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Today, I played with styrofoam gliders with my kids; I made an asymmetric wing, asymmetric horizontal tail, symmetric vertical tail, and attached these to one stick for the fuselage. I've had big problems with pitch stability, so I played with adding mass to the nose, hoping to solve the problem.

Every time I throw a glider with my hand, the nose lifts up or falls down (which one depends on where I set the glider's center of gravity), and the glider crashes. I set the tail to produce downforce, but I can't find the right pitch balance to make the glider fly well.

I know that, for level flight, the CG has to be in the same place as the glider's overall center of lift (both wing lift + tail lift).

Should the horizontal tail have a symmetric or asymmetric profile, and what should the angles of incidence for the wing and tail be?

Maybe the best way is to put the CG at the wing's center of lift (at the 25% chord line) and set the tail to a neutral position (producing no positive or negative lift)?

What is the easiest way to calculate where to place the CG if I don't know how much lift is produced by the wings and tail?

  • What I would do is put the CG at 25%. Then I'd make an adjustable elevator out of thick paper. The way to build this adjustable elevator changes with how big the plane is, so can you tell me that? Also, this question might be better on drones.se – ThatCoolCoder Jan 13 '21 at 20:33
  • @ThatCoolCoder How do you mean adjustable elevator,my "glider" is from styrfoam ,dont have electrics...If I put CG at 25% chord then tail must be neutral? –  Jan 13 '21 at 20:36
  • I can explain better if you tell me how long the plane is (from nose to tail). – ThatCoolCoder Jan 13 '21 at 20:37
  • @ThatCoolCoder Nose to tail maybe 80cm, I must tomorow meassure,but I can change dimensions,because I sctoch taype wing and tail with stick..This is just for fun for kids,not proffesional.. –  Jan 13 '21 at 20:40
  • I think the best thing for this plane is to cut a strip of thick paper that is as wide as your tail and about 5cm long. Then attach this to the back of the tail with half sticking out. diagram. First throw the plane with the paper flat. If the plane goes down, bend the paper up a bit. If it goes up, bend the paper down a bit. After a bit of experimentation, you should be able to trim the plane perfectly. I have used this method for years on small foamboard gliders and it worked well. – ThatCoolCoder Jan 13 '21 at 20:44
  • You can also use this method to control the plane going left and right if you bend one side of the paper more than the other. – ThatCoolCoder Jan 13 '21 at 20:50
  • @ThatCoolCoder With CG at wing lift (1/4 chord)? –  Jan 13 '21 at 20:55
  • yes, everything I said presumes 1/4 chord – ThatCoolCoder Jan 13 '21 at 20:56
  • @ThatCoolCoder Is better to put more mass at nose to align CG with 1/4 chord or move wing close to tail?If I move wing too close to tail I will reduce directional stability.. –  Jan 13 '21 at 22:14
  • I would probably add more weight to the nose, since this plane is made of light materials. – ThatCoolCoder Jan 13 '21 at 23:46
  • You cannot fix a speed related pitching issue by "moving weight", without understanding static stability trim. – Robert DiGiovanni Jan 14 '21 at 10:36
  • My rule of thumb with model airplanes was symmetric tail, cambered wing, cg at one third of wing chord. Start with a few degrees of incidence (maybe 2°) on the wing and zero on the tail and launch at its desired gliding speed. Adjust tail incidence until it flies well. Only try to throw harder once the smooth launch results in a smooth glide. – Peter Kämpf Jan 15 '21 at 17:24

2 Answers2

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Joe Ayoob's world record throw of the John Collins designed paper airplane is worth watching over and over again.

Every time I throw the glider with my hand ...

This caused me to split my free flight hobby into throwing gliders and launching from a hill.

A thrown glider experiences a wide range of speeds in flight, therefor it cannot be set with a staticly stable "tail downforce/weight forward" arrangement. This type of glider will loop if thrown hard, but will be fine if thrown off a hill. Try to gently release it at its trim speed. Moving the weight/changing tail decalage will produce the best glide.

But for throwing on flat ground for distance the glider must be slightly staticly unstable to keep flying as it slows down. You want it to gradually pitch up (increasing AOA) as it loses speed to generate adequate lift for level flight.

The plane flies straight out until it stalls. Notice on Joe's throw the glider went around 150 feet, from 40 mph to maybe 5 mph, stalled, then recovered from the dive, flying on to the world record.

Glider has asymmetric tail, which produce downforce, then glider is "tail heavy"

An asymmetric tail will produce pitching force aerodynamicly, not by weight. This is why "classic" aircraft design has an "upright" vertical stabilizer (many glider designers also benefit by putting the horizontal stabilizer on top in "T" fashion, increasing pitch torque without adding drag). The tail must produce a down pitching force to "set" wing AOA if CG is forward.

For "throwing" gliders, you need tail upforce, weight aft (while maintaining directional stability), and some luck to go 226 feet. If one can control the elevator (by R/C) in flight, then discus launched gliders (DLGs) are worth a look!

Robert DiGiovanni
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    Plase, not unstable! But the thrown glider needs near neutral stability, or it will loop when thrown. An unstable plane will only dive into the ground or stall early in the flight. – Peter Kämpf Jan 14 '21 at 21:11
  • @Peter Kampf Agree with "near nuetral", and Collins talked alot about "launch angle". For his world record it was critical, as excess lift will cause the plane rise vertically, which creates down force on the tail. If the wing is slightly forward of CG and the tail has upforce, pitching at higher speeds can be minimized. The plane will rise, but not loop. The potential deformations of paper under these stresses drove me to balsa free flight from hills. – Robert DiGiovanni Jan 14 '21 at 23:19
  • @Peter I have positive stability, CG infornt wing.But when throw glider fast it loop.Tail is symetric set for neutral lift..Is this because center of mass is under center of pressure(lets call it drag or frontal pressure point)? –  Jan 15 '21 at 14:48
  • @EBV821 perhaps you would like to tell us a bit about your wing. – Robert DiGiovanni Jan 15 '21 at 15:50
  • @RobertDiGiovanni asymetric airfoil, wingspan 100cm, chord 14.5cm, thick 4cm, last 4cm flow is separated, incidence angle wing and tail zero degrees.. –  Jan 15 '21 at 16:16
  • @EBV821 … but a cambered wing, so effectively you have different incidence between wing and tail. If it loops when thrown it has too much stability. Shift the cg back and adjust the tail incidence accordingly. – Peter Kämpf Jan 15 '21 at 17:13
  • @PeterKämpf It loops only when I throw hard.Isnt CG too much in front, put nose down,not up? –  Jan 15 '21 at 17:18
  • @EBV821 Yes, that's right. When thrown hard, it flies faster and wants to change to a slower speed. This it does by pitching up. When thrown at an upwards trajectory, this pitching will result in a loop. With less stability the desire to fly more slowly becomes weaker, so it will pitch up less hard. – Peter Kämpf Jan 15 '21 at 17:20
  • @PeterKämpf Glider will loop only if center of mass is below overall drag center? –  Jan 15 '21 at 17:52
  • @EBV821: No, the vertical distance between mass and drag has only a minor influence. It is the lengthwise position that counts. – Peter Kämpf Jan 15 '21 at 18:14
  • @PeterKämpf But what then cause pitching up torque when glider accelarate ?I dont understand –  Jan 15 '21 at 19:02
  • @EBV821 Read this – Peter Kämpf Jan 15 '21 at 19:04
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I know that, for level flight, the CG has to be in the same place as the glider's overall center of lift (both wing lift + tail lift).

Not quite. You want your CG forward of the NP of the aircraft for static longitudinal stability - meaning that you will have a nose down stabilizing moment when perturbed during flight. Typical acceptable margins for static stability are 5-20% non-dimensionalized from main wing chord -> suffice it to say a smidge in front of the quarter chord.

For simplicity sake I'd say pick a symmetric airfoil and play with incidence angle to achieve the correct lift characteristic for your main wing.

But if you want to get super fancy with your analysis check out the Appendices in this text! Dynamics of Flight Stability & Controls

spacegirl1923
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