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Thermals - and how to Catch them.

TRIMMING YOUR PLANE

SUMMARY - Twisted wings; Washout; Removing the Warps; Centre of Gravity; the Dive Test; Control Throws; Loose Gear; Tail & Wing Incidence; Lateral Stability; Tow Hook Placement; Motor Thrust Line; General Airworthiness;

So, you've purchased a model and are now flying solo and your trainer or others in the club have helped ensure your plane is trimmed to the degree that you can fly it safely. So how did they achieve this and how do you do it yourself for your next plane or after repair or to tweak your existing plane to fly longer and more predictably?

TWISTED WINGS

A common problem with gliders in particular is that the wing is twisted. This has occurred in the building process or, more commonly, in the covering process or subsequently owing to being left in the sun or stored badly. Wings MUST be symmetrical or they will cause the plane to twist as it flies. It is hoped that this is not required in your model but if it does then I had better cover it as otherwise you will simply not be flying. My first response to this was - no worries, I'll simply compensate with rudder. This sounds fine but doesn't work because your plane will be flying at a variety of speeds and the effect of rudder over the warp will be different at different speeds.

For the bungee launched glider, this is a disaster as it is rapidly pulled into the sky - or perhaps smack back down on the ground because, at the speed of the launch, the rudder has insufficient compensation over the warped wing. The good news is that un-warping a wing is usually easily done. But, before we cover how, let's look at a further characteristic that we may want to ensure is present in the wing.

 WASHOUT

A stall occurs where, because of insufficient airspeed, the airflow over the wing becomes turbulent as it goes over the rear of the wing and lift is no longer provided. If this occurs on one wing before the other, that wing can drop suddenly (called a tip-stall) which can be a disaster if you are close to the ground and an annoyance if it happens further up and you keep losing height and direction and keep having to recover.

As a light glider usually flies at its most efficient speed only a little faster than the speed at which the wing would stall and because we may not be used to maintaining speed, tip-stalling is a real danger.

Washout is the raising of the outer rear tips of the wing about two or three degrees. This causes the angle of attack to the airstream of the outer tips to be less than the angle of attack of the central section of the wing. So, as your plane slows, the centre stalls (it loses lift first) while both tips continue to provide lift. The nose of your plane will drop a little but lateral stability is maintained. So, if the design requires it, we want to ensure washout is built into the wing (or re-inserted in our un-warping process).

 REMOVING THE WARPS

Start by establishing what parts of the wing are warped and which of the outer panels (if any) need adjustment to provide washout. To check the overall integrity, place the wing on a stool or a rubbish bin and look at it from the rear and the front. Take note of what needs to be un-twisted to make it all straight and symmetrical, but (if washout is supposed to be included) with the rear tips of the outer panels about 2 to 3 degrees higher than the centre.

 With a flat bottomed wing you can also test the straightness of the respective panels by placing them flat onto an un-warped table. You may need an assistant for this. The inner panel(s) must be flat but the outer panels, when placed on the table, can be easily measured for any required washout and compared.

A simple way of measuring one degree is to measure 90 millimeters along and up one. So if your wing tip has a chord (distance from leading edge to trailing edge) of about 180 millimeters, then 4 millimeters up is 2 degrees.

You will need an iron to take out the warps, preferably a modeling iron but it can also be achieved with a domestic iron on a lowish heat setting. If you spit on the iron and the spit sizzles, the iron is too hot. Note that being caught spitting on to a heated domestic iron may lead to a heated domestic dispute, as will putting the hot iron down on the bare table top. Remove any colouring from the covering that makes its way onto the iron with turps.

Place your wing on the bench or table and use a telephone book to weigh down the centre section. Have a trusty assistant hold this down firmly but safely or add a couple of extra phone books. Twist the wing and you will notice wrinkles appear in the covering. Iron these wrinkles A LITTLE and let go. It is VERY easy to over compensate so take it steady and CHECK REGULARLY on your progress. Also use this technique carefully on the tail, if required, and re-check everything before packing up.

As an alternative to the iron, you may have success with a heat-gun used CAREFULLY, initially from a distance or you will burn holes in the wing. Wear gardening gloves for protection against burns.

Experience is a great asset so seek assistance from someone who's done this before. If the problem is too great is may be necessary to remove and replace the covering on one or both sides of a panel.

CENTRE OF GRAVITY

The plan that your plane was built from will indicate the recommended centre of gravity (CG) and that is precisely where you should start. However a plane flying poorly (or one you have acquired without instructions or plan) may require some experimentation. Further, any repair that you effect, particularly on the tail, may affect the CG and the plane may require re-trimming in the manner described below. The CG mark on a plan is a small circle divided into four, with the opposite quadrants coloured black..

For wings that stick straight out, the CG will generally be a third of the way back from the leading (ie front) edge, and will usually correspond with the placement of the main spar. If the wings are swept back you will need to calculate the total wing area (exclude the tail from this calculation) and then calculate how far back from the front you would need to draw a straight line perpendicular to the fuselage to capture the forward 33% of the wing area. This then becomes your starting CG mark.

Flying Wings or Deltas do not have elevators, but instead have combined ailerons / elevators - sometimes called elevons. These are trimmed slightly up to give the wing a positive angle of attack to the airstream (ie it points slightly upwards as it flies along). The centre of gravity in a plane such as this is more likely to be closer to a straight line marking off the front 30% of the total wing area*. (*Exclude the area of the ailerons from the calculations for these planes.)

THE DIVE TEST (This is good stuff!!)

Now I will let you into a little secret that not everyone is aware of. (I would point out that this is ahead of where the raw beginner will be up to but you may as well know now!) If the plane flies but you wish to more finely test the trim, try this.

Set your trims so that the plane is flying level when the sticks are centred. Get plenty of height then, facing into the wind, drop the nose so the plane is heading down at about 30 degrees and starting to accelerate. As the speed builds, without you inputting any up or down elevator, the nose should start to rise until the plane is gently climbing and the speed will drop off and so will the climb, hopefully resuming a horizontal flight path.

If the nose lifts too quickly for your liking, there is TOO MUCH weight in the nose and you may wish to move the centre of gravity BACK by removing some weight, moving the battery back or adding a very small amount of weight to the tail.

This is all the opposite to what one might expect and the reason for it is that an excess of nose weight is compensated for in level flight by elevator being trimmed up. When the speed increases in a dive, the elevator is having more effect than the nose weight so the nose quickly lifts. Reduce the nose weight a little and add a click of down trim on your transmitter so that the plane flies level and then try the dive test again.

This time the nose should rise a little more slowly and you may find the handling of the plane and its ability to turn a little more to your liking. However if, as the plane accelerates, the dive rate increases, then the centre of gravity is too far back. To move it forward, move the battery forward or add a little weight to the nose. This test works for flying wings as well as conventional planes.

A plane with a CG too far forward will fly poorly. One with the CG too far back will be difficult or impossible to fly and may be positively unstable and dangerous.

Many of us use coins as weights as they are always a known quantity. They can be easily taped to the nose or placed inside until a more permanent arrangement, possibly using lead of the same weight, can be effected back at the workshop.

If adding weight to the nose, feel free to make some of that weight up as glue, to add strength in the nose at the same time. Keeping the added weight well forward means not requiring as much weight. On a plane with an internally pointy nose, some lead shot (such as shotgun pellets) poured in with some epoxy will do the job and won't flop around.

Ensure the metal weight does not short out the battery by using tape or some sort of spacer as an insulator between the weights and the battery holder.

Once the preferred position of the CG is determined, mark it on the under side of the wing with a couple of thickness of tape cut into a narrow strip. This enables you to FEEL the CG mark with your fingertips at the point of balance.

Some more experienced fliers pilots like the extra sensitivity and feedback and differing turning characteristics that having the CG slightly back gives. However the beginner is best to have a greater amount of stability.

In a higher wind, a couple of extra clicks of down trim and / or a coin or two of nose weight may be required to provide adequate penetration.

CONTROL THROWS

This refers to the amount of travel in the control surfaces, being elevator / rudder / ailerons / elevons. Obviously sufficient amounts of travel are required to ensure that the plane will change direction when you instruct it to but sometimes a plane may fly like a dog simply because the amount of travel is too great. To reduce travel, move the clevis to the next outer hole on the control horn(s) and / or to the next inner hole on the servo arm(s). The feeling of excessive throw is exacerbated when the CG is a little far back and it is sometimes not immediately obvious whether the poor flying characteristics have been caused by too much throw or a CG too far back or both. This is where the dive test is handy and the input of an experienced pilot is of assistance.

A computer radio with an exponential function will allow you to move your stick a reasonable amount in the centre range with a gentle response by the servo, increasing in sensitivity at the outer edges of the movement in the stick - thus combining stability with enough range of movement when required.

Some flyers have their multi-channel transmitters set up with dual rates so that with the flick of a switch the rates change in severity. I would not recommend this for the inexperienced glider pilot as it is too easy to accidentally flick the switch before or during launch and have the plane react in an unexpected manner. Better to keep it simple and be safe at this early stage in your flying career.

THE MOVING CENTRE OF GRAVITY

An all too common cause of crashes is the moving of a heavy weight, typically the battery, in flight - causing a movement in centre of gravity. This is particularly prevalent in electric planes with their heavy batteries but can also occur where the receiver battery, the receiver or ballast is not sufficiently restrained. Pack the battery / receiver in or use Velcro as a means of holding it in place. Ballast (sometimes required if the plane is too light for the wind) should be appropriately boxed / packed in.

TAIL INCIDENCE

The horizontal stabiliser (that's the horizontal part of the tailplane that the elevator is attached to) needs to be parallel with the central fuselage line (an imaginary line down the centre of the fuselage). This is often overlooked by beginners rebuilding tails after hard landings. It must also be free of warps. To have it wrong will mean having to fly with the elevator trimmed constantly up or down, which creates drag but, more importantly, will lead to different flying characteristics at different speeds.

WING INCIDENCE

This refers to the difference in angle between the wing (an imaginary line between its leading and trailing edges) and the abovementioned central line of the fuselage. The wing needs to be sticking up about 2 degrees (that's about 4 millimetres on a wing with a 200mm chord wing) to fly at its best.

This should be correct on the plan / kit but may not be or subsequent repairs may have altered it. Bring some packing material (a bit of folded cardboard or some balsa) to the field to enable you to experiment before making the adjustment permanent. Some expert input may be of assistance here.

As a test, if you have successfully trimmed you plane for level flight, land it and see whether the elevator is pointing up down or centred. If it is centred (and you have zero tail incidence as described above) you are probably okay. The overall aim here is to get fuselage and wing both flying at their respective best angles to the airstream, giving maximum lift and minimum drag and the plane flying in a consistent fashion, regardless of airspeed.

LATERAL STABILITY

Unless you wish your plane to spend its life flying in circles, it is also important that the wings on each side weigh the same. This cannot be overcome by rudder or twisting of the wings as, again, the effect will be different at different speeds. String up the plane or balance it on its centre line. If it keeps toppling over to one side, add weight to the opposite wing until it is even.

For some planes it is a simple matter of inserting screws or nails as weights. When complete, cover the heads so as to not highlight that you have built the plane out-of-balance(!).

TOW HOOK PLACEMENT (unpowered gliders)

If your bungee / winch launched glider is now flying well but the launch requires the holding in of up or down as it goes up the line, the tow hook is possibly in the wrong place. This is often overlooked but is one of the most important variables as the launch is the riskiest part of the flight.

The hook will generally be about 20 degrees in front of the centre of gravity, so that is not a bad place to start. If you have the drill out, drill a series of very fine holes about 5 millimeters apart to enable you to make adjustments to your plane out on the field. (But don't drill through the bottom of your servo or receiver!!). If the plane requires up stick as it goes up the line, move the hook back. Conversely, if the plane stalls up the line or flips backwards (very dangerous) and requires down elevator, move the hook forward.

As you become more experienced, and in different wind conditions, you will find that some small amount of control input will maximise your launch height, particularly if you give a small amount of down then up at the top of the launch so as to gain (in lesser form) some extra height from the "whip" effect that the lads with the winches achieve.

If your plane is flying well normally but still turns to the left or right when it goes up the line then you may still have a little more work to do on un-warping your wing or perhaps your tail is not straight.

MOTOR THRUST LINE (powered planes)

If your glider / plane is has an electric motor, you will probably not want to have to constantly ride the sticks when the motor is on, but would rather be able to leave the sticks centred whether under power or gliding. To achieve this, a plane with the motor in the nose will need to have the motor set 2 degrees to the right (to compensate for the twist of the propeller) and 8-10 degrees down.

Alternatively, if you can't or have got it wrong and you are fortunate enough to have a computer radio with mixing, you can build in throttle to steering and throttle to elevator compensation mixing, so when throttle is applied, a SMALL amount of down elevator is applied. (Too much and you will send your plane into the ground when you were trying to accelerate away from it!)

Note that the more symmetrical the wing, the less down trust is generally required. This is because on a non-symmetrical wing the increase in wind speed across the wing is increasing the lift of the wing - leading to the nose coming up. The down thrust will keep the nose down. On a symmetrical wing the extra speed will not generally create as much extra lift - and so less down thrust will be required.

GENERAL AIRWORTHINESS

There should be no slop in your controls (generally caused by poor control linkages or the tubes containing the control rods not being adequately secured in the plane). Servos should be healthy and correctly centreing and wings and removable tails must be held firmly and correctly in place and not move during the launch or in flight.

Your aerial should be well placed and not at risk of being torn off during launch (straws down the fuselage may work) and your radio in good condition (and certified) and not affected by or a cause of interference.

Internl Combustion powered planes will have special requirements, in particular prevention against damage caused by vibration and fuel.

Electric motor should be fitted with three capacitors to avoid "motor noise" interfering with the radio reception and that the receiver is well separated from the speed control and not mounted beside or under servos, as these can cause interference.

Most importantly, your brain must be in gear with the safety of others being at the forefront of your concerns.

IN CONCLUSION

I hope this article will help you to successfully trim your current or next plane to your preferred flying style. If your plane is currently flying, adjust only ONE variable at a time and only a LITTLE bit at a time. Start with what the plan / instructions suggest, seek the input of those with experience and, to avoid your brain curdling, fly as often as you can.

NOW CHECK OUT .......................... THERMALS - AND HOW TO CATCH THEM.

 

 

 

 

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