous post, even with hook there, it is rock solid on tow.

So, does that mean CG should be around 108? But thats with 10-20mm camber and still stable.

I was off a little from my previous post . Hope this helps.
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Old Jan 26, 2009, 09:15 AM
agony sweetns the victory
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Quote:
Originally Posted by fnnwizard
from all my reading on here, using rudder instead of diff is a little more efficient...

tuan
tuan, I just don't see how that can possibly be the case.

Given that a ship is designed to have it's lowest CD is when no surfaces are deflected, not using differential and increasing the amount of rudder deflection can only increase the CD of the ship. And if you're not ultra smooth on your rudder inputs, all the more so. Whereas less rudder and less TE deflection (via differential) must have less CD, hence more efficiency. Perhaps no differential and more rudder input works for your flying style, but I don't see how it can possibly be more aerodynamically efficient.

Todd
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Old Jan 26, 2009, 09:49 AM
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Quote:
Originally Posted by atjurhs
tuan, I just don't see how that can possibly be the case.

Given that a ship is designed to have it's lowest CD is when no surfaces are deflected, not using differential and increasing the amount of rudder deflection can only increase the CD of the ship. And if you're not ultra smooth on your rudder inputs, all the more so. Whereas less rudder and less TE deflection (via differential) must have less CD, hence more efficiency. Perhaps no differential and more rudder input works for your flying style, but I don't see how it can possibly be more aerodynamically efficient.

Todd
Hi Todd,

When I first started out, I used to use the standard 2:1 or even 3:1 diff as all my local experts told me. I also see lots of people holding trans in just one hand and operating right stick only as they had A/R mixed in. I thought, man these guys are good.

Then I found markdrela and read everyone of his posts. I can't find the exact post now, but I remember he wrote something like using diff is using drag to counter the yaw and using rudder is using lift so using rudder, drag is smaller than using diff. Something like that.

I will try to look for the post later.

Anyhow, I still use A/R mix but mixed at rate that makes rudder move lots in beggining of ail movement, but it's on a switch and it gets turned on when I am at limits of my vision without height and for when I let others try my plane. Also, in normal flying I will start the turn will rudder just before adding ail as needed to keep bank angle. So far the X needs almost zero ail once in a thermal bank or even opposite, I just thought about this now, when in opposite your diff is actually working against you.

Tuan.
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Old Jan 26, 2009, 09:55 AM
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Todd - Check out some of Dr. Mark Drela's posts. He states that no aeirleron differential is best for sailplanes. I fly my Shadow with no differential, and I like the results.

George
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Old Jan 26, 2009, 10:27 AM
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United States, CA, Midway City
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Ah, I found it!
Quote:
Originally Posted by markdrela
Aileron differential uses drag to create a yaw moment, while the rudder uses lift. By definition, a drag force sucks energy from the glider, while a lift force does not.

The lift force on the rudder does have some associated profile and induced drag. But for a given yaw moment, this additional drag on a typical rudder is much smaller than what's required on the upward-deflected aileron. Increasing the aspect ratio of the vertical tail makes it even better in this regard.
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Old Jan 26, 2009, 10:48 AM
agony sweetns the victory
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Quote:
Originally Posted by fnnwizard
Then I found markdrela and read everyone of his posts. I can't find the exact post now, but I remember he wrote something like using diff is using drag to counter the yaw and using rudder is using lift so using rudder, drag is smaller than using diff. Something like that.
Tuan.
Tuan,

If I understand the aerodynamics of aileron inputs correctly, the aileron going down is the one that has more induced drag and the aileron going up (although it travels more) sees less airflow and therefor has less drag in comparison to the down aileron. Also, I believe the aileron that goes up should be sufficient in and of itself to roll the plane and counter adverse yaw, HOWEVER the roll rate would be really bad. So we include a bit of down aileron (even with it's higher drag component) on the opposite wing and we gain back the roll rate. So the idea of aileron differential is to maintain as much roll capability as we need with as little down aileron input as possible.

I do agree with your comments that in a coordinated turn the rudder does act as a lifting surface, but if I have to move it twice as much to keep the plane in a coordinated turn that's got to be twice as much induced drag, no other way around it.

I haven't read all of Mark's writings (although I've read many of them, but not the one you've posted), but I do see in his Supra setup, he doesn't use aileron differential??? I don't know how specific that is to Mark's Supra and if all Supra pilots fly theirs that way or not. I do know that in near dead air no lift conditions when I stuck my stick in the corner and held it there, and held it there, and held it there, my Xplorer didn't fall out of the sky (tip stall) it just slowly decreased in altitude. I also know that going into a normal turn the fuse tracked very nicely with the turn radius (no adverse yaw). These two "tests" said to me (in my young experience) this plane is setup right (or very close to) relative to aileron differential and aileron/rudder coupling. Of course there's more to learn and tinker with...

Todd
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Old Jan 26, 2009, 11:06 AM
4 wheels move a body; 2 a soul
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You know, Ive experimented with differential setting and I swear I could never see much if any adverse yaw in a model sailplane, even when trying to deliberately induce it. Frankly I just put in differential cause many say thats the way to go! Have to start paying closer attention I guess during my "testing."
Walter
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Old Jan 26, 2009, 11:22 AM
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United States, CA, Midway City
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Most of the planes coming out now mirror the Supra very closely.

I am no expert either, but I tried to explain this to a friend of mine.

The it's something like this.

Let's just say if normal, most efficient flying you need equal 5mm up/down ail with 15-20mm rudder. I don't know, it could be any number.

Now lets say you want to use diff, then say 5mm up 2.5 down with 10-15mm rudder? This will not produce same roll rate as previous, so need to move up ail to say 6-10mm ( again it could be any number) then down becomes 3-5mm, don't really know where rudder throw would be.

But from Mark's comments, I assume the 15-20 mm rudder throw + whatever ail needs to be causes less drag than the smaller deflection of rudder + extra up movement needed for same control authority.

I have tried it a few times and at height it is hard to tell, but when lift is really really light and I am nervously circling low this is where I have seen a difference.

I plan on attaching a camera to this X to see how flying surfaces act and also attach a yaw string just so I can be sure I am flying with correct rud/ail inputs. I will post that when I get it done for all to see and benefit.
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Old Jan 26, 2009, 11:29 AM
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Quote:
Originally Posted by atjurhs
I do know that in near dead air no lift conditions when I stuck my stick in the corner and held it there, and held it there, and held it there, my Xplorer didn't fall out of the sky (tip stall) it just slowly decreased in altitude.

Todd
Also add that using Mark's theory, if you do this with no diff, more rudder, the "decresase in altitude" should happen over a longer peiod of time, so in essence x% better. The important part is; is this x% better worth the extra effort of using left thumb more and having more pilot workload?
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Old Jan 26, 2009, 12:58 PM
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FIRST, in this discussion it's not my intention to be insulting in any way. Please don't read my posts with any sort of malice or insulting at all. Just a polite discussion of some aero-theory applicable to our mutually enjoyed hobby.

Quote:
Originally Posted by fnnwizard
Most of the planes coming out now mirror the Supra very closely.
Sorry Tuan, but I don't I agree with that statement at all. They are definately different airfoils, and I think the planform is rather different too. I'm pretty sure they fly differntly too, but given that I don't have a Supra, it's just my opinion.

Quote:
Originally Posted by fnnwizard
Let's just say if normal, most efficient flying you need equal 5mm up/down ail with 15-20mm rudder. I don't know, it could be any number.
Sorry, but I thought from an aero-drag perspective wev'e already established it can't be the most efficient configuration?

Quote:
Originally Posted by fnnwizard
Now lets say you want to use diff, then say 5mm up 2.5 down with 10-15mm rudder? This will not produce same roll rate as previous, so need to move up ail to say 6-10mm ( again it could be any number) then down becomes 3-5mm,
Tuan you're stating that you would need to have more throw (and thus more drag) using differential to effect the same radius turn. I don't think you really know that. That's just how you feel, your opinion, which is perfectly valid, just not a fact. I would say that all along all you needed to execute that specific radius turn was 5mm up 2.5 down with 10-15mm rudder using differential, and without differential you would need to add to that (and therefore add drag) the 5mm up/down ail with 15-20mm rudder. I would say that by going to the 6-10mm (again it could be any number) then down becomes 3-5mm you would get a tighter radius turn. So I would say that you are comparing two turns of different radi - apples and oranges. But this is my opinion, what I believe. It is I believe the accepted convention, so probably the opinion of hundreds if not thousands of other pilots think this way. I think I would need to see facts/data to change my mind.

Quote:
Originally Posted by fnnwizard
don't really know where rudder throw would be.
That one I agree with and I think we might also agree that rudder would need less throw with the differential?


Quote:
Originally Posted by fnnwizard
I am no expert either, but I tried to explain this to a friend of mine.
I'm not an expert either, so I stick with convention. So far it seems to have yielded a really nice flying plane!
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Old Jan 26, 2009, 01:16 PM
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Originally Posted by atjurhs
FIRST, in this discussion it's not my intention to be insulting in any way. Please don't read my posts with any sort of malice or insulting at all. Just a polite discussion of some aero-theory applicable to our mutually enjoyed hobby.
Oh, no worries Todd, none of what you posts taken as insult. I learn too from just about every post.

I think you read me incorrectly. I was trying to explain using the assumption that if Dr. Drela is right about using more rudder throw vs more up ail in a turn (keeping same speed, bank etc). The 5mm u/p 20mm rudder was just a baseline for comparing.

I will have to measure my avg throws to find out what I am using right now without any diff.
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Old Jan 26, 2009, 01:26 PM
jrerickson
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Quote:
Originally Posted by fnnwizard
Oh, no worries Todd, none of what you posts taken as insult. I learn too from just about every post.
Tuan,

Taking a video of your yaw string is an interesting idea. I've got news for you, however. To keep that string straight you can't just do it with your right thumb. Unless you can program your radio to take speed and diameter of turn into account you are not going to have a fully coordinated turn.

I fly uncoupled with very little differential. I use my left thumb a lot. Sequence for a turn is right thumb to establish bank, add rudder, reduce rudder and bank out.

Flying at really high altitude or when you are just flying casually you can throw on the rudder/aileron mix, but I really think it makes you a better pilot to work on the coordination of thumbs. In hand launch this becomes really apparent, especially when turning close to the ground.

Excessive yaw, either in or out, is as bad as excessive bank. Both are lift killers.

John
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