Cause of death spiral?

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Just a fast question: what causes a glider to suddenly go into a death sprial? It started out in a nice stable glide then suddenly went into a tight nose down spiral into the ground.


This was not a scratch built but an Apogee Cirrus Breeze kit glider that I got trimmed out nicely after 45 minutes of minor adjustments.
 
PULL UP! PULL UP!

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Sounds as if it was trimmed to fly a bit too shallow, airspeed slowed below the stall speed and a stall occured leading to a spin.
 
Did the kit give you guidelines on where the glider's Center of Gravity should be? If the CG was too far aft, it could contribute to an unrecoverable spin after stalling...
 
The inside wing lost lift. This occurs when the glider's inside wing tip is not "washed out".

The principle is documented in an ancient model aviation text "Spiral Stability" written by Frank Zaic, IIRC.

A couple of things you can do: Warp the inside wing tip DOWN by breathing/steaming it and holding it down a bit till it takes a set, you might also try adding a bit of modeling clay to the outside wingtip which will open up the turn. More complicated adjustments such as warping the stabilizer and/or vertical fin can also be tried but the results may be detrimental to the powered climb.
 
Did the kit give you guidelines on where the glider's Center of Gravity should be? If the CG was too far aft, it could contribute to an unrecoverable spin after stalling...


It came with a construction DVD but neglected to pinpoint the CG location. The odd thing was that if performed well after hand trimming, including a few hard tosses upward at about 45°.
 
The inside wing lost lift. This occurs when the glider's inside wing tip is not "washed out".

The principle is documented in an ancient model aviation text "Spiral Stability" written by Frank Zaic, IIRC.

A couple of things you can do: Warp the inside wing tip DOWN by breathing/steaming it and holding it down a bit till it takes a set, you might also try adding a bit of modeling clay to the outside wingtip which will open up the turn. More complicated adjustments such as warping the stabilizer and/or vertical fin can also be tried but the results may be detrimental to the powered climb.

The inside wing had a small bit of clay on the wingtip to encourage a circular glide so it wouldn't fly out of sight. I'm wondering if that might have somehow caused the problem?
 
Good advice given, but note that non-RC models tend to be unable to recover once they start "spinning in".

Correct Center of Gravity location is a major concern. In general, the CG location is roughly 1/4 to 1/3 the width of the wing, as measured from the leading edge. For example, if the wing is 3 inches wide, the CG is roughly 1 inch from the leading edge. (All this varies, depending on wing shape; just trying to give you some rough info to play with.)

Sometimes, when I don't have CG info, I have looked at the Tower Hobbies website to find an aircraft of similar shape and size. Generally, Tower gives a Tech Sheet that usually lists the CG location of the aircraft; I have used the Tech Sheet of a similar model to estimate the CG location of my model. Not the best method, but it is a way of getting info, when you don't have the CG.
 
Long time competition B/G flier here...the "death spiral" is a spin condition, caused when the inside wing loses lift since it is flying slightly slower and stalls first. if you have weight on the inside (of the turn) wing, definitely take it off, the weight is pro-spin. You want to make aerodynamic adjustments instead.

Washout (up warp NOT down) on the wing tips is very helpful since reduces the angle of attack of the wingtip, causing it to resist dropping by not stalling as soon. Warping the inside tip down is the wrong thing as it causes the tip to stall sooner and thus is also pro-spin. it may seem like it helps a little but, but it makes the wing drop more abrupt and severe (BTW that is how you intentionally spin an airplane using opposite aileron and rudder).

Put in the circling turn with stab tilt, which provides a small yaw force rather than roll. To get a left turn (suitable for right handed toss), tilt the right side of the stab (as seen from the rear) up several degrees. Remember that the aero force on the stab is downward, so the yaw component is backwards from what you'd expect.

Having enough dihedral is crucial; it provides roll stability by decreasing the lift of the "up" wing and increasing the lift of the "down" wing when the glider rolls.

Also check the stab incidence. If it's negative (raising the tail and pushing the nose down) the glider will always nosedive (severe) or spin (subtle).

There's a ton of great information on https://www.amaglider.com/ . I'd recommend that anyone interested in flying gliders should inhale that site totally.

There is a simple but very good explanation of building HLG's here: https://f4bscale.worldonline.co.uk/building.htm . The website look-n-feel is awful but the info is great. Take note of the instructions for sanding washout into the wings. You should be able to retrofit your glider even if already built.
 
Good advice given, but note that non-RC models tend to be unable to recover once they start "spinning in".

Correct Center of Gravity location is a major concern. In general, the CG location is roughly 1/4 to 1/3 the width of the wing, as measured from the leading edge. For example, if the wing is 3 inches wide, the CG is roughly 1 inch from the leading edge. (All this varies, depending on wing shape; just trying to give you some rough info to play with.)

Sometimes, when I don't have CG info, I have looked at the Tower Hobbies website to find an aircraft of similar shape and size. Generally, Tower gives a Tech Sheet that usually lists the CG location of the aircraft; I have used the Tech Sheet of a similar model to estimate the CG location of my model. Not the best method, but it is a way of getting info, when you don't have the CG.

Long time competition B/G flier here...the "death spiral" is a spin condition, caused when the inside wing loses lift since it is flying slightly slower and stalls first. if you have weight on the inside (of the turn) wing, definitely take it off, the weight is pro-spin. You want to make aerodynamic adjustments instead.

Washout (up warp NOT down) on the wing tips is very helpful since reduces the angle of attack of the wingtip, causing it to resist dropping by not stalling as soon. Warping the inside tip down is the wrong thing as it causes the tip to stall sooner and thus is also pro-spin. it may seem like it helps a little but, but it makes the wing drop more abrupt and severe (BTW that is how you intentionally spin an airplane using opposite aileron and rudder).

Put in the circling turn with stab tilt, which provides a small yaw force rather than roll. To get a left turn (suitable for right handed toss), tilt the right side of the stab (as seen from the rear) up several degrees. Remember that the aero force on the stab is downward, so the yaw component is backwards from what you'd expect.

Having enough dihedral is crucial; it provides roll stability by decreasing the lift of the "up" wing and increasing the lift of the "down" wing when the glider rolls.

Also check the stab incidence. If it's negative (raising the tail and pushing the nose down) the glider will always nosedive (severe) or spin (subtle).

There's a ton of great information on https://www.amaglider.com/ . I'd recommend that anyone interested in flying gliders should inhale that site totally.

There is a simple but very good explanation of building HLG's here: https://f4bscale.worldonline.co.uk/building.htm . The website look-n-feel is awful but the info is great. Take note of the instructions for sanding washout into the wings. You should be able to retrofit your glider even if already built.

Thanks for the advice and reference websites gents, really appreciate it since I want to try my hand at designing RGs. Will check out those sites to glean a more thorough understanding of glider dynamics.

One thing that I suspect contributed a lot to the sudden death dive is that the slide box is a bit loose, allowing the wing tips to wobble up/down maybe 3° or so. What do you think?
 
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