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Apr 25, 2010, 08:45 PM
G_T
G_T
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loop test for dialing in elevator->camber mix


Hi Folks,

I typed this in an attempt to answer a question on another thread, but thought it might be of general interest. This is a method I use at times to get in the ballpark for elevator throws (in the up direction, not the down) and for elevator->camber mixing ratios. I've been using it for a couple of years or so.

-------------------

There is a test I use but I'm not going to try to defend it. It gets me pretty close. I call it the loop test.

Before doing the test, have the plane already trimmed out balanced out and flying well. This test is for fine tuning the total elevator throw and the elevator to flap coupling.

Put the plane in cruise mode and build up a little speed. Not super fast. Basically a short dive is fine.

Now pull full up elevator. If the plane does big loops then add a little more max up elevator. If the plane tries to loop tight but mushes out quickly, then there is too much elevator available. The wing can't support the AOA the elevator is trying to force. It is stalling. Reduce the elevator slightly.

Now lets suppose the max elevator travel is reasonable so a tight loop can be done without the plane mushing out immediately. It might not be efficient and may only make a few loops before dying, but that is fine.

Now, generally one doesn't use full up elevator except in emergencies. What you have done is set the emergency elevator setting. More than that doesn't accomplish anything so more than that is prevented. But there is as much as we can get away with.

In normal flying even in tight turns and aerobatic playing, we are using less than 100% throws. We try to stay in the more efficient range to conserve energy. So write down your up elevator max position and trim position, and then back off the max up elevator to perhaps 90% of what it was. Or perhaps even 80%. Just pick your poison.

Now the max up elevator should be in the efficient range, more or less. Now we want the wing to match up with this, to be as efficient as it can be.

So pick some starting number, such as a 3% mix. Now the flaps don't move very far with the reduced full up elevator. Put it in cruise and dive it to build up a bit of speed. Now from this point on, for each dive in the test, duplicate how long you dive. Consistency will matter a little for the test.

Suddenly pull full up elevator. Count the number of loops that you get before the plane fails.

By the way, if the loops tilt over towards a side as they continue, then there is some cross aileron and rudder trim that should be removed, or perhaps there is a wing/fuselage alignment issue, or perhaps there is a wing warp. Fix those things.

Now make a slight increase, to perhaps 4% mix. Count the number of loops. Now 5%. Get the picture? Find what percentage mix gives the best energy retention -> the most number of loops. For a modern unlimited for instance, the number of loops can be unlimited. For a DLG, it should be greater than 6 but it may not be for all planes. It probably won't be greater than 9.

Now set the elevator max back to the previously determined value. Do the loop test one more time, but this time hold the stick at about the 80 to 90% full up elevator position, to duplicate the previous test position. Count the loops. If it does fewer or slightly larger, then add just 1% to the elevator->flaperon mix.

That's it.

By the way, the mix can be different for different default camber settings. For instance, one might want less up flaperon for down elevator in speed mode, and less down flaperon for up elevator in float/thermal mode. And in speed mode one would likely be aiming for bigger faster loops.

That's one non-modeling based field setting approach. I hope it is of some use. I won't call it perfect and technically it is not correct but it certainly gets one in the ballpark. It is quick and easy to do.

Gerald

PS - forgot to mention - you want to do the tests in neutral air away from trees and not at super low altitude when the wind is blowing. Otherwise air velocity gradients will alter the loop behavior. Essentially a minor bit of dynamic soaring, either plus or minus.
Last edited by G_T; Apr 26, 2010 at 01:59 AM.
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Apr 28, 2010, 11:40 PM
Registered User
Cool! Thanks for explaining it - I was wondering how much E->F mix to add - I'm surprised that you get more loops with this mix - I thought that it essentially slows the plane down as the main benefit, but it seems that increased energy retention is also in the books. One thing is that it seems that the initial speed is a very important variable and a 20% variation may result in 1-2 more loops.

BTW, that minor dynamic soaring - can it be exploited? I always wondered, but I'm too chicken to try with a real DLG .

M.
Apr 29, 2010, 01:01 AM
Your user title is suggestive,
Xptical's Avatar
I think I understood about 60% of what you said. Still, I'm bookmarking this for when I get back and start tuning my Blaster and whatever my new DLG turns out to be.

I, too, wonder about the dynamic soaring. I understand it involved loops through differential air masses. Or something like that.
Apr 29, 2010, 02:13 AM
Team WC2013 F3K
oakman7004's Avatar
Good writing Gerald,

I'm sure many will have good use of a "way of setting" like this.

Thinking how I set all this is in some sort of simular behavior, but I do not loop...I just pull elevator after picking up speed and can with experience se the behavior.

One BIG thing differs though, might be fine tuning for most! I have a non linear mix on the flap vs elevator. This is probably coming from a feeling in turn behavior I'm looking for when just pulling let say 20-30% elevator.

/Jonas
Apr 29, 2010, 08:21 AM
wishes this caption was longe
will_newton's Avatar
Quote:
Originally Posted by G_T
By the way, if the loops tilt over towards a side as they continue, then there is some cross aileron and rudder trim that should be removed, or perhaps there is a wing/fuselage alignment issue, or perhaps there is a wing warp. Fix those things.
or the wind blowing the plane around.
Apr 29, 2010, 10:19 PM
It's a mere flesh wound!
Buddy Roos's Avatar
Quote:
Originally Posted by will_newton
or the wind blowing the plane around.
Or one wing weighing more than the other!!

I discovered this when trimming a Pattern plane. The loops kept screwing off to one side but tracked perfectly after adding a penny to the light tip!

Buddy
Apr 30, 2010, 01:04 AM
I do this for fun!
Nice!!

thanks.

Ken.
May 03, 2010, 03:09 PM
G_T
G_T
Registered User
Thread OP
As a comparison to what you should aim for, I did one quick test at IHLGF with my Edge prototype, and from a [email protected]#[email protected] low launch I got 12 loops before aborting due to a tree being too close. Otherwise I'd have gotten one more before being too low to risk it, and it wasn't done yet.

I have seen as few as 5 loops from a poorly tuned setup.

Jonas,

Yes, a non-linear mix may make sense. But I was trying to keep things simple and reproducible here! Some highly experienced pilots can feel their way through the setup quite well without doing any specific test such as the one outlined here. I was just trying to provide something on the order of a "dive test" for the elevator->camber mix.

Mihai,

"BTW, that minor dynamic soaring - can it be exploited?" - We do minor forms of dynamic soaring with DLGs all the time. Many probably don't realize it. For instance, suppose one is 10' off the ground (~3M), trying to stretch a float for time, and the wind is blowing. Your float time will be greater if headed into the wind than away. The reason is the velocity gradient in the wind at low altitudes. Essentially the upper surface of the wing sees a slightly higher airspeed than the lower surface. So it generates slightly more lift.

So a gust hits when we are floating into the wind in this scenario. What do we do? We let the plane rise some, and when the airspeed slows down we pop it into float camber as necessary until we restore the airspeed by losing altitude. But in the process we have extracted energy from the gust. This shows up as additional time on the flight. In contrast, if one prevents the plane from ballooning some in the gust then one essentially just gets the additional drag without the altitude improvement. The additional drag is going to reduce airspeed and in turn groundspeed. Then when the gust dies down, the low airspeed will still require some altitude loss to get back to a reasonably efficient floating speed. One ends up lower.

Many pilots are quite good at doing this bit of low altitude dynamic soaring, even if they don't think of it as dynamic soaring. It is harder to do at altitude and distance as one can't study the plane as intently.

If one can use pulsations in the motion of an airmass or the relative motion of airmasses to achieve a boost in airspeed or in altitude (either increase the total energy of the airplane - altitude giving potential energy and airspeed giving kinetic energy), then that is dynamic soaring.

G_T - not a DS expert.
Dec 15, 2013, 12:30 PM
Detail Freak
target's Avatar
Quote:
Originally Posted by G_T
Jonas,

Yes, a non-linear mix may make sense. But I was trying to keep things simple and reproducible here! Some highly experienced pilots can feel their way through the setup quite well without doing any specific test such as the one outlined here. I was just trying to provide something on the order of a "dive test" for the elevator->camber mix.
G_T/Jonas-

I am assuming that you would both be referring to a mix where the snapflap comes in and stops adding camber before all the elevator travel is added....
Is that correct, or is there something else that either of you are suggesting?

Thanks,
Target
Dec 16, 2013, 12:14 PM
G_T
G_T
Registered User
Thread OP
What I'm suggesting for most pilots is a linear mix, not really a "snap flap". It gets often called a snap flap but it isn't. A sufficiently good pilot can possibly do a little better than a linear mix but the difference in flight isn't going to be large. But not having the mix in any form, or having it badly set up, will result in a DLG feeling sluggish and/or bleeding energy in turns.

Some foil families and wing designs can use a lot of camber efficiently. Some cannot. Do not expect that the mix which works well for plane A will also work well for plane B.

Gerald
Dec 16, 2013, 12:41 PM
Detail Freak
target's Avatar
So when you speak of using a non-linear mix, is it camber all in before elevator all in, or the other way around?
I am assuming it's the former. (?)

Or, maybe no dead zone, but more camber at the beginning of the stick pull?

R,
Target
Dec 16, 2013, 01:15 PM
G_T
G_T
Registered User
Thread OP
As you pull up elevator, you increase the angle of attack of the wing. For each angle of attack, what is the optimal camber setting?

Gerald
Dec 16, 2013, 05:36 PM
Detail Freak
target's Avatar
I'm not sure....
I suppose one could spend time doing the loop test that you have outlined with various known amounts of up elevator, but I'm pretty certain that would be quite time consuming, and that maybe you'd already done just that.
I was hoping that you'd reached a general conclusion that works on most of the current wing profiles of DLG's?

R,
Target
Dec 16, 2013, 06:34 PM
G_T
G_T
Registered User
Thread OP
There will not be a general conclusion. Camber behavior with the various wings out there is too different.

With a bit of practice, it doesn't take all that long to do the loop test. Not really all that much longer than doing the dive test, cg change, retrimming work to get into a good CG range. It might even take less time than working out a good starting CG.

Gerald
Dec 16, 2013, 06:45 PM
Detail Freak
target's Avatar
OK, thanks.

R,
Target


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