FIXING POP-OFF PROBLEMS
10-25-2007 | 06:48 AM
#1
FIXING POP-OFF PROBLEMS
UNDERSTANDING AND FIXING POP-OFF PROBLEMS
by Ed Anderson
aeajr on the forums
If you launch your glider with a hi-start or a winch and are having pop-off
problems, consider this a cry for help from your plane. It has a problem that
needs to be fixed.
What is a pop-off?
Pop-offs occur when your plane over rotates on launch and comes off the
hi-start or winch tow line too early. This often happens shortly after
launch when the plane has not achieved a lot of height. So a plane that has
popped off can be very hard to handle.
Since the plane is going through a rotation and essentially starting a loop
to the rear, I have found the best way to deal with a plane that has popped
off is to help it finish this backward loop. I will add up-elevator,
usually a lot, to help the nose over the top. Sometimes it will continue in
a nice smooth loop, be it 6 feet over my head, or it will flip over at the
top, in which case I get off the elevator and try to get the plane back to flying
speed, usually with slight down elevator, then work back to level flight,
sometimes only a few feet off the ground. Then the priority
is to set up for a safe, clean landing .... somewhere away from people.
Hook location relative to CG -
A hook location that is in front of the CG will not cause pop-offs, but
understanding hook location is the first step to fixing pop-offs. For hi-start
or winch launching, your hook should be in front of the CG by up to a 1/2". You
will want to move this back, closer to the CG point as you gain confidence with
the plane and as you work toward more aggressive, higher launches. The closer
to the CG, the steeper the angle of climb and the higher the launch. Some
planes have adjustable hooks to make it easy to adjust this. My Spirit has 3
hook positions. I consider them Beginner, Sport and Competition, with the last
one being right on the CG. My Thermal Dancer has an adjustable hook and my
Legend has 2 spots. Note that as you move the hook back, the more unstable the
plane becomes on launch and the more important it is to give it a good hard
push/throw rather than just letting it go. Typically you will want to move the
hook back over time, as you gain confidence with the plane in order to get the
highest launch.
Note also that if you add or remove weight from the nose of the plane, as you
adjust the CG, you are also changing the hook position in relation to the CG.
Most glider pilots move the CG back over time as this usually leads to better
handling and better thermaling. However as you move the CG back, the hook
position is moving forward in relation to the CG. So, consider moving the hook
to keep your launches high. Adjustable tow hooks make this easy.
Hook angle -
Your tow hook should be parallel with the bottom of the plane or bent slightly
up toward the plane. There should be plenty of clearance so that the ring can
come out when you want it to. Sometimes, a light weight hook can become bent
and angled away from the plane. This will contribute to pop-offs. Push it back
into shape. If this keeps happening, replace it with a stronger hook. Also,
some people put a small notch in the hook right behind where the ring rides.
This can help reduce pop-offs too.
H-Stab or elevator setting - The dive test
This is a test for trim and balance, called the dive test. This is best done on
a calmer day to eliminate influence from the wind.
Take your plane up high and get into a smooth glide into the wind. Now put the
plane into a dive of about 30-45 degrees and let go of the sticks. Does it tend
to stay in the dive, coming out slowly over several seconds? Does it hold the
dive, requiring you to pull out. If it does, that is good. However if it tends
to pull out of the dive quickly, on its own, that is bad.
A properly balanced and trimmed plane should hold a dive of 30-45 degrees and
come out of it very slowly. If it tends to pull out quickly, you have a
problem. Your plane may be too nose heavy which will require you to trim in up
elevator to hold the nose up. Or, your h-stab is up in the back. Your elevator
may be trimmed neutral but the up angle of the stab is giving you the effect of
up elevator. Either of these conditions will cause the plane to want to climb.
In a normal glide you may not notice it but at the higher speeds of a dive, or
on launch, the plane will want to go into a loop which, on launch, causes a
pop-off. This also causes more drag which hurts your planes performance.
When I first got my Spirit I was having terrible pop-off problems. I thought is
was my launch technique. I was new, what did I know. However one of the club
experts performed the dive test described above. After we removed some nose
weight, things got better but still the plane would pull out of a dive too
quickly and it still tended to pop-off too easily. Temporarily I put in some
down trim on launch to control this so I could fly that day, but we knew I had a
problem.
Over time we determined that my h-stab was not parallel to the wing. I had to
do some wood working to correct this, but once I got the h-stab right, the
pop-off problem went away, the plane had a much better glide and thermaling
became a lot more fun.
NOTE: If you have a fully flying stab, that is the entire stab moves on a
pivot, then you do not need to go through this shim process. You automatically
fix any wing to stab alignment problems when you adjust your trims. This is an
advantage of a fully flying stab.
If the stab is glued on, one way to test for this condition is to shim the wing
if you can. This works with top mount wings, but not with side mount wings. Do
this after you feel you have made all of your balance adjustments or you will
mask a nose heavy plane.
When you mount your wing, put a shim under the back of the wing. Start with
something like 1/32 and keep adding till your dive test looks good. Chances
are your pop-offs will stop too. Rather than aligning the stab with the wing,
you are aligning the wing with the stab. This is not the best way to fix the
problem but it does work and it is quick and easy. If correcting the stab is
not an option, then adjust the wing to match it. Note that this will cause a
gap under the wing. If you are going to use this as a permanent fix, then fill
that gap with foam tape or some other filler or this will lead to drag, which is
something to be avoided.
If you are putting in down elevator trim on launches to prevent pop-offs,
chances are you have a trim problem. A properly balanced and trimmed plane,
where the tow hook is in front of the CG, does not tend to pop-off on launch.
The Throw -
Until your plane reaches flying speed, it is in a stall state. In stall, there
is little or no control on the surfaces and not enough lift from the wing. If
you just let go, then your plane is a rock till the hi-start gets it flying.
One of the side effects of this will be a tendency of the plane to drop left or
right causing a wild launch or a crash. It can also cause over rotation,
leading to a pop-off.
By throwing the plane, you get it up to speed quickly giving you more control
and a smoother safer launch. Rotation will be smoother and there will be less
tendency toward pop-offs. At first you can throw out flat or at about a 20
degree up angle. Over time you can bring that angle up to 45 degrees or even
higher. The stronger the throw, the steeper you can launch resulting in higher
launches. Competition pilots, using winches or strong hi-starts, will throw the
plane with great force at angles over 70 degrees to get maximum height launches.
Summary
Pop-offs are a cry for help from your plane. It has a problem and it needs your
help. It may have multiple problems. It may be nose heavy AND have a bent tow
hook AND have an h-stab that is not properly set AND a pilot who is not giving
it a good throw. Don't ignore it, don't fix it with down trim and don't add
weight to the nose to prevent it. If you understand your plane, you will see
that the problems are not hard to fix. Not only will addressing these issues
fix your pop-off problems but you will get higher launches and have a better
flying plane that handles better and thermals better and more easily.
Clear Skies and Safe Flying!
RESOURCES
Learning to use a hi-start
http://forums.flyesl.com/topic.asp?TOPIC_ID=216
Learning to use a winch
http://www.flyesl.com/News-y-Article...7&TOPIC_ID=112
The New Glider Pilot's Handbook
http://forums.flyesl.com/topic.asp?TOPIC_ID=251
Gordy's Balancing System
http://forums.flyesl.com/topic.asp?TOPIC_ID=255
Performance Tuning Gliders - Video review
http://forums.flyesl.com/topic.asp?TOPIC_ID=227
Test Flight Procedures
http://forums.flyesl.com/topic.asp?TOPIC_ID=226
by Ed Anderson
aeajr on the forums
If you launch your glider with a hi-start or a winch and are having pop-off
problems, consider this a cry for help from your plane. It has a problem that
needs to be fixed.
What is a pop-off?
Pop-offs occur when your plane over rotates on launch and comes off the
hi-start or winch tow line too early. This often happens shortly after
launch when the plane has not achieved a lot of height. So a plane that has
popped off can be very hard to handle.
Since the plane is going through a rotation and essentially starting a loop
to the rear, I have found the best way to deal with a plane that has popped
off is to help it finish this backward loop. I will add up-elevator,
usually a lot, to help the nose over the top. Sometimes it will continue in
a nice smooth loop, be it 6 feet over my head, or it will flip over at the
top, in which case I get off the elevator and try to get the plane back to flying
speed, usually with slight down elevator, then work back to level flight,
sometimes only a few feet off the ground. Then the priority
is to set up for a safe, clean landing .... somewhere away from people.
Hook location relative to CG -
A hook location that is in front of the CG will not cause pop-offs, but
understanding hook location is the first step to fixing pop-offs. For hi-start
or winch launching, your hook should be in front of the CG by up to a 1/2". You
will want to move this back, closer to the CG point as you gain confidence with
the plane and as you work toward more aggressive, higher launches. The closer
to the CG, the steeper the angle of climb and the higher the launch. Some
planes have adjustable hooks to make it easy to adjust this. My Spirit has 3
hook positions. I consider them Beginner, Sport and Competition, with the last
one being right on the CG. My Thermal Dancer has an adjustable hook and my
Legend has 2 spots. Note that as you move the hook back, the more unstable the
plane becomes on launch and the more important it is to give it a good hard
push/throw rather than just letting it go. Typically you will want to move the
hook back over time, as you gain confidence with the plane in order to get the
highest launch.
Note also that if you add or remove weight from the nose of the plane, as you
adjust the CG, you are also changing the hook position in relation to the CG.
Most glider pilots move the CG back over time as this usually leads to better
handling and better thermaling. However as you move the CG back, the hook
position is moving forward in relation to the CG. So, consider moving the hook
to keep your launches high. Adjustable tow hooks make this easy.
Hook angle -
Your tow hook should be parallel with the bottom of the plane or bent slightly
up toward the plane. There should be plenty of clearance so that the ring can
come out when you want it to. Sometimes, a light weight hook can become bent
and angled away from the plane. This will contribute to pop-offs. Push it back
into shape. If this keeps happening, replace it with a stronger hook. Also,
some people put a small notch in the hook right behind where the ring rides.
This can help reduce pop-offs too.
H-Stab or elevator setting - The dive test
This is a test for trim and balance, called the dive test. This is best done on
a calmer day to eliminate influence from the wind.
Take your plane up high and get into a smooth glide into the wind. Now put the
plane into a dive of about 30-45 degrees and let go of the sticks. Does it tend
to stay in the dive, coming out slowly over several seconds? Does it hold the
dive, requiring you to pull out. If it does, that is good. However if it tends
to pull out of the dive quickly, on its own, that is bad.
A properly balanced and trimmed plane should hold a dive of 30-45 degrees and
come out of it very slowly. If it tends to pull out quickly, you have a
problem. Your plane may be too nose heavy which will require you to trim in up
elevator to hold the nose up. Or, your h-stab is up in the back. Your elevator
may be trimmed neutral but the up angle of the stab is giving you the effect of
up elevator. Either of these conditions will cause the plane to want to climb.
In a normal glide you may not notice it but at the higher speeds of a dive, or
on launch, the plane will want to go into a loop which, on launch, causes a
pop-off. This also causes more drag which hurts your planes performance.
When I first got my Spirit I was having terrible pop-off problems. I thought is
was my launch technique. I was new, what did I know. However one of the club
experts performed the dive test described above. After we removed some nose
weight, things got better but still the plane would pull out of a dive too
quickly and it still tended to pop-off too easily. Temporarily I put in some
down trim on launch to control this so I could fly that day, but we knew I had a
problem.
Over time we determined that my h-stab was not parallel to the wing. I had to
do some wood working to correct this, but once I got the h-stab right, the
pop-off problem went away, the plane had a much better glide and thermaling
became a lot more fun.
NOTE: If you have a fully flying stab, that is the entire stab moves on a
pivot, then you do not need to go through this shim process. You automatically
fix any wing to stab alignment problems when you adjust your trims. This is an
advantage of a fully flying stab.
If the stab is glued on, one way to test for this condition is to shim the wing
if you can. This works with top mount wings, but not with side mount wings. Do
this after you feel you have made all of your balance adjustments or you will
mask a nose heavy plane.
When you mount your wing, put a shim under the back of the wing. Start with
something like 1/32 and keep adding till your dive test looks good. Chances
are your pop-offs will stop too. Rather than aligning the stab with the wing,
you are aligning the wing with the stab. This is not the best way to fix the
problem but it does work and it is quick and easy. If correcting the stab is
not an option, then adjust the wing to match it. Note that this will cause a
gap under the wing. If you are going to use this as a permanent fix, then fill
that gap with foam tape or some other filler or this will lead to drag, which is
something to be avoided.
If you are putting in down elevator trim on launches to prevent pop-offs,
chances are you have a trim problem. A properly balanced and trimmed plane,
where the tow hook is in front of the CG, does not tend to pop-off on launch.
The Throw -
Until your plane reaches flying speed, it is in a stall state. In stall, there
is little or no control on the surfaces and not enough lift from the wing. If
you just let go, then your plane is a rock till the hi-start gets it flying.
One of the side effects of this will be a tendency of the plane to drop left or
right causing a wild launch or a crash. It can also cause over rotation,
leading to a pop-off.
By throwing the plane, you get it up to speed quickly giving you more control
and a smoother safer launch. Rotation will be smoother and there will be less
tendency toward pop-offs. At first you can throw out flat or at about a 20
degree up angle. Over time you can bring that angle up to 45 degrees or even
higher. The stronger the throw, the steeper you can launch resulting in higher
launches. Competition pilots, using winches or strong hi-starts, will throw the
plane with great force at angles over 70 degrees to get maximum height launches.
Summary
Pop-offs are a cry for help from your plane. It has a problem and it needs your
help. It may have multiple problems. It may be nose heavy AND have a bent tow
hook AND have an h-stab that is not properly set AND a pilot who is not giving
it a good throw. Don't ignore it, don't fix it with down trim and don't add
weight to the nose to prevent it. If you understand your plane, you will see
that the problems are not hard to fix. Not only will addressing these issues
fix your pop-off problems but you will get higher launches and have a better
flying plane that handles better and thermals better and more easily.
Clear Skies and Safe Flying!
RESOURCES
Learning to use a hi-start
http://forums.flyesl.com/topic.asp?TOPIC_ID=216
Learning to use a winch
http://www.flyesl.com/News-y-Article...7&TOPIC_ID=112
The New Glider Pilot's Handbook
http://forums.flyesl.com/topic.asp?TOPIC_ID=251
Gordy's Balancing System
http://forums.flyesl.com/topic.asp?TOPIC_ID=255
Performance Tuning Gliders - Video review
http://forums.flyesl.com/topic.asp?TOPIC_ID=227
Test Flight Procedures
http://forums.flyesl.com/topic.asp?TOPIC_ID=226
10-25-2007 | 09:21 AM
#5
RE: FIXING POP-OFF PROBLEMS
As always, your input is very valuable. I realize that it takes a lot of time to write something like that (I do it for our club sometimes), but it is much appreciated.
RRW_SOAR,
Congratulations! Those are some enormous accomplishments! I hope to get there some day.
Happy flying!
RRW_SOAR,
Congratulations! Those are some enormous accomplishments! I hope to get there some day.
Happy flying!
10-25-2007 | 09:30 AM
#6
RE: FIXING POP-OFF PROBLEMS
RRW_SOAR.
I would love to hear about your plane, and how you accomplished this task. If you would start a new thread and tell us about it, that would be great.
If you could edit your post and add a link to that discussion, that would be wonderul.
However,
Let's stay on topic in this thread so it can become a resource to those who have a problem with pop-offs.
I invite those who have an issue with pop-offs, or those who can offer helpful advice to please post here.
Thanks!
I would love to hear about your plane, and how you accomplished this task. If you would start a new thread and tell us about it, that would be great.
If you could edit your post and add a link to that discussion, that would be wonderul.However,
Let's stay on topic in this thread so it can become a resource to those who have a problem with pop-offs.
I invite those who have an issue with pop-offs, or those who can offer helpful advice to please post here.
Thanks!
10-25-2007 | 10:06 AM
#10
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Joined: Jan 2003
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From: Lost Andes, CHILE
RE: FIXING POP-OFF PROBLEMS
aeajr,
Have you found a relation between pop-offs and camber control, like adding flaps or full trailing edge camber?
Another one, does the wind have any influence with pop-off? I've seen guys to pop-off frequently when wind appears.
Have you found a relation between pop-offs and camber control, like adding flaps or full trailing edge camber?
Another one, does the wind have any influence with pop-off? I've seen guys to pop-off frequently when wind appears.
10-25-2007 | 12:08 PM
#11
RE: FIXING POP-OFF PROBLEMS
Since camber does add lift, if the plane is very agressively balanced and if the hook is back I suppose that added wing camber contribute to a tendancy to pop off on a very agressive launch. However, on a properly balanced and trimmed plane launched at something under 45 degrees, I would not expect camber to be an issue. But I have never tested that so I could be wrong.
I think wind can influence pop-offs as you now have a highly variable factor in the launch. Exactly when and how it would promote pop-offs I can't say. Again, on most sport launches I would not expect it to be much of a factor but on a high angle agressive launch I could see it pushing the nose over and releasing the line. Again, that is subjective with no practical experience.
In both cases we are talking about agressive, high angle launches where I think these might be a issue. In more "sport" oriented launches I would not expect these to be major factors.
I think wind can influence pop-offs as you now have a highly variable factor in the launch. Exactly when and how it would promote pop-offs I can't say. Again, on most sport launches I would not expect it to be much of a factor but on a high angle agressive launch I could see it pushing the nose over and releasing the line. Again, that is subjective with no practical experience.
In both cases we are talking about agressive, high angle launches where I think these might be a issue. In more "sport" oriented launches I would not expect these to be major factors.
10-25-2007 | 04:01 PM
#12
RE: FIXING POP-OFF PROBLEMS
I never thought I'd be posting a reply in this thread, but here it goes...
I am a complete newbie, but I figured out my hi-start problem. It sounds simple, but it was enough to catch me up.
If the sailplane is set up right, then let the darn thing fly!!! Don't over control it!!!
I am a complete newbie, but I figured out my hi-start problem. It sounds simple, but it was enough to catch me up.
If the sailplane is set up right, then let the darn thing fly!!! Don't over control it!!!
10-25-2007 | 04:41 PM
#13
RE: FIXING POP-OFF PROBLEMS
Nathan,
That is a VERY valuable observation. Whether hi-start or winch, the plane should not need any correction if it is properly trimmed and thrown firmly and flat. It should just track up the line.
Congratulations!
That is a VERY valuable observation. Whether hi-start or winch, the plane should not need any correction if it is properly trimmed and thrown firmly and flat. It should just track up the line.
Congratulations!
