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Jan 19, 2013, 04:18 PM
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Build Log

FMS T28 Part 3 sound and other systems


Part 3: The sound (and other) systems

Only 6 days after I ordered the MrRCsound system, it was delivered across the Atlantic, for less than 150 Euro, airmail included. It consists of the basic budget system B (with Wright Cyclone sound) and a pair of extra 1,22” speakers. I opted for that layout because of the models’ limitations in CG, limited available space facing down due to retracts and wing attachments, and the possibility to hide the small speakers in the engine exhaust stripes. Straight out of the box the complete 3 speaker system is very compact.

The speakers are well protected in solid casings, and the black electronic parts on the sides can be easily removed if necessary. The tiny heat dissipater indicates that contrary to the stock system in the Art Tech T6, this system efficiently converts power into noise instead of producing exaggerated heat. The adjustable potentiometer arm sticking out will be Dremeled off once the sound volume has been adjusted. A dry fitting on the bare fuselage was made to find out about necessary wire lengths, also for system power to be tapped between battery and ESC connection.

After careful considerations and looking at the pictures of the finished airplane I took a fresh knife and carved out the openings to receive the small speakers. I didn’t dare to open their casings to find out if I could temporarily disconnect the wires, and choose to make the openings conical (wider inside as out) so I could insert the wired speakers from the inside against an outside grille.

I then had to decide how to place the main speaker and the amplifier behind the wing. To maintain structural integrity of the fuselage/wing assembly it seemed impossible to insert the complete assembly from the inside. Looking into the back of the fuselage I also saw a crossframe in the way, but concluded removing the bottom part of it wouldn’t be too dangerous. I thus opted to make a 45 inside angled cut to make a removable square panel in the bottom of the fuselage. On the picture you can see the speaker is still protected by a its white plastic transportation cover.

Further carving out of the removed square panel to allow the circular speaker noise to pass, and the fuselage frame to allow the system to fit tightly in the new opening. I then glued an outside grille in the fuselage speaker openings. Although MrRCsound provides thin black clothing to cover the openings, I prefer to use a more solid material that can be painted to match its surrounding. I use scissor cutouts of 3M abrasive silicon carbide mesh material (sold to sand plaster surfaces) that can be flush mounted following single local curves.

Following this I cut pieces of the MrRCsound supplied Velcro to fix the main speaker (elastically) on the bottom panel. I also painted the cones of the small speakers flat black because they otherwise would be too visible through the side grilles. The bottom grille was then painted white, the small side ones black. Everything is now ready for the later final insertion in the completed fuselage but before that the Wright Cyclone sound was selected on the little knobs and the system was powered up as a final test (although it had been tested during 10 minutes before being shipped).

I was very pleased by what came out of the speakers. Startup sound is adequate but does not include the whining sound of the speeding up inertia starter. Initial firing is realistic and idle sound much better as the Pratt & Whitney of my DC3. Opening up the throttle reproduces the cylinder firing speed well, but not the big increase in overall noise. I don’t expect that to be a problem because the main speaker being mounted facing down, the sound for people’s ears positioned higher as the aircraft during startup will be attenuated, whilst pointing directly at them during overflights. With the Dakota I already noticed having sufficient decibels during startup and tickover catches the attention of the audience, even if they were looking at something else, and the tiny side speakers will help that. I’ll just have to make a mental note to align the aircraft parallel to the public line instead of pointing at them. To avoid damaging the system I keep it out of the fuselage till further completion.

I then filled up the so called panel lines on the fuselage. In stock form these are sufficiently wide and deep for an aircraft carrier to navigate in. After a few layers of body putty and careful sanding they are much less visible and suitable for vinyl covering. Making them completely flat is feasible but breaking up the large surfaces up a bit seemed a better idea. My previous paint decorated foam aircraft were usually covered by one or two coats of cheap but strong wood floor varnish. I did that to protect the not so good adhering paint and to harden the soft foam for transportation damage. The better adhering vinyl does not need that, but after vinyling the tail I noticed how the thin layer could easily be damaged due to the softness of the foam. Because I also use the T28 as a test airframe for vinyl covering techniques (for my future Windrider B737), the prepared fuselage got two coats of that varnish before applying the vinyl. It makes a much harder underground, but vinyl adherence to the fuselage is degraded. It adheres very well to the varnish, but the varnish to foam adhesion now becomes the weak point. Vinyl application now becomes even more difficult because lifting it for corrections has the tendency to keep the varnish on the vinyl, leaving no adhesion material anymore for a second application. Every piece thus has to be applied correctly without creases or bubbles on the first go. I will evaluate the results of the various techniques used on different aircraft parts after a few months of use and hangar rash. The fuselage being the part getting the most manual handling on the field, I opted for that part to be hardened the most.

Before further covering work on the aircraft I wanted to test all the electrical systems and circuits. I changed the ESC contact into a Deans plug and hooked up a 4s4000mah battery. Awaiting delivery of a separate 10A BEC I connected the 65A ESC to my Spectrum AR10000 receiver so I could test all servo’s, lights and engine. During the substitution of the stock 50A ESC with the stronger 65A FMS version, The motor cables got reversed. After binding I heard the motor run, but after test fitting the assembled prop I noticed it ran backwards. With the fingers deep into the fuselage I reversed 2 out of the 3 cables and everything ran as it should.

Because I like to precisely control engine RPM during the final and landing portion of my powered approaches, I modified the linear throttle response into a throttle curve, allowing me a rather gradual buildup of RPM in the low power regime, and a faster buildup in the higher regime only used for short max power bursts. The prop was removed again till final assembly for safety. I then connected the battery with wide open throttle and waited for the two beeps that indicate the system recognized that position, then pulled the throttle idle and waited for the additional beep that confirmed the ESC memorized and calibrated that position as well. All that was done with the throttle trim neutral. I disconnected the battery and reconnected with the idle position for a normal power up of the aircraft system, then adjusted my throttle trim so the engine wouldn’t rotate in neither full idle, nor first click position, only starting at very low rpm on the second click.

The reason for that is the sound system I run on a separate channel and gets mixed with the throttle channel with offset and values needed to have the correct engine sound production according to the lower prop RPM’s. It requires some fuddling to get everything in line (especially with that throttle curve). I set it up that with the throttle full idle nothing happens, with the throttle at one click the sound system produces the startup sound but the prop doesn’t move. As soon as the startup sound changes into idle sound, I open up the throttle to the second click and the prop starts turning at the lowest possible steady speed, the sound system producing that beautiful slow tickover noise. As the engine (proverbially) warms up, I open to the 3rd click and both prop and noise go into a slightly higher rpm range. The slaving of the noise to the prop rpm is important in the lower (taxi) regimes, because that is what people will hear best, and will allow you to better control power in final approach because the engine sound system gives you audio clues electric motors do not. For shutdown I reduce the throttle till the second click allowing the engine to idle for a while (as any radial engine aircraft does to stabilize temperatures), then reduce to the first click with the system producing that typical shutdown sound, the prop being allowed to freewheel to a stop thanks to ESC reprogramming. I later might also experiment with reprogramming of the ESC into slow start (helicopter mode) during the startup engine sound in the first click.

All lights got checked separately in the battery connector of the receiver and all worked properly, including the correct colors and flashing of top and bottom fuselage red beacons. Next was the landing gear. With the wings not yet completed I tested all 3 gears separately. After having read the possible jamming of the system due to sequencer initiation I decided to first test the doors separately (plugs labeled 5C), then leave them open and test each gear operation by only connecting the 5B connectors, and activating all those with a two position switch on my DX10t Spectrum transmitter. I closed everything again as they came out of the box then per gear connected the 5B and 5C plugs via the sequencer. Everything worked nicely in sequence first time around, and a few complete cycles were made to ensure system consistency. I noted even without cycling, the gear sequencer got quite warm and will take that into account for mounting it in a cooled area. With everything properly tested, I now move individual servo’s without transmitter, but using a servo tester hooked up through the receiver to the esc and battery to get the appropriate voltage. Gears and doors thus can be positioned to paint the assemblies before further work on the wings.

The owner keeping his aircraft in an immaculate condition (his T28 is a typical example of an overly restored warbird to compete for awards), I had no choice as to paint the wheels and main gear wheel wells in a clinical gloss white as well, the original plastic or foam being too yellowish. To paint the interior of the doors, the large one has to be removed. To do that I first opened the separate servo controlled mid doors, then operated the gear down and disconnected the link between the gear leg and the outer narrow door (which is left open during all next steps). Raising the gear again you now get access to the screw keeping the large gear door fixed to the leg. After unscrewing and removing it you get better access to the screws holding the complete retract mechanism into the wing. When those screws are removed, lower the gear again to pull it out of its location. Now check the inner Alen screws that they are firmly locked to keep the gearleg tightly aligned in their brackets. Because I only fly from smooth tarmac surfaces I didn’t bother changing the original plastic ones by replacement metal pivot brackets but now would be the moment to do that. On my model I noted excessive play between the legs and wheels. The axles seem 4mm and the mainwheels 5mm, the nosewheel is a large 4mm and does not have too much play. I then removed the wheels to paint them (but didn’t risk damaging the tires by removing them from the rims), and cut lengths of 3/16th brass tube to fit over the axles to reduce the play of the wheels.

I also used that moment to cut some foam of the sides of the wells (as indicated elsewhere on the FMS T28 forum thread). Apparently that reduces the risk of jammed doors causing the gear to remain stuck in the up position. After two coats of white over the well interior, inner part of doors, and exposed parts of the gear retraction mechanism in the down position, vinyl could be applied and trimmed to the out side of all gear doors (inner servo controlled gear doors have to be temporarily closed again to do that). When everything is dry and completed, the reverse assembly operation can be completed including the insertion of spacer roundels so the complete retraction mechanism gets tilted just a bit from its plastic holder, and prevents the axles or wheels to push against the wing interior when fully up. Make multiple cycling to test and adjust (doors) so you are sure of consistent jam free operation. Now is the easiest time to correct anything abnormal. When finished, raise the gear and close all doors for the remainder of wing modifications and finish. Sounds tedious and complicated, but it is the only way to get everything mechanically and aesthetically correct.

Click here to read part four of the T28 build log
Last edited by BAF23; Jan 25, 2013 at 04:07 AM.
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