Construction d'un avion de sport monoplace Van's RV-3

Think ahead !

Posté par Yann le 23 septembre 2010

En suivant quelques constructeurs américains, j’ai donc compris que la construction du RV-3 était, certes, un long fleuve…mais pas un fleuve très tranquille. Le maître mot apparemment est qu’il ne s’agit surtout pas de « suivre » les plans et le pauvre manuel de construction; il faut les anticiper à chaque étape pour éviter de compromettre une autre étape 2 ou 3 steps plus loin.

Alors sans vouloir mettre la charrue avant les boeufs, il paraît sage d’envisager avant tout mouvement le type d’installation moteur envisagée et son intégration globale tant les répercussions sur l’ensemble de l’avion ne sont pas seulement « firewall forward » !…Tout est terriblement étriqué sur le RV-3 et notamment sous le capot ce n'est rien de le dire !!...

Il faudra par exemple absolument considérer entre autres  :

– la position du régulateur hélice qui, sur l’arrière du moteur, n’aura pas la place nécessaire contre le pare-feu et qui nécessite donc d’ajourer cette cloison pour y ajouter un « firewall recess » non prévu

– le système d’injection Precision Airmotive Silverhawk EX ne rentre pas sous le capot inférieur standard du -3 et la commande du papillon de gaz interfère avec la FAB fournie par Van’s…ce qui nécessitera d’adapter un capot de -4…en revanche il ne nécessite pas de ligne retour carburant vers les réservoirs…

– l’utilisation voltige de l’avion impose l’installation de plongeurs carburant souples dans les réservoirs, pour l’alimentation en vol inversé, ce qui interfère avec les jauges carburant mécaniques qu’il faudra déplacer dans les 2 réservoirs..(le RV-3 n’était à l’origine pas prévu en réservoirs structuraux d’ailes mais avec un simple réservoir de fuselage…).

Je commence donc à « stocker » toutes les informations utiles sur ce sujet»

Fuel injection: Precision Airmotive Silver Hawk EX (Courtesy of Roee Kalinsky’s RV-7 project)

The certified ancestors of this engine use the venerable Bendix RSA-5AD1 fuel injection system. This system is currently produced by Precision Airmotive, and continues to be used on certified Lycomings. Precision Airmotive also produces the Silver Hawk EX, which is a non-certified clone of the RSA-5AD1. Physically, the Silver Hawk differs from the certified RSA-5 in its method of manufacturing. The main housing of the certified RSA-5AD1 servo is cast aluminum, whereas the non-certified Silver Hawk’s servo (known as model EX-5AV1) is machined from aluminum billet. According to Precision, modern CNC technology makes the latter no more expensive than the former, and eliminates the RSA-5’s problems with casting porosity. There is also another superficial difference in that the certified RSA-5 is black anodized, where as the Silver Hawk EX is clear anodized (hence the silver appearence). And that’s about it. Otherwise they are identical in operation, and their parts are interchangeable.

The variant of the Silver Hawk EX that I am getting actually has one more difference. In the standard variant of the Silver Hawk (as well as the RSA-5), the mixture lever is oriented downwards and swings below the bottom of the servo, which is known to interfere with Van’s standard airbox. A common solution that is now available as an option from Precision is an « alternate rotation mixture » variant, where the mixture lever is oriented up instead of down, and rotates in the oppositve direction so that it is still rich-forward / lean-aft without having to use a bellcrank or other mechanical contraption to reverse the direction of the control cable. Reportedly, with this option, Van’s standard airbox can be used without modification, as well as Van’s carburator mixture bracket. The alternate rotation mixture variant for my engine is kit # EX360-3 (whereas the standard kit is # EX360-1). The physical differences are limited to a couple of internal parts, and the servo could be converted back and forth if needed (by Precision, not in the field). So the part numbers are as follows:

Kit # EX360-3:
1ea Servo p/n 3015010-1 (model EX-5AV1 with alternate rotation mixture)
1ea Flow Divider p/n 3015004-1
4ea Nozzle p/n 2524864-2

Regardless of the mixture rotation option, the fuel inlet fitting can go either on the left next to the mixture lever (typical) or on the right next to the throttle lever. The fuel outlet fitting of the engine-driven fuel pump is on the left side, so it probably makes for more direct routing of the fule hose to have the fuel inlet fitting of the FI servo on the left side as well.

This fuel injection system is relatively low-maintenance. The only scheduled maintenance items are to clean out the inlet finger screen and replace three o-rings in the inlet cavity at each annual. The only other field-adjustable item is the idle mixture. And otherwise it is expected to be maintenance-free until overhaul, which is specified at the engine’s overhal (2000 hour TBO) or 10 years, whichever occurs first.

All « rubber » components (o-rings and diaphragms) are now made of fluorosilicone, which is compatible with all known fuels, including ethanol. A good thing as a contingency, looking pessimistically into the future of aviation fuel availability.

Note that I had also considered some newer fuel injection systems from Airflow Performance. These are still continuous-flow all-mechanical systems, but claim higher performance than the older RSA-5 style systems. By all accounts the Airflow Performance FI’s are good systems. However, the performance gains are minor, the installations more complex, they are more expensive, have less of a service history, have single-source parts, and are unfamiliar to most A&P’s in the field so they can only be service by Airflow Performance. For those reasons I decided to stick with the RSA-5 based Silver Hawk.

In the past I had also considered the Aerosance FADEC and other electronic fuel injection solutions. I still believe that EFI is a better solution in principle, and I hope that solid viable products will emerge in this space in the future. But to face facts, I have to say the the EFI products currently availble for this class of engine are just not quite there yet. So, sadly, the prospect of EFI will have to wait, perhaps until my first engine overhaul.

Ignition: Dual P-Mags

The P-Mag (form company E-Mag) is a self-contained electronic ignition system that fits into the form factor of a traditional magneto. These deliver a stronger spark than magnetos, support variable timing advance as a function of MAP and RPM, and should enjoy a long maintenance-free service life with no mechanical wear (no « points »). This product has had its growing pains when it first arrived on the market, but has since proven itself to be reliable and robust in the field over the past couple of years (even Lycoming is now OEMing it on their new light sport engines). The P-Mags, as opposed to the original E-Mags, are also self-powering above idle RPM via a small internal alternator. Ship’s power is only used for start-up, at very low RPMs (lower than flight idle), and as a backup in case the P-Mag’s internal alternator fails. In my case, I will have dual electrical systems anyhow, and so each P-Mag will receive its ship’s power from a different electrical system for added redundancy.

Automotive spark plugs, aviation bosses, adapters

The P-Mags support either traditional aircraft spark plugs or automotive spark plugs. The consensus seems to be that automotive spark plugs are the best choice all around. Having been designed with modern automobiles in mind, they are designed to support a stronger spark, and therefore don’t erode as quickly as aviation plugs when using electronic ignition. Automotive plugs are also available in more heat ranges. And as an added bonus, as anything automotive vs. aviation, the automotive plugs are far less expensive. To their credit, aviation plugs are RF-shielded, whereas auto plugs are not. But with modern resistive plugs and modern avionics (anything newer than ADF…) this doesn’t present any problem with RF interference.

Aviation plugs have 18mm threads, whereas auto plugs have 14mm threads. As an available option, the bosses on the cylinders can be threaded for 14mm so that automotive plugs could be used directly. But although I plan to use auto plugs, I opted for the bosses to be threaded for the standard 18mm aviation plugs, and I will then use 18mm-to-14mm adapters along with the auto plugs. That gives me the flexibility to use either automotive or aviation plugs in the future without having to re-machine the bosses.

Starter: Sky-Tec 149-NL

Sky-Tec model 149-NL « High-Torque Inline » starter is reputed to be strong, durable, and reliable, and is a popular choice on these engines. It also has a unique feature called the « Kickback Protection System », which is essentially a field-replaceable sacrificial shear pin that is meant to fail before damage could be done to the engine’s ring gear in case of kickback.

The other common choice is Sky-Tec model 149-12LS « Flyweight » starter, which is both a pound lighter and a bit less expensive. But this model is reputed to suffer from occasional problems with cracking and deformation of its housing, which can also lead to damage of the engine’s ring gear by engaging it off-axis. So in this case I decided to go with the more reliable solution at the expense of an extra pound of weight and a few bucks, and chose the NL.

Straight spin-on oil filter adapter

Several styles of spin-on oil filter adapters are available for these engines, including straight right angle variants. While the right angle adapters look like they may make it easier to remove the filter, I’ve been advised that the straight adapters actually make the job easier and less messy. Just slip a « bread bag » over the filter when you remove it, and the bag will capture any oil that may drip out. According to Bart, the right angle adapters are the only choice for Cubs due to the limited space between the engine and firewall, but RV’s have ample space and so a straight adapter is possible and preferred. So a straight adapter it is.

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52 ans, ancien pilote militaire Aéronavale...à présent pilote de ligne...un jour peut-être pilote de RV-3 !...En attendant, concepteur de ce site...



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