Multiplex EasyStar Review: Part 1

A few weeks ago, I found myself feeling a bit fed up with my GWS SlowStick - pictured. Not because of the plane - I'd recommend it to anyone, it has a loyal following for a reason. No, it was the realisation that I prefer to fly it with the engine off! I would climb up, switch off that buzzing prop and then fly about looking for lift. I'm a glider junkie, it's true. Plus, in a bit of wind, she didn't have the oomph to make much headway with the stock brushed motor. So, it was time for a change. I could upgrade to LiPo and brushless, but that wouldn't solve the constant "I want to glide and thermal" desire in my bones. I wanted to take the battery and radio gear off the SlowStick and re-use it quickly on something else. So, what to do? The Multiplex EasyStar ought to be suitable, because it:

• looks like a glider, so should soar reasonably well with the motor off!

• has a great reputation as an ideal newbie's plane, also pre-fabricated

• comes with a brushed motor and is 3 Ch

• can be upgraded, and modified for aerial photography

• should handle a bit of wind better than the SlowStick (including better penetration)

• has loads of resources and info about it on the internet

Eagerly, I ordered one online from rcmodelcentre.co.uk. When the box arrived, I was like a kid opening it up. However, to my horror it had been bashed in transit and the photos below show what I saw.

Oh ,those beautiful wings, so crushed and crumpled!

Thankfully, Mike at rcmodelcentre.co.uk was very good about it and sent a replacement very quickly. The pack included the wings, fin, stab, fuselage, canopy- all in Elapor foam (not regular foam like Depron, but Multiplex's stuff), pushrod wires, plastic tubing of various diameters, a main spar tube, horns and other small parts, a bit of velcro, motor, prop, spinner, a decal sheet and some thorough, well illustrated instructions.

So, EasyStar is the name, but just how easy was she going to be to build and fly? In Part 2 of this Review, I'll tell you the rest of the story.

FREE MODEL GLIDER PLAN: QUEEN BEE 8" Tiny glider

This is a conventional pod and boom design with V dihedral. A notable feature is the use of natural carbon fibre for the boom. Natural carbon fibre? Yes, I mean kebab skewers! These can be bought for peanuts (satay sauce?) from a supermarket or online for less than 2 pence each. Search through a pack and you may find a decent number that are straight, stiff and suitable for building with. Any hard outer skin on the cane should be positioned underneath the boom. The wonky ones can be used for, you guessed it, kebabs! Of course, if you cannot find real bamboo, please do feel free to substitute with carbon fibre instead.

For ease of construction, the wing has a straight high point. In the normal HLG manner, after the wing has been shaped, finished and cut, the ends are bevelled and glued into the correct V dihedral setting. A matching V is sanded carefully into the fuselage top - I make a tool for that using a coffee stirrer, balsa and Al-ox paper. Use epoxy for the dihedral joint and wing-fuselage junction. After the wing has set on the fuselage, you can cyano thread to the LE. It may act as a turbulator, but even if not, it protects the LE. I could not detect a change in performance after adding it. That said, if you leave the LE bare with no thread and it receives a bang or dent, then you can sponge it out with a tiny bit of water. It is really up to you.

The QUEEN BEE plan shows the original dayglow yellow and black stripe colour scheme. Finish in the same way as DOGEAR in the previous blogpost.

For some previous notes about flying this model click here. My "still air times" (as described in the DOGEAR blogpost) for QUEEN BEE were in the range ~28-30s.

FREE MODEL GLIDER PLAN: DOGEAR 6" Tiny glider

This is DOGEAR, a development of DOGCHEW (see previous blog post, again, just click and save the plan). DOGEAR differs mainly in the wing design. The planform is elliptical at the front with a basswood leading edge (or use lime wood which is also genus Tilia). Trailing edge is mostly straight with single taper near the tips. The wing is also thicker, 2.5mm (~5%) compared with the 1.6mm of DOGCHEW. By feel, I carefully sand a small "Phillips entry" under the LE, but the rest of the underside is flat. It is a typical HLG aerofoil. The wing is assembled as a straightforward three panel structure using epoxy at the breaks - the tips are raised by 15mm from the flat centre.

The three panel wing and the fact that dogs appear to find my models "tasty" were the reasons for the name DOGEAR.

Fuselage is laminated using pva (e.g. Titebond II). The thin glue layer increases stiffness and strength a great deal. A lighter way is to use spots of cyano. Alternatively, forget about the laminating completely and use hard 1/8 balsa instead. Please do feel free to experiment with nose length. In other words, cut it shorter if you wish. The size shown on the plan seemed work well in calmish conditions. I use epoxy for the wing fuselage joint and throw tab. Fin and stab should be sanded as thin as you dare, so that you can breathe and bend for trim.

My finish is the traditional 2 coats of thinned down sanding sealer with very fine Al-ox paper between coats. I finish the wing with sanding sealer before it is cut into three panels and the stab and fin before they are glued to the fuselage. A splash of dayglow colouring helps enormously for visibility. Apply Al-ox paper grips on the fuselage sides. In the photo below, you can see some inked stripes that also help to see the plane against the sky (er, I mean clouds, cos that's wot we av ere most in England).
By my reckoning, DOGEAR will stay up for about 5s longer than DOGCHEW in "still" air; by that I mean in very calm conditions outdoors early morning or late evening. I recorded consistent times of ~22-24s for DOGEAR in such conditions (I am sure a lighter version would do better).

FREE MODEL GLIDER PLAN: DOGCHEW 6" Tiny glider

The Small Flying Arts site has changed dramatically. I think they are in the process of transferring all the plans and articles over to the newly designed site. However, the plan and build notes for my DOGCHEW design are currently unavailable there. So I thought I'd publish them here. In future blogposts, I'll also publish some other tiny glider plans. Click and save the plan below. The build notes, which I wrote in 2006, are shown in full below the plan.

Build notes for DOGCHEW 6" glider

These notes accompany the plan for a simple 6" span free flight HLG. Small gliders do not have much momentum so tend not to break easily. Another advantage is that they can float on a whisper of lift. You get maximum fun with minimum fuss!

Now this is the first 6" glider of mine to fly over 30 seconds. When I was testing the prototype in a local park, there was nothing for me to do but laugh while I watched a "friendly" Dobermann Pinscher pick it up tenderly with its jaws, manouvre it about and then crunch it, very gently. I said to the dog-owner: "Well, I guess it looks a bit like a stick." He replied, "You’ll be needing a bit of glue then. A pity, cos it flew really well." I thought "yeah FLEW." At least the dog seemed to enjoy the flavour of dope on balsa. I spent the lonely walk home carefully spying the ground for other deeds by my new best mate Dobie….

That is why this design is called DOGCHEW. Its best flight so far is about 63 seconds and still air time around 18 seconds. In the 2006 Tiny Gliders postal contest (see http://www.windandwavemodels.com), average flight time was about 32 seconds.

It is a straightforward build. The plan is to scale, so you may find it easiest to use a photocopier or scanner to enlarge or reduce it in accordance with the fuselage dimensions.

Instead of laminating the fuse with PVA, you can use spots of CA – this is lighter. Alternatively, forget about the laminating completely and use hard 1/8 balsa instead. The wing is made in the usual HLG way. Make a template from the plan. Use it to cut out the whole wing planform, sand in the airfoil with the high point as per the dotted line and apply 2 coats of sanding sealer. Then cut the wing in half, sand the dihedral angle at the root chord and glue together with epoxy. Sand a “v” in the fuse for a wing seat and use epoxy here and for the throw tab. Fin and stab should be sanded as thin as you dare, so that you can breathe and bend for trim. (The prototype had trim tabs, but these were dropped in the final design because they were easy to break). Apply sandpaper or similar for grips on the side of the fuselage.

Feel free to play around with the length of the nose – e.g. chop it shorter if you like. The length shown on the plan seems to work well in calm conditions. Also, you may find it beneficial to sand a bit of washout under the starboard wing tip TE.

Using gentle glides from the shoulder, trim for a left turn, with a slightly “stally” glide. Full power throws are to the right of the wind with a slight right bank. As with most HLGs, avoid throwing with less than full power! Add left rudder if it stalls in the glide, but not too much otherwise it will spiral dive. When properly trimmed, it should transition at the top quickly and start turning nice flat left circles.

Thermal in peace….. :)

Berkshire, U.K., 2006

Silly Putty Timer DT (Dethermalizer)

The silly putty timer has been around for well over a decade in free flight aeroplane model making.

Over the last few months, Tony Mathews, free flighter extraordinaire, has been experimenting with micro versions of the timer. He has come up with a timer that is amazingly small - 1/8" x 0.7" (3 x 18 mm) - and light at about 0.3g.

Here is a photo of his excellent Stinger 8" CLG with a micro silly putty timer installed. It operates an aluminium drinks can foil DT. Notice that the foil is on the inside of the circle turn, causing a steep downward spiral. When I saw that, my initial reaction was more "Death Trip!" than De-Thermaliser! Encouragingly, he reports that small gliders like this suffer no damage on DT deployed landings.

Tony has put together an absolutely superb pictorial build guide here. You must look at it. I've made some timers according to his plan and found them to be quick and easy to build. Cutting aluminium tube with a scalpel is fun! You need to roll the blade over the tubing to make one cutting groove around the circumference and perpendicular to the axis. As you continue to roll, it then pings off at air pistol speed, so one trick is to thread some wire or string through the aluminium tubing before cutting it, to catch the piece.

I have some ideas to try to improve the consistency of the timer, which is not as good as a commercial viscous button timer. If I find success I will report it.

Here is more background on silly putty timers. Useful sources of information include articles by Peter Michel (on Ramon Albon's great free flight site) and Pensacola free flighters. Thayer Syme's super page and diagrams dating back to 1996 are here. Martin Gregorie has posted some useful and detailed information here.

Drag explained for model aeroplanes

This is my attempt to provide a simple explanation of drag for the model builder.

DRAG is the force that resists the forward motion of an object through the air. For model aircraft, there are essentially two types of drag (refer to Figure 1 which is a plot of Drag force against air speed, v):

1. PARASITE DRAG is the resisting force that is due to the shape, roughness and form of the aeroplane. As the model moves through the air, the flow is disrupted by the shape and texture of the model. Its magnitude is given by the expression shown in Figure 1 (red, think of A as a constant). Parasite drag increases as the plane speeds up. Think of it as "High speed drag".

2. INDUCED DRAG is simply the drag force caused by the circular flow of air around the tips of the wing, that is, the vortex. An alternative name is "vortex drag". To get an idea of what it "looks" like, just search on wake turbulence or wing vortex in Google Image Search. Notice also the dependency with the lift force squared, L^2. That is why it is also known as "drag due to lift". So, the induced drag also increases with lift, for example with increasing angle of attack. Its magnitude is given by the expression shown in Figure 1 above (black, think of B as a constant). Since it is inversely proportional to v^2 it means that as the model speeds up, the induced drag decreases. Conversely, as the plane slows down, induced drag increases. Think of it as "Slow speed drag".

The total drag is the sum of the parasite drag and the induced drag (see blue curve in Figure 1 and Equation (1)).

Note for readers who like a bit of mathematics: Equation (2) is the "drag polar". To understand where it comes from you would need to look at an aerodynamics book (see References). The full expression for Equation (1) is Equation (6). You can easily derive Equation (6) by substituting equations (3) and (4) into (2) and recalling the relationship between aspect ratio (AR), span (b) and area (S). Equation (5) is the usual expression for dynamic pressure (q).

So much for the theory...

What does this mean for model aircraft performance and design?

1. It is generally desirable to reduce drag. Lower drag means a flatter glide (see the discussion of glide angle in a previous blog post here). In other words a higher L/D, which is a key performance indicator. Modern open class gliders can achieve L/D of 60 or better. In contrast, the Wright Flyer of 1903 had L/D of 5.7.

2. Mathematically, Equation (1) means that the total drag is a minimum at the air speed where the parasite drag equals the induced drag. That is the air speed where the best L/D is achieved. So if one of these drag contributors (parasite or induced) is low at that speed, then the total drag will be two times that low number. While that's a good thing, the plane's behaviour may suffer at one or other extreme of speed.

3. Take for example a small span pylon racer model. It has a small cross section area, smooth, clean, polished surfaces and therefore low parasite drag. Due to its high wing loading, it will have a high induced drag (this will be explained further below). Since it is fast, it will generally fly on the right hand side of the drag-speed curve shown in Figure 1. The good news is that parasite drag is lowish for this model, so it will perform fine under normal operating conditions; it is not impaired by the high induced drag.

4. For slow flyers for example, thermalling gliders and free flight rubber planes, induced drag is much more important. At low air speeds, parasite drag does not have any appreciable influence - this is the left hand side of Figure 1.

5. For many types of aircraft however, both parasite drag and induced drag should be minimised, for instance hand launched gliders and catapult launched gliders. These travel quite fast on release, so low parasite drag means a higher launch. After the transition to glide, they fly slowly, so low induced drag is required for a flatter glide. Another example is the RC glider. It needs to glide well at slow speed in order to climb in thermals. Then after the climb, it needs to be able to glide at shallow angle to cover lots of ground with little loss of height, in order to catch the next thermal. Induced drag is very important for free flight models too, including rubber power.

Reducing Induced Drag and Parasite Drag

6. The wing is the biggest contributor to both induced drag and parasite drag. So concentrate on the wing before the fuselage and tail feathers!

7. The biggest factor for reducing induced drag is the span loading W/b. This comes from Equation (6), noting that L~W, and see also previous blog. It is not as simple as just increasing AR for reasons explained in that blog post. (The over-emphasis sometimes placed on increasing AR to reduce induced drag probably arises from the dimensionless expression in Equation (2) above).

8. That means keep her light and make her span as big as allowed!

9. Another factor to reduce induced drag is wing planform design (elliptical and similar shapes are good). Non-planar surfaces can also reduce induced drag compared to a same span planar wing. For example, winglets, polyhedral configurations and span-wise camber. Some efficiencies can also be gained from multi-surfaces (e.g. boxplanes), but there is obviously a parasite drag and weight penalty.

10. For reducing parasite drag the biggest factors are the apparent cross section area and the wetted area (the area of the plane that is in contact with the air). Keeping the fuselage as narrow and small as possible is a good start. Sharp corners and junctions between wing and fuselage could be smoothed or "filleted" to reduce the drag. Surface roughness also plays a part (but its not as simple as smoother the better: sometimes a rough surface can keep air flow attached to the wing - "turbulators" are a PhD study on their own!).

11. Note that adding weighty fairings and cowlings in an effort to reduce parasite drag could be counter productive because it may increase induced drag! Fairings and such like may help the high speed flight, but could ruin the low speed glide.

CONCLUSION

I hope this blog has helped you to understand drag. Think about what your plane will be doing most of the time. Flying fast or flying slow? What kind of drag would be most relevant to your model? Having decided that, work to reduce the predominant source of drag. However, concentrate on the wing first. As ever, weight is a major factor especially for induced drag.

REFERENCES

1. Anderson J D (2005) Introduction to Flight, McGraw Hill, 5th edition

2. Simons M (1999) Model Aircraft Aerodynamics, Special Interest Model Books, 4th edition

3. Kroo I (2001) Drag due to lift: Concepts for prediction and reduction, Ann. Rev. Fluid. Mech 33:587-617

Queen Bee 8" HLG: A simple hand launched glider

Nothing in aeromodelling gives me more pleasure than flying the humble chuckie. Lately, I've been messing around with RC: gliders and electric planes. They're fun, but for some reason, the enjoyment can wear off. I always come back to the chuck glider. Photos here of a simple model that I enjoyed flying yesterday and today, in the light wind and sunny blue sky of September.

Best flight was a 66s today, but alas, that was not one I'd declared for the tinygliders postal competition


I guess it's obvious why she's called QUEEN BEE. She has a bamboo fuselage and simple wings.

Another thing. Chucking gliders is great exercise. I can tell that it has been a while since I did some throwing, because my body really feels it - especially the bicep and shoulder!

Yesterday, I lost WEE BO WEEVIL. It went OOS at ~210s. The DT had triggered - I could see the elastic hanging, but the wing failed to pop up.... :( All the more upsetting because the DT worked perfectly for maxes earlier that day.