Thursday, September 14, 2006


Key Firewall Forward Decisions

It is finally time to order everything I will need in front of the firewall, so I must be making some progress. For me, this is the exciting part since I most enjoy the electrical and systems aspects of building.
We are all different in what we want with our aircraft. I want it to be light in weight, and uncluttered, with no more instrumentation than necessary. I have banged on about weight elsewhere, but I really believe it is worth concentrating on this issue, particularly on a -4.

I believe an important point to understand is that the things you do not have, can't go wrong! A statement of the obvious perhaps, but widely ignored.

The significant decisions I have made so far:

Engine - I will order a carburated XP-O-320-D1A from Aerosport power. They can also supply a 40 amp alternator which they say is slightly smaller and lighter than the 60 amp VANS unit.

[Note - added 4/10/06 - several people have been puzzled I did not order an injected engine. Putting aside the issue of the different RV4 cowls for the injected engine, covered elsewhere, I think before you decide you should read Stein Bruch' s article for a balanced view. I did, together with a lot of other input, and it saved me a lot of money.]

Mags - I can not build this aircraft and then use early last century technology to ignite the spark. I plan to install an e-mag and a p-mag. If you are reading this in the UK you might be saying "The PFA wont let you". I have been working with the PFA and by the time this is ready to fly I think they will. (Lets be honest. Traditional mags are the least reliable part of our aircraft!)

Exhaust - I spoke to Vetterman. He steered me towards the 4-pipe. The reasons being:
a) Although in theory a cross over is more efficient, the pipe lengths are all wrong under the -4 cowls, so the most power (probably marginal) will come from the 4-pipe.
b) The weight is about the same.
c) I like the simplicity.
d) Noise could be an issue but this can be more than compensated by thoughtful use of the pilots left hand!

Prop - A 2-blade hydraulic MTV11C. It will help me stop in my short strip.

Cabin Heat - I am going to make a serious attempt to provide cabin heat by using the waste heat off the oil cooler. The UK does not have the bitter cold of the American mid West. For the rear passenger I plan to provide electric elements in the seat to keep her warm (and friendly).

Why use the oil cooler heat?
a) The heat is there for free and it reduces the risk of CO2 leaks.
b) It reduces the parts count - less to go wrong.
c) It reduces the weight.
d) If I never manage to get enough heat, it wont be too difficult to add a heat muff on the exhaust. (I will try to remember to leave room for it.)

Primer - I will use the accelerator pump on the carb to prime the engine. With the new batteries and starters the engine spins so well, a stroke of the throttle will, I am sure, fire her up beautifully. I used to fly an O-540 Pawnee glider tug that was started this way, and it worked just fine. Just remember not to over prime! The reason again is simplicity and weight. This might not be appropriate in climates much colder than the UK.

Carb Heat - On my -9A, I followed the VANS design which results in minimal carb heat. It worked, but I was never happy. Since summer and winter here in the UK, particularly the Vale of York, we have frequent high humidity, I have decided to install a powerful carb heat system. I have ordered the muff from Robbins Wings. Nice to deal with by the way.

Parking Brake - I will dispense with this.
a) I have never trusted the parking brake for doing my run ups. I always at least cover the brakes with my feet so I may as well use them.
b) I do not trust them to hold the aircraft parked on a slope; for that you need chocks.
c) Without the joints there is less opportunity for brake fluid leaks, and again it reduces the weight. You no longer need the Matco brake unit, the four fittings or the operating cable. It is one less control to clutter the cabin.

It might not be to your taste, but that's just fine. It will suit me!

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