After many years spent flying in Kent, I retired to Dorset and joined the Phoenix MFC, a very friendly and well-organised club. Electric gliding has proved to be pretty popular here, and having competed in many glider events up and down the UK over many years I decided to design a model that fitted the bill for the clubs electric thermal competition. So, Apollo was born, and to date I've built 20 of them for club members, the criteria being minimum cost and easy build techniques using basic materials (with the exception of carbon spars for improved strength). My choice of wing section is a modified Selig Donovan 3021, as this gives good penetration under most conditions but will slow down and perform well in calmer air. There are probably many readers who haven't experienced the joys of thermal flying, so lets have a quick look at what its all about.
A thermal is a rising current of warm air, a natural phenomenon that can be exploited by the correct type of model to achieve outstanding flight times. You may have witnessed a thermal when a small piece of paper gets tossed around the air, even though little or no wind is present. This free product of nature can be elusive to modellers however, and finding a thermal can be a challenge in itself, let alone making the best advantage of it.
Ok, you've launched your model, climbed to a good altitude and powered the motor down to serenely waft around the skies. You cant see a thermal, so how do you know that you're in one? You'd probably expect to see the model immediately lift without touching the controls, but its more likely that you'll first see it waggle its wings as it comes into lift - watch the model closely at all times when hunting thermals, small indications such as this could lead to a real boomer!
The model may be flying in a straight line when a wing tip suddenly raises - this is an indication of being on the very edge of lift, in which case turn towards the raised tip and into the lift. Fly 30 - 50 in a straight line and turn either left or right to try and stay in the warm air. Should you fall out of the lift and be unable to relocate it then fly a traversing pattern crosswind, to see if you can find more. Of course wind has a bearing on proceedings, as the position of thermals will vary with wind speed and direction.
Ah, the simple pleasures of thermal soaring!
Other places to look for lift are along a line of trees, or above a slope where you may find ground effect thermals. And keep an eye out for the birds, too - if you see them soaring and circling then there's some lift to be found. Generally speaking there'll be more lift on a bright, cloudy day than in cloudless conditions. Particularly under dark clouds, which can yield extreme amounts... too much on some occasions, and indeed some very fine, strong models have been lost through being literally sucked up, the resulting forces tearing the model apart in mid-air. Scary indeed!
So, what if you're in a thermal and want to get out? First and foremost, simply try to fly away from it. If you're having difficulty then try to disturb the models flight pattern by looping or spiralling. This is all well and good at an altitude where the models reasonably visible, but if you're unprepared for such an event at high altitude then you could be in trouble. Far better to practice such an escape at lower altitude (not too low, of course!) so you can put this into successful practice should you get caught out.
There you go then, a whistle-stop introduction to basic thermal flying. It can be challenging and addictive, and at some time you may even fancy pitting your flying skills against your clubmates. The electric thermal competitions we run at the Phoenix MFC are well supported, and the Apollo really is a superb model to use, even though I do say it myself. Have a quiet word with your competition secretary, and maybe your club can join in the fun!
Construction begins by making the rib-cutting jig and cutting the required number of basic ribs as oversize rectangles. Then, five at a time, drill both the 10mm and 3mm holes using the rib template for alignment. Place each rib blank individually on the cutting jig and cut the rib profile around the template. Repeat this operation for the tip ribs, then cut the centre rib and the four dihedral ribs.
To build the centre-section, first cut both the 10mm and 3mm carbon tubes to 1m in length. Slide the centre rib onto both tubes, position exactly in the middle, and fix with cyano. Lay this assembly over the plan, slide on the ribs, place the dihedral ribs at either end and cyano all in place, letting the glue run around the carbon tubes. Incidentally, its a good idea to weigh down the assembly at this stage.
Choose the lightest wood you can find and aim for a max AUW of 2.5lbs.
Next, fix the leading edge (l.e.) and trailing edge (t.e.). The t.e. slots can be marked and cut using four hacksaw blades glued together, which gives the correct size slot for the 3/32 ribs. Keep the assembly weighed down until complete. Bevel the dihedral ribs and put the centre-section to one side. The outer panels are constructed in a similar fashion, up to the tip. Glue the wingtip to the end rib (R5) and put the l.e. in place, pulling the soft-to-medium section around to meet the wingtip. Fix with cyano, then mark and cut the t.e. in the same way as the centre-section, whilst also cutting 1/8 blocking pieces as per the plan. Bevel the dihedral rib and put to one side. Bend the wing joiners using 18swg and 12swg wire to the angle shown on the plan.
Okay so far? Good! To assemble the wingtips to the centre-section place a piece of closed-cell foam inside the 10mm tubes and push down to allow enough space for the joining wire to have free access into the tube (the foam is there to prevent epoxy from running right down the tube). Before joining the tips to the centre-section (51/2 tip dihedral, as per the plan) wrap insulation tape around the two ends to be joined to give a nice, clean joint. Remove the tape when the epoxy has just set, and repeat this procedure for the other tip.
Cut the tailplane from light, firm 1/8 sheet balsa, drill the lightening holes as shown and then sand the parts to shape, rounding the leading edges. Bevel the elevator leading edge sections and shape their trailing edges to 1/16. Pin the centre-section down and bevel the two tailplanes to give the correct angle of 110°, as shown on the plan. Place these against the centre-section at the correct angle and cyano together. Remove from the building board and turn upside-down, then cut the two slots across the assembly to the required depth and slide in a piece of 1/16 birch ply. Mark the shape, top and bottom, then remove the ply and cut to shape before replacing and securing with cyano front and back.
Locate the 1/32 ply onto the centre-section, glue in place, and drill the holes for the fixing screws. Cover the assembly and elevators (Easi-Cote is a good choice), then join using diamond tape top and bottom to form the hinge (the plastic angled horns are fitted later).
Scarf together 1/8 balsa sheets to obtain the required length for the fuselage. Cut the fuselage sides and mirrorlite ply doublers to shape, and glue the doublers in position. Add the top and bottom stringers and the triangular pieces, being sure to make opposite sides! Cut ply former F1 (located at the leading edge of the wing) and the parts for the motor tube and block; these latter assemblies are explained on the plan, whilst a photo of the motor tube being rolled can be found over the page. I used cyano to construct the tube, indeed this has proved to be an extremely strong and reliable method that's capable of surviving heavy impacts.
With all the parts to hand, lay the right-hand side flat on the bench and glue F1 into position, followed by the motor tube and block. Place the left-hand side directly on top of this, then glue and clamp together with the assembly base flat on the bench and the sides upright. Leave to dry. Pull the end of the fuselage together, placing 1/8 square spruce between the two sides for the length of the tailplane seating, then position the 1/4 liteply fixing plate. Next, place the fuselage assembly on top of the balsa base and glue, making sure the fuselage is straight; weigh down until dry. Tack-glue the front top sheet onto the upper part of the nose and shape down to include the hatch and former F1, then cut across to form the removable hatch. Carefully remove the hatch and fit the catch and front location pin.
Place the wing assembly into position, against the rear of former F1, and mark the centre positions of both wing and former. Drill the wing to take the retaining peg and drill a corresponding hole in F1, then fit the peg into the wing. Place the wing retaining plate in position, drop the wing back into place, locating the peg, and mark the spot for the wing fixing hole. Drill through, remove the wing and fix the metal retainer for the nylon bolt. You can make up the thickness of the retaining plate with 1/8 balsa. So far so good. Replace the wing and tighten the nylon bolt, then place a block at the front of the wing above the peg and fair into the fuselage at F1. Fix this to the wing.
Right then, while we have the wing attached, lets firm-up the tailplane location. Holding it in position, check the alignment with the wing, adjust if necessary, then drill through for the fixing screws before bolting it down. Remove the wing and put aside for covering.
Moving to the back of the fuselage and the rear hatch, cut out the servo tray and the former for the pushrod tubes, and glue into position. Open out exit slots in the fuselage sides to take the pushrod tubes and fit them in place. Fix the servos in position, line up the 20swg wire pushrods, put a Z bend on the servo ends and fix them to the output arms. Next, check that everything's lined up, locate 1/8 balsa spacers to hold the tubes straight and glue them to the fuselage at their exit points. Place a length of 1/8 balsa sheet on the top of the fuselage from the front of the tailplane to the wing t.e., cut the hatch as shown on the plan and glue the rear section of the fuselage, shaping and rounding off the fuselage sides when dry. Add the tab to the hatch and drill for the retaining screw.
Remove the tailplane and cover the fuselage, hatches and wings. Easi-Cote is an ideal material here, though you'll doubtless have your preferred versions; a red or blue scheme plus white flashes towards the tips of the upper wing surface and fluorescent yellow flashes on the lower wing surface make for good visibility when Apollos at altitude. Replace the tailplane and make the fit final by gluing and screwing into place. Fashion Z bends at the control surface end of the pushrod wires and fix the plastic angled horns (available from Pete Tindal's Airplanes) to the elevators, making sure the arms on the servos are at neutral and the elevators are level with the flying surface.
With the airframe complete, Apollo can now be fitted with a Permax (or similar) 600 7.2V motor, which gives good results on a 7 x CP1200 cell battery pack and Graupner 9 x 5 propeller set (this includes the spinner).
Solder on the suppression capacitors and a 35A (minimum) BEC speed controller / switch assembly before fitting the motor into place. Rather than fit a connector, use about 2 long leads from the ESC direct to the motor as the connector will only get in the way and will be a possible weak point in the system. Slide the battery pack into the fuselage noting that on the prototype I fitted EPP foam either side to form a tight fit. Incidentally, I found the C of G to be just about spot-on with three cells of the 7-cell pack forward of F2. The R/C installation is completed by installing the Rx (a small, square unit such as the GWS 8-channel is ideal), the aerial of which is fed down the fuselage to exit out through the rear end. As a finishing touch the Apollo logo can be applied to both the wing and fuselage using decal sets that are available in a variety of colours from Plus-Decals - see panel.
The day of the maiden flight was set fair, with a reasonable 10mph wind blowing. After charging the batteries and making final checks there was nothing left to do other than launch Apollo into the wind. The climb rate was excellent for a model of this type, especially considering the low-cost powertrain used. Turns were very nice and the stall was very gentle; early on in these explorations I realised the model was climbing in lift and enjoyed myself for 14 minutes of flight, extracted from a 1.5 minute climb. The second flight was a very similar experience, and was exactly what was required. The collection of gathered club members were suitably impressed, and all of them expressed a wish to own one. Subsequently a number of Apollos were built, and have been flown on a regular competitive basis for the last 18 months.
Shes very capable indeed, with the longest flight to date being 47 minutes following a 1-minute climb! As far as climbing is concerned, the installed set-up will give a 23/4 to 3-minute run... more than adequate, as this will take the model out of sight. Shes quite well mannered, too, and I see no reason why a beginner shouldn't be successful with Apollo, providing he or she has adequate tutoring from an experienced pilot. The V-tail configuration requires mixing, which may not be available on cheaper transmitters, however GWS supply a dedicated on-board V-tail mixing unit that will overcome this.
Before I leave you to rush off to your building board, a word of advice. Although the wing is extremely strong and thus far hasn't broken on any of the current models, its best to loop or spiral the model down from a great height rather than drive it down at great speed. So, that's it folks. Whether you choose to build your Apollo purely for yourself, or as part of a friendly club organised competition, I sincerely hope you enjoy it. Truth is, I know you will!
Electric-powered thermal glider
: Pete Kessell
4.9sq. ft. (0.45sq. m.)
: 7 - 8oz / sq. ft.
2.2 2.4 lb (1 1.1kg)
Permax 600 7.2V
7 x CP1200
Propeller: Graupner 9 x 5''
- IMPORTANT Please note, Pete has advised us that the original plan was published showing positive incidence on the tail which should not be the case. The plan should be amended but builders should be aware. The tail seat should run parallel with the bottom of the fuselage. This article was originaly published in RCM&E May 2006 and so the powertrain items mentioned may no longer be available. Cheap (brushless motor and Li-Po battery) power systems have since proliferated since the article was published and this would be the suggested route - Ed.