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aeajr
477 Posts |
 Posted - 10/03/2006 : 11:08:22 PM
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Maximizing Launch Height Using a Hi-Start
by Ed Anderson
aeajr on the forums
While the winch is the preferred method to get the most height on a launch,
you can get great launches off a hi-start if you know what to do. The
advantage the winch has is that you can control the pull throughout the
launch. With a hi-start, the pull is maximum on initial release then
decreases throughout the launch. But that does not mean that you can not
use some of the same techniques the winch launchers use to gain extra height.
ELEVATOR AND CG
The first thing we want to do is be sure your plane is set-up correctly and the
CG is one of the key set-up areas. If your plane is nose heavy then it is not
going to launch as well as it could. It will also not thermal as well as it could,
so working on the CG will help your launches AND your thermaling.
I am not suggesting you move the CG to improve your launches. What I am saying is that you move your CG back to make your plane fly and thermal better. As a result of that work, it will launch better.
The further back the CG, the easier it is for nose to come up during the
launch. If your plane tends to launch fairly flat and you find you need up
elevator to get it to rotate, try moving the CG back slightly. Then fly the
plane. It will be more responsive and will read lift better. You may find
that you have to add some down trim after moving the CG back. This is
because you have been using up trim to hold up that heavy nose. This
process will reduce drag and make for a better flying plane.
I usually start my gliders at the most forward CG recommended by the MFG,
then over a series of flights, perhaps weeks, I move it back till it gets too twitchy to
handle, then I move it forward a little till it gets stable enough for me to
fly it.
If you have a computer radio, try adding some expo. This will soften the
response around the center of the controls making it easier for you to
manage smooth flight with a plane that has a more rearward CG.
HOOK AND THROW
Hook placement will make a big difference in your launch height. The closer
the hook is to the CG, the steeper the rotation of the plane during launch.
The optimum situation is that your plane climbs at about a 70-80 degree
angle presenting the most resistance to the contraction of the rubber. This
takes the plane through the largest arc and produces the longest time of
launch, extracting more energy from the pull and translating it to height.
Naturally this also places the most stress on the wings, so there is a balance that must be maintained. However few planes will be damaged by the pull of a hi-start, but just the same, watch those wings. If you are concerned that your wings may not be strong enough, start with a less agressive hook position, then work it back over time, watching for wing flex on each launch.
Just as we did with the CG, as a starting point, move the hook to where it is about 1/4" in front of
the CG. Now, over several launches, or over several weeks, move that hook
back. Note that as you move it back the plane will tend to rotate faster and will
be less stable on the launch so do this gradually so that you remain comfortable
with controlling the plane.
If there is a breeze and your launch angle is steep enough, the rubber may
never fully contract during the launch giving you the greatest height
possible. You will use that remaining tension in the line when I cover the
zoom, later in the discussion.
When you launch initially, if you have enough pull on the line, throw the
plane up at about a 30 to 45 degree angle rather than out flat. This gets
the plane through the rotation faster getting it into this max lift position
sooner. The throw really makes a difference. It gets you up to flying
speed faster which is important the closer you have the hook to the CG.
Typical hook position is about 1/4" in front of the CG, but having it closer
will usually give you higher launches.
Note that, as you move the CG back, as advised above, you are moving the CG
further and further away from the hook, so you may need to adjust hook
position just to maintain your current hook/CG position. And, as stated above, the
further back the hook placement, the closer to the CG, the harder the plane
is to control on launch. It will stall easier on initial release, so it is
vitally important to give it a good throw and not just let go.
When moving your hook back, you may wish to have someone help you by either
throwing the plane for you, or working the radio. This way the hands are on
the sticks at all times. If the throw goes bad or if you have moved it back
too far, you will be better able to respond. If the plane stalls on the
launch, a little down elevator for a moment will help it gain speed and you
may be able to save the launch. Once you have it where you like, you should
be able to launch it by yourself.
ZOOM
It is possible to "zoom" off a hi-start, in a similar fashion to what is
done with a winch. The bounce is not as dramatic, but still, you can gain
additional height if you get the timing right.
When the model is about 70% through its arc, and while there is still
tension on the line, rather than just flying off the line, you nose down for
less then a second, then pull up hard. The weight of the hi-start, and any
remaining tension, will help to accelerate the model to a higher speed. The
down/up pull then translates this speed into altitude. I do this with all
my planes. The Spirit, the most fragile of my gliders, picks up at least 50
feet this way. with a moderate breeze and a fast plane, you can do even
better.
If you have an aileron plane or aileron/flaps, and a computer radio
that will allow you to reflex the wing, this can help you gain even more
height. That's next!
CHANGING THE WING
If you have flaps try adding a little on launch. If you have ailerons and a
computer radio, then drooping the ailerons also will add even more lift upon
launch.
By creating more under camber, more bottom curve, you create an airfoil that
produces more lift. It also produces more drag, but during the launch, when
you have the pull of the hi-start, you can afford this drag to gain the
lift. This will create a longer arc and a higher launch on many planes. However
some people find that their plane launches better without flaps. You have
to experiment.
Launch - Flaps or flapperons down for initial launch: Usually 10-20 degrees
is enough, but every plane is different, you have to experiment. I have seen
launch positions as high at 45 degrees on the flaps. If you have the
ability, droop the ailerons too.
Throw the plane hard with the nose up at around 45 degrees or higher.
Mid arc - If you have a computer radio and can set a reflex position, about
70% up the line you switch to reflex. This is where the ailerons/flaps are
actually moved up slightly from their normal position. On most airfoils,
the RG15 foil being a know exception, this creates a high speed/low
drag/lower lift setting that is great for gaining speed. Now you want to
accelerate the glider to gain speed.
Zoom - Around 70% though the arc you do the down/up while at maximum speed,
still in reflex. The down lasts less than a second and the up is strong.
Go for a climb angle of 45 to 80 degrees, depending on the plane and the pilot.
You have to experiment to see what works best for you.
Level - As it climbs it will lose speed. Before the plane loses all its
speed, you level the plane, go to normal wing position and start your hunt.
If you are stalling at the top of the launch, resulting in a drop in the
nose and loss of height, then you are waiting too long to level the plane.
It should look like the plane just leveled onto a table with no drop of the
nose. This is your best launch height.
Using a winch, this sequence can gain 100-200+ feet, if you
have a strong plane. It can be quite dramatic.
Using a full size hi-start with enough pull, you can do about 25-50% of
that. The limiting factor is that you can't increase the power at the end
of the launch like you can on a winch. However you are also unlikely to
overstress the wings off a hi-start. I have seen planes fail on zoom, using
a winch, when the pilot was too aggressive on the zoom. I
have never seen a plane fail when zooming off a hi-start, but I am sure it can
be done.
GIVE IT A TRY!
If you go through all the steps above, you may find that you are getting
higher launches without the zoom. This is great!
Then you can try adding the zoom, but tune the CG and hook positions first
as these are more important. But you can try a zoom on any plane, the
benefits will just not be as great as when the plane is tuned properly.
If your plane drops at 100 feet per minute during its glide, and you can
gain 50 feet on the launch, you just gained 30 seconds of flight time. And
thermals tend to be larger in radius the higher you go, so you have a larger
target. If you can gain 100 feet even better.
The more sophisticated the plane and the more flexible the radio, the more
you can tweak the launch. The longer and stronger the hi-start, the higher your launches.
Even with a simple R/E plane, you can improve your launch height by going
through these steps. You will get higher launches, longer glides and likely
find more thermals than you did before.
Other resources can be found in the handbook. I hope this has been
helpful.
The New Glider Pilot's Handbook
http://www.flyesl.org/forums/topic.asp?TOPIC_ID=251
The angle method for placing the tow hook
http://www.rcgroups.com/forums/showpost.php?p=7107908&postcount=15 |
Best regards,
Ed Anderson
Long Island Silent Flyers
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Edited by - aeajr on 11/02/2009 09:15:20 AM |
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aeajr
477 Posts |
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aeajr
477 Posts |
Posted - 04/22/2008 : 4:32:30 PM
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PLAYING WITH YOUR LAUNCH SETTINGS
This past weekend we were flying in about 12 MPH winds with gusts to 20. Lift was there and we were having fun. However the winch battery gave out.
No more fun? Not a chance!
I always carry my hi-start in my car just in case we have a winch problem. So out comes the hi-start. This was going to be the first time I had launched my Supra off a hi-start. This is a NE Sailplanes XL hi-start so it is very strong, giving me about 24 pounds of pull on the launch, or about 6X my 4 pound plane.
With a 12+ mph breeze I expected a good launch.
I hit my launch setting and just as I threw the plane the wind speed picked up. The Supra went up at about an 80 degree angle but the wind was pushing back against the hi-start. The plane was now slightly behind me as it climbed and, as luck would have it, directly in the sun. It was an interesting launch, to say the least, and I got pretty good height. But I can do better!
Next time I launched I left the launch mix off. I did this twice. The climb was not quite as steep but it was faster. I actually got a better launch and a little zoom at the end. But I can do better.
For the next series I launched without the launch mix, throwing up at about 45 degrees, then once the plane was about 50 feet up, moving quickly, I flipped on the launch mix. With the greater speed and the later application of the flaps my launch was higher still. Before I went for the zoom I switched to reflex, gained speed and up it went! This is the same zoom techniques I use on the winch. It did not match the super zoom of the winch, but this was my highest launch.
After a couple more launches like this I was getting pretty high launches off this 100 feet of rubber and 400 feet of line. But I can do better!
So I added another 100 feet of line to the hi-start. I continued to add the launch mix at about 50 feet. The wind carried the extra line with no problem and I got the highest launch yet. I also tried leaving the launch mix off and using my thermal camber mix instead. That was an improvement over no launch mix but I am not sure if it was as high as the late application of my flap/aileron launch mix.
By time I had completed about 12 launches with various settings and line configurations I think I was getting about 80% of my winch launches. Basically I lacked some the powerful zoom that I can get out of Supra when using the winch. But I was very satisfied with the experience and learned a lot about the effect of when and how to use the launch mix for best effect.
The net net of this story? Experiment! Play! Try new things and see what works for you. And if you find a good combination, please post it here so others can give it a try.
Clear skies and safe flying!
Best regards,
Ed Anderson
ESL Content Editor
aeajr on the forums |
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Maximizing Launch Height Using a Hi-Start
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