Wednesday, May 24, 2006


If you are going to build a -4 there are a few issues which you need to think about.....

1) The empenage's moving surfaces are made of .016 aluminium. However, there is a standard option you can order from VANS, to upgrade these to .020. It costs only a few dollars. The reason you may want to do this is because many owners have experienced cracks in the lighter weight skins after a few hundred hours of flying. Having said that, many have not. In the 'RVator 24 years' there is a suggestion as to how you bend the trailing edge, and how you put a blob of Proseal to stop this happening. On this issue, I am a belt and braces person, and have the thicker skins and the Proseal. The disadvantage is simple; weight in the tail. If you are going for a large engine, or a 2 blade prop, resulting in large pressure pulses, I would at least spend 5 minutes making a decision deciding what you want..

2) I am told there is also a standard option for thicker skins on the first section of the fuselage - .040 - behind the firewall. Again it is a cracking issue I believe, and possibly again associated with larger engines. I know less about this issue, and am somewhat surprised since the cowl cheeks must provide significant stability for this area of skin.

3) Buckling of the firewall and cracking of the steel weldements that join the longerons to the engine mount have been a historic problem with the -4. It is apparently caused by heavy landings and flying off rough strips. The design was considerably beefed up in the mid '90s by VANS. Most of the relevant plan changes are dated 9/94. However, it is not clear that the new design is entirely proof against this problem. There is a rumour that there is an option to order a beefed up lower section of the firewall itself. I have not found any part number for this, though in the plans I think a heavier grade of steel is mentioned. Certainly a few individual builders have added strengthening strips. An example can be seen here.

The left hand picture above is of the top left firewall weldement, delivered 2006, and the right hand picture is the lower right hand weldement.

4) The exhaust system will be another area where you have to make a decision though this can be left until you are making the other firewall forward decisions. The two main options to consider are a crossover system resulting in two exit pipes, or a cheaper and simpler 4 pipe. The down-side of the 4 pipe is noise - though many argue that there is little difference. Talking to Vetterman though, as I understand it, on the -4, he believes the 4 pipe produces a little more power. This comes about because in the crossover system there is not enough room to get the necessary pipe length to produce a tuned system. It's more a question of making a system that fits under the cowl.

5) If you want a 5 point harness for the rear passenger, VANS leave you somewhat on your own. It is not discussed in the plans. Some have put the 5th belt around the flap tube. Alternatively, if you think about it early in your build, you could attach a bracket to the bulkhead under the floor like this . You will have to work around the elevator push rod. There is an argument that a 5 point harness for the passenger is useless, since it is hard to see how you stay within the weight limit for aeros with fuel and a passenger. A 5th point is reassuring though, in heavy turbulence.

6) Another decision to make is footwells for the passenger (comfort), or rudder pedals. They seem to be mutually exclusive. I am building my aeroplane so if you want rudder pedals you have to be me! Lots of people leave the rear pedals out but include the footwells.

7) Heat to the rear seat is another frustration. VANS say you can cut a hole in the spar to put SCAT tube through. However when you ask where to cut it the answer is "It is OK to put the 2" duct through the spar, but if you want to do it that way you will have to cut the hole yourself. We don't have exact dimensions for the appropriate place.... ". My own decision is to give the passenger an electric seat heater and a switch, since if VANS don't know where to cut the hole, I certainly don't! I am also thinking of putting all the heat into the forward compartment between your feet and providing a path from there into the area with the stick. If these two enclosures are sealed the air will have no option but to move aft, beside the elevator pushrod toward the passenger. (The Romans invented this approach though not in an aeronautical context. They called it a hypocaust!)

Postscript Feb 5th '08
There was some interest in the rivet layout etc for the firewall weldements shown above. Not sure where to put them I think I will put them here since there are similar pictures above.

I think the weight limits for aerobatics is counted without the fuel. To calculate if you are within limits you simply take the actual weight of the plane and subtract the fuel. The CG has to be within limits though.

The fuel is in the wings, and does not add any forces to the to the main spars.
The above comment raises a frequently discussed issue with respect to the RV4. For further discussion you may want to review the thread at and draw your own conclusions.
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