Angle of Incidence and Decalage in HLG

This is one of the real secrets of HLG flying. A stubborn glider that does not launch well can often be transformed into a fine performer by changing the angle of incidence of the stab or the wing. The photo (left) is DOGEAR, a 6 inch span hand launched glider that I designed and built. It did not launch well until I replaced the stab and now it's great. Its first ever timed flight was 38 seconds.

Why and how?

Angle of Incidence is the angle between the wing's surface and the fuselage axis. (Usually for polyhedral wing models, you can place a ruler flat with the underside of the wing near the root chord to get an idea of the incidence).

Decalage, geometrically and simply speaking, is the angle between the wing's surface and the stab's surface. If you were to use a ruler under the wing and another ruler under the stab as described above, then it is the angle between the rulers. See the diagram here.

0-0 (said "zero-zero") is when the two surfaces (or rulers in the example above) are parallel.

Some HLG modellers swear that 0-0 is the best setting. On the other hand, many others (including me) say that a HLG should have a small decalage. When I say small, I mean small. For example, if the stab has zero angle of incidence, then the wing should have just a few degrees positive, e.g. the LE of the wing is 1-2 mm higher than the TE. What I can say is that the vast majority of HLG pilots say that "large" decalage is bad. Now, bear in mind that it is not easy to measure AOI accurately even with rulers and "large" is really a referring to a small measurement.

How to recognise large decalage?

Usually, you see a loopy launch. Instead of the plane arrowing up in a straight-ish launch, it loops round and may even execute a full loop frighteningly close to the ground. In addition, such a model may balance well-forward of the ~2/3 rule: generally, for traditional HLGs the Centre of Gravity is at around 66% of the root chord (i.e. closer to the TE than the LE). To compensate for the large decalage, the CoG of such a model may be at 40% or less. Too far forward.

Your throwing energy is converted into a pretty but dangerous low level aerobatic manouvre, not into height!

How to recognise small decalage?

Straight up and straight down! It goes up like a rocket and down like an arrow. Sadly, there is little or no glide bit in between. Hopefully, it will not land with a sickening crunch. You may see quite a lot of "up" elevator bent into the stab to compensate.

If you have a HLG that does not launch well, try altering the decalage. Yes, that may mean removing the stab, sanding and/or planing the rear of the fuselage and fitting a new one, but it is well worth it. For a pod and boom model with tip up wing or tipping boom, you can insert a set screw or thin shim that for fine adjustments.

There is nothing like seeing your glider rocket up and flip over beautifully into a stable soaring pattern high in the air.

Free Plan! Indoor Fun Glider: Man-o-Straw

Fancy something to chuck around the house during winter? Need a quick glider fix? Man-o-Straw takes just a few minutes to build out of paper and straw. It's just a bit of fun. However, on another level, you can use it for prototyping larger models. Since it can be made quickly, the design provides a convenient way to explore aerodynamics and to experiment. You can try out wing planforms, mess around with dihedral, polyhedral and wing tips. Most importantly you can experiment with tail moment length, rudder and stabliser sizing. And the kids love it (including the adult ones)! Just click on the plan to download the .jpg file:

Get a Load of this! Glide Angle, Wing Loading & Span Loading

You wanna fly model planes, so why bother with all this glide angle and loading mumbo jumbo? Well, because it is helpful and, as Mr Spock famously said, "fascinating". Glide angle is a fundamental concept. Wing Loading and Span Loading are important design considerations that affect the model's performance. In this post, I will explain the terminology and try to describe what it means. There is some maths, but don't let that put you off!

GLIDE ANGLE

Figure 1. Showing a plane in steady glide and how the lift force and drag force relate to the glide angle.

L is the lift force generated by the wing.

D is the drag force experienced by the model.

W is the model's flying weight.

h is the height above ground.

x is the distance travelled.

Glide Angle or L/D (pronounced "ell over dee") is an indicator of a glider's performance. From the geometry of the forces acting on a glider in steady flight, you can see that the glide angle relates directly to the ratio of the lift force over the drag force, regardless of the weight of the plane. The maths is set out in Figure 1 (above). High lift and low drag means a flatter (more horizontal) glide. The plane flies forwards, not down like a brick!

WING LOADING & SPAN LOADING

Wing Loading is simply the weight of the plane divided by the wing area. Span Loading comes in two "flavours". The first is the weight of the plane divided by the span. The second is the weight of the plane divided by the span squared. Unfortunately, both are referred to as "span loading". For convenience, I will call these Span Loading v.1 and Span Loading v.2 respectively.

Referring to Figure 2 (below):

• Wing Loading or WL = W / S
• Span Loading v.1 = W / b
• Span Loading v.2 = W / b^2

Figure 2. Showing the important areas for Wing Loading and Span Loading

where:

W is the flying weight of the model.

S is the area of the wing (in blue in Figure 2).

b is the span, and S = ab, where a is the mean chord of the wing. The red area in Figure 2 above is b^2.

Qualitatively speaking, lower Wing Loading means that the model:

• climbs better, both under power and when gliding: it has a "floatier" glide
• flies slower with respect to the air
• requires a shorter take off and landing for rise off ground models
• is more prone to being bumped around by turbulence

Lower Span Loading v.1 means:

• less drag at lower flying speeds, that is less "induced drag

Lower Span Loading v.2 means:

• better glide performance, that is, a flatter more horizontal glide angle

Taken all together, the above indicates that a light weight plane with big span would be ideal. Just consider a typical full-size soaring glider (or 'sailplane' for readers in the USA). For example, the famous and beautiful Duo Discus. I've flown one of these and it was absolutely lovely. In these glass ships, when you push the stick forward, the plane just whooshes forwards (not downwards!).

Note that you can also reduce the Wing Loading (WL) of a given design by increasing the wing area while trying to keep the weight increase to a minimum. However, if you do this by increasing the wing chord alone while keeping the span the same this may not result in a flatter glide. Hopefully, these intricacies will become clearer after you've read the rest of this post.

LEARNING FROM MATHEMATICS!

I suggest that there are three equations worth considering, playing with and understanding. This is quite rewarding and helps to develop a feel for some basics of aerodynamics generally. They are:

Or, when limited by my keyboard:

L = q S CL ......(1)

Di = q S CDi ......(2)

CDi = (CL^2)/(pi AR E) .....(3)

where:

L is the lift force. For a decent model in a steady glide it is essentially equal to the weight.

q is 1/2(rho v^2), also called the "dynamic pressure". Rho is the air density, v is the airspeed.

CL is the coefficient of lift (which depends on angle of attack of the wing).

S is the area of the wing (see above).

Di is the induced drag force. The total drag is this plus the drag from other sources (skin friction and form drag). At lower speeds, the induced drag dominates.

CDi is the induced drag coefficient.

pi is its usual 3.14...

AR is the aspect ratio, that is the span over chord: AR = b/a = b^2/S.

E is a factor relating to the efficiency of the wing. It depends on the design and shape of the wing, 1 for perfect shape, otherwise less than 1, 0.7 is typical for a rectangular planform.

Eeek! What does all this mean?

First, there are some basic ideas contained in these three relationships.

The lift force depends directly on the wing area. Bigger wing, more lift. When the plane goes faster, the lift increases as the square of the airspeed. That's why jet airliners look as if they have smallish wings in relation to the size of the whole plane. Lift from the wing is greater at sea level than in the mountains, where density is lower. So your model may not fly well if you move to a flying location at a different altitude. The drag increases if you decrease the aspect ratio while keeping all else the same (which goes part of the way in explaining why increasing chord to reduce Wing Loading as discussed above, may not lead to a flatter glide).

Substituting (1) and (2) into (3) gives the induced drag:

Di = L^2 / (q pi E S AR) = (L/b)^2 / (q pi E)

This tells you that the induced drag depends on the quantity L/b squared. As mentioned above, in a steady glide, L is essentially the weight of the plane, W, so L/b is the same as Span Loading v.1. A small increase in Span Loading v.1 increases the induced drag significantly because of the squared relationship.

Dividing both sides by L gives the very important Di/L ratio (often referred to the other way round as L/Di):

Di/L = (L/b^2) / (q pi E)

From the geometry of the forces acting on a gliding aircraft, it is a measure of the glide angle (please see Figure 1 to see why). The above relationship shows that Span Loading v.2 is a key determinant of the glide angle (again using the relationship that L ~ W in a steady glide). The smaller Span Loading v.2, the flatter (more horizontal) the glide angle. Also, the more efficient the wing design (E gets closer to 1.0) and the flatter the glide angle.

Another thing to note from the above equations is that Span Loading v.2 = W/b^2 is just the same as WL/AR (just divide top and bottom by wing area S). This explains the point that I made above that reducing the Wing Loading by increasing the chord alone and not the span, may not flatten the glide because although the Wing Loading reduces, the AR decreases as well. We can now see that it will not flatten the glide unless the ratio WL/AR decreases overall.

CONCLUSION

Summing up, if you want to improve the Glide Angle of your model, then concentrate on reducing Span Loading v.2 and on improving the wing's design and efficiency (airfoil, planform, etc). In free flight, good models also tend to have low Wing Loading, at least for calm conditions.

I hope this gives a flavour of how powerful this stuff can be!

Top Tips for Free Flight model making

Free Flight Aeromodelling is not new by any stretch of the imagination. One of the most celebrated models of the 19th Century was this rubber power job by the amazing Alphonse Penaud called the Planophore or Planaphore, c.a. 1871:
All the modern aspects are there. This was about 20 years before Otto Lilienthal's daring gliding exploits and those of Percy Pilcher. Both aviators died as a result of plane crashes. The Wright brothers began to solve the control problems in the first few years of the 20th Century.

With so much development over a great deal of time, you'd think we may have learned something by now!

Here are five things that may help you to improve your free flight aeromodelling skills:

1. Build light. This means that within the relevant rules, the model should be as light weight as possible. If a model is built so tough that it is crash resistant it will be too heavy to perform really well. Generally glue is heavy. Choose materials wisely. Balsa is amazing in terms of the strength to weight ratio, but there is a great variation between planks as well. I had a balsa board that was as dense as spruce. I've not been able to utilise it in a model plane.

2. Stability is really important. It's no use having the flattest glide ever, if the plane spiral dives to earth whenever it hits a bit of turbulence. Learn about stability. Try to adjust or re-design planes that are not stable.

3. Learn to "pick good air". Get a mylar streamer or a 10 ft piece of old audio tape and attach it to the top of a tall pole. Stand it upwind of your launch point. Watch it carefully and work out what it does as the air changes. It will move around to show the wind strength and direction. Importantly, you may be able to feel cool air when the streamer points downwards and warm air when it points upwards. Thermals are often marked by a rise in temperature, a drop in the wind strength and a change in wind direction. It takes a lot of practice to learn to launch your model into lift.

4. Do try to minimise drag of the plane. However do not increase weight to do so. Putting it simply, at low flying speeds, one type of drag dominates while at high speeds, another type of drag is more important. The low speed one is due to the disturbance created by the model, mainly the wing and is called induced drag or vortex drag. The high speed one is to do with the form or profile of the plane, and its skin friction. Read up on drag and design accordingly.

5. Keep excellent records, notes, plans and sketches. For example, your model's flying weights and "still air" times (taken for flights in the early morning or late evening on calm days) are invaluable resources later. Design ideas can come out of the blue, so it may be a good idea to have a pocket book to jot them down in.

To close this post, here is a photo and some info about a century old, patented, rubber powered canard, by T.W.K Clarke that I snapped from the London Science Museum. I guess it was around 50 inches in wing span. Those years would have been exciting times for aeroplane lovers...

FREE CHUCK GLIDER PLAN!

"Jellybean" 18" Hand Launched Glider - "HLG" model glider or "Chuck Glider"

I enjoyed designing and flying this model glider. The name "Jellybean" comes from the appearance while it is flying. From underneath, the tissue is backlit by the sky and the colours reminded me of jellybean sweets. Use it as a second model, or decent beginner's competition HLG. It can be fairly competitive - I've 1 minute maxed with it quite a few times and thankfully, it has not yet flown away!


No DT, but one could be fitted (e.g. side or top flap made from an aluminium drinks tripped by a button timer).

I haven't figured out how to make and attach a .pdf file, but hopefully, you should be able to copy and save the above .jpg file on to your computer and use some image manipulation software to enlarge it.

If you try this model, I hope you enjoy it as much as I have.

The photo below shows some of my other designs:

• Dogchew 6" span HLG (for plan and build notes click here)

• Oompah 16" span HLG

P30 - Ideal for Improvers

Did you enjoy building and flying your first rubber powered model? Are you hungry for more? A P30 class model may be the perfect answer. This class is a few decades old and is very popular. It suits the developing modeller and also provides a real challenge for experienced fliers. The rules vary slightly country to country, but in essence are:

1. Wingspan and overall length each less than 30 inches.

2. Plastic commercially available non-folding prop less than 9.5 inches in diameter.

3. Rubber to be enclosed within the fuselage.

4. Rubber including lubricant to weigh less than 10g.

5. Model without rubber to weigh 40g or more.

These things can fly! Two minute maxes are not difficult to achieve. Flyaways are common, so you need a good dethermaliser. The minimum DT is the pop up stab - see the photo at the bottom. While these are ok, they may be insufficient to bring the plane down from a strong thermal. More drastic DT is the pop off wing. The wing tip is connected with a line to the rear end of the fuselage. When the DT releases, the wing comes off. The fuselage hangs down and the wing spirals around as the plane descends.



These are photos of my first P30. It is based on the Roger Dodger design, but I could not help modifying it in various ways.

The photos also show a simple design for a "stooge". This is basically an anchor point to hold the plane while you wind the rubber with a mechanical winder. It is also advisable to use a "blast tube". Mine took 5 minutes to make out of a piece of 20mm plastic piping that I had lying around.

Below is a brief video of the model at the top of the power climb. It flew OOS on that occasion, but was later recovered (thank Hung I wrote the phone number on it!) That happened because I set too long a time period on the DT. Doh!

Free Flight models can fly OUT OF SIGHT!

Flights that go OOS (out of sight) create mixed feelings. In competition, they can mark the end of your flying day. In recreation, they are a little upsetting, but in an odd way satisfying. After all, the model has performed very well to stay in lift for so long. There is a theory among aeromodellers that the thermal God, known as "Hung", takes only the best models away. This is so that good people can play with them in the afterlife :)

Here are some tips:

1. WRITE YOUR MOBILE NUMBER CLEARLY ON THE FUSELAGE AND WING

It is worth it! Two of my models have flown OOS in the last year. One, an 8" span HLG, was found months later a few miles away in the woods by an old lady out walking her dog. The other, a P-30 rubber plane, was found about a week later in front of a house. It must have been on the roof for a while, because the prop was still stuck up there. In both cases, the finder rang the mobile number and the models were back in the air after just a little repair work.

2. ALWAYS GIVE THE FINDER A LITTLE GIFT

Chocolate is good, wine for adults is ok too.

3. FIT A DETHERMALISER.... OR BUILD LOTS OF MODELS!

DT's are a subject in themselves. Examples are:

• Pop up flaps (side or top)
• Dropping weight
• Dropping streamer
• Pop up stab
• Pop up wing
• Pop up tailboom, also called "broken back"
  
• Pop off wing

However, even if you have one, it does not mean that it will bring your plane down safely. Things can go wrong. For example, the DT may not be powerful enough to make the model descend out of strong thermals. Or the DT may fail to operate. Some people just don't bother with DTs, but instead just make lots of models. If you are into competition, I think you need to learn how to make and fit DTs and use them consistently.

Make your own Propeller

Why?

Why bother with carving a propeller when you can buy a plastic one cheaply? Surely a balsa prop would break easier than a plastic one, I hear you say. Perhaps that is true, but I was surprised by the robustness and lightness of my first self-made prop. Less weight spinning around means easier trimming, not to mention a lighter model.

How?

I followed the inspirational instructions provided by Dannysoar here. I did not have a block, so laminated bits of 6mm balsa that were lying around with PVA glue. One piece was a "dog" because it was so heavy. It is almost as dense as spruce and I could not find any use for it in my models. So I used that piece as the topmost sheet, that is, to be the front or the leading edge of the prop. Also, I was careful about aligning the grain of each sheet so that one piece was not running opposite. This prevents "snagging" during the carving. I sawed the block into triangles, marked and glued it (photo above) this time with epoxy. Then started carving with a normal craft scalpel. Balsa cuts easily and it was really quick.


The photo on the right shows the initial carving. Basically, you carry on, try to keep it even, then shape the tips. Every now and then, put a piece of wire through the hole and check the balance of the prop. Remove material from the heavier side. Carefully glue some tubing in the hole, check balance, sand and smooth it off. You will need some form of free-wheeling device. I used a simple Z bend clutch - called a Garami. This prop was for my Cloud Tramp. It flies much better than it did with the plastic prop.

The photos below and on the left show details of the Garami freewheel clutch and the finished prop installed on my Cloud Tramp. As a final job, I superglued thin cotton thread to the LE for protection. So I've found a use for that heavy balsa after all! Even though this laminated prop had lots of glue in it and the heavy bit of balsa, it was still 20% lighter than the plastic prop. Wood is a difficult material to beat...

It is a very satisfying thing to do and much easier than I expected. Since the pitch of the wooden prop is higher, the duration of the power run is longer.

I think it also looks prettier.

Trees

Trees attract balsa. Even if there is only one tree around, you can guarantee that your model will end up in its highest branches. Here are some tips for recovering your work from the clutches of a "balsa magnet" otherwise known as a "tree":

• Don't fly where there are trees!

• It is not worth risking your life to get it down, even if did take a long time to build. This means: do not try to climb the tree

• Watch the model carefully. Binoculars are useful. Note where the model landed - not just which tree, but where in the tree

• For lower level stuff, use a 6m telescopic fishing pole. In England, these are called a "whip". Buy from eBay or online for just a few pounds. Put some white tape at the end, so you can see the tip. Use it gently, or you may put a hole in the wing!

• For higher level stuff, use the ball and line method. This involves looping strong fishing line over the correct branch, taking hold of both ends and shaking until the model falls out. To get the line up there, attach a weight to it. A heavy-ish ball the size of a tennis ball is good, because you need to be able to throw it accurately. Throw it as close as you dare to the model over the branch that it is snagged on. The fishing line should be heavy duty (at least 10kg (approx. 20lb) strength). Attaching the line with loads of plastic parcel tape to a tennis ball can work fine. I used it to get a plane down from the tree in the photos here..

The plane was stuck just next to the ball that you can see here. It was at about 10m high. This is a lovely oak tree. However, we got down eventually, with no harm to tree or any person. I'm afraid, a ball was left behind, but it did fall out later.

Jargon Busting!

Ah, Jargon! Love it or hate it, there is a lot of it. Some is really important to know. For example, bits and pieces of information that relate to "trimming", that is, getting the best performance out of your model. Other terminology is more esoteric, and arguably, not necessary for you to enjoy the sport of Free Flight (and it is officially a "sport" in the UK). I'm not proposing to explain all of the jargon. Instead, I'll list a whole raft of it now. Later, I'll explain some of the important terms, or at least give you a link to more information. So, in no particular order, here's a list:

1. Lift
2. Drag
3. Best L/D
4. Min Sink
5. Polar
   
6. Centre of Gravity
7. Centre of Pressure and Aerodynamic Centre
   
8. Angle of Attack
9. Angle of Incidence
10. Dihedral
11. Decalage (sometimes called Longitudinal Dihedral) - great diagram here
12. Moment Balance
13. Stab Up, Stab Down
14. Stab Tilt
15. Stab Twist
16. Stall
17. Sideslip
18. Dutch Roll
19. Wash Out
20. Wash In
21. Tip Weight
22. Wing Offset
23. Wing Planform
24. Aspect Ratio
25. Wing Area
26. Wing Loading
    
27. Chord and Mean Aerodynamic Chord (MAC)
    
28. Aerofoil section
29. Camber
30. Thickness
31. Winglet
32. Dethermaliser, or DT, or Dethermalizer
33. Span Loading (two flavours)
    

Watch out for "those that prowl in them thar Parks!"

Because, this is what can happen to your model aeroplane!

Cloud Tramp

My Cloud Tramp. This 22" span classic is one of the rubber models that I would recommend for a beginner (see previous blogs). It is fairly easy to build. Powered by six strands of 3/32" rubber (three loops), it climbs beautifully especially in lift. This gives a motor run of about 50 s. Best flight so far is around 90 s. It's OK under power, but the glide is not as flat as a decent hand launched glider.

And here is a photo earlier in the build of the fuselage, undercarriage legs and stabiliser: