Handlaunched Gliders

[Ignition Override]

Here is an observation of something a bit different.

When you throw a fairly large styrofoam glider (over four feet wingspan), with the wings level and into a steady wind over smooth ground, it seems to always turn away from the wind, and I can not figure it out. These were launched at about chest-height. With no wind, these will fly fairly straight. Do they throw frisbees in the UK? These will always bank to one side if you throw them at a high angle.

It is not clear to me why this happens to such a glider, when launched in a level attitude (there are no moveable control surfaces), with the fuselage parallel with the ground. I've never seen one stay pointed into the wind.

:= :confused: :hmm:

[Field In Sight]

Throwing a model aeroplane gives it a certain amount of initial groundspeed e.g. 10 kts.

In no wind this will give it an initial airspeed of 10 kts, if this is close to it's "trimmed speed" then all of it's designed in stability will help it fly straight.

Into a head wind of say 10 kts, it's initial airspeed will be 20 kts or twice it's trimmed speed. The inbuilt stability will maneouvre the aircraft to hunt for the trimmed situation.
This will probably be an initial climb which will slow it down (a bit too much at first) this will reduce the directional stability until the speed builds back up again as the nose drops.
Eventually the trimmed speed will be found but the initial direction will have wondered.

That makes sense to me anyway.

FIS.

[Milt]

Its all very simple really

Every take off and landing is a transition from one medium into another.

When you have transitioned into the air medium then ALL flight is with reference to the air you are in.

A submarine does the same in water when it submerges.

Granted there remains confusion when you are at the interface of the two mediums and there are cross over effects such as gusts.

As for momentum and inertia. Why has no one considered that the parcel of air we fly in is going around at the equator at about 1000 Kts. But it doesn't matter. The aircraft is aptly named. It flies relative to the air ONLY.

The chopper drivers have a thread going on this subjest also. They seem to be more confused that the fixed wing guys.

[Oktas8]

Edited - Field in Sight beat me to it. But I'll leave this post as it goes into a wee bit more detail.

Might it have to do with launch airspeed?

A very light glider will be trimmed for a low glide speed.

If you launch into a strong wind, it will tend to pitch up briskly, and any slight yaw at time of launching (ie your hand not holding it perfectly straight) would give a strong roll tendency - roll being the secondary effect of yaw, and slight yaw at high airspeed usually leads to quite strong secondary effects.

Strong pitch up would lead to loss of airspeed and subsequent pitch down. If the glider rolled at the same time, it would tend to end up in a descending turn, which may be a stable state depending on its design.

Compare this with a launch downwind or in nil wind - lower airspeed so aerodynamic response is reduced, and it would neither pitch or roll very fast. To the observer, the glider might appear more stable, and perhaps only slowly pitch / roll into a descending turn.

Hmm... Comments welcome on this theory.
Oktas8

[Ignition Override]

Good points, gang. When this styrofoam glider (at least 4' wing span) is thrown, even slightly nose down into a steady headwind, there must be some bit of assymetry-either a little Dutch Roll or maybe one wing has more scrapes and resulting drag-or both wings are not held at equal angles in the wing slots.

As for uneven forces, even on our older planes we fly, either the turn indicator ball is not installed perfectly in the ADI or a roll spoiler (or two) are bent from too many years of use (maybe a slat?), and we need a little rudder and aileron trim quite often, no matter what the airspeed change. Never mind with intermixed engines "power by the hour". :rolleyes:

[EchoTango]

Seems to me that the glider will not be launching in a clean air flow if someone is standing immediately under, and slightly to one side of its flight path in a 10kt breeze ?

ET

[aviate1138]

Ignition Override said......
"Good points, gang. When this styrofoam glider (at least 4' wing span) is thrown, even slightly nose down into a steady headwind, there must be some bit of assymetry-either a little Dutch Roll or maybe one wing has more scrapes and resulting drag-or both wings are not held at equal angles in the wing slots."

Aviate 1138 says......

Any unswept high aspect ratio wing suffers from large moments of adverse yaw. That is why we pilots who trained initially on gliders, have plenty of rudder practice in maintaining the ball somewhere near its centre line - the hand launched glider with no moving surfaces has no such luxury.

... "Take up slack......."

Aviate 1138

[ShyTorque]

A toy glider launched in still air probably won't fly in perfectly straight trim due to a number of reasons. Due to the inbuilt stability of a free-flight model this will result in yaw and roll. However, any deviation from straight flight in still air will quite possibly be imperceptably small to begin with.

However, if a headwind is introduced, any slight turn away from the wind will induce a drift component which will greatly exaggerate the deviation from the original straight ahead, especially at the low airspeed of a hand-launched toy glider. The yaw and roll will continue and the drift will increase to a maximum as the aircraft flies to the full crosswind heading. As the wind becomes a crosswind, any gust will cause the into-wind wing to be affected more because of fuselage blanking, causing an increased angle of bank away from the gust. As the model turns out of wind, the effects will reduce again and the glider will likely as not fly off downwind. (Some will forget that the glider has inertia, so it is affected differently by a change in relative wind)

This happens to birds as well, they often swoop away downwind but always make a point of turning back into wind to land. My model gliders never had this ability, hence a trail of broken balsa wood when they landed downwind with a high groundspeed.

[cwatters]

Does your car go straight if you take your hands off the wheel on a bumpy road? Why do you expect an aircraft to do so.

[Field In Sight]

Does your car go straight if you take your hands off the wheel on a bumpy road?
Mine wouldn't because it is a pile of junk. I would expect a reasonably well maintained car to go relatively straight on a motorway, otherwise why would you pay for tracking.

Why do you expect an aircraft to do so.
Because some Boffin who understands aerodynamics, designs in a nice amount of directional stability for me so I can fly and eat sandwiches at the same time.

FIS.