Tuesday, June 10, 2008


Weight......the final result!

Looking back through my blog, I see that 21 months ago I 'shared' my feelings about weight. (5th Sept. '06) See below. A few days ago I finally weighed it. I was lucky to be able to borrow a set up, which is normally used for weighing saloon cars being prepped for racing on the Nürburgring. It has 4 pads and automatically sums the weights, and does many other magic things quite unnecessary for weighing a -4.

The final weight is, to some extent, one of many judgements on the builder's workmanship. I had set my heart on it being below 990lbs, so when it checked in at 970lbs. with the CofG at 68.1" I was naturally delighted.

It is equipped with a Superior O-320-D1A, a 72" MT c/s prop, and I have the heavier grade empenage skins. I also have oversize tires which add a few pounds. The panel you can see here, and this is where I am sure I saved a few pounds though I am wanting for nothing.

Becki Orndorf supplied her very comfortable RV4 seats and I added VANS 5pt belts back and front.

It means that I and my wife, a full fuel load and 50 lbs of luggage check in at a gross of 1496lbs, 4lbs below the design limit and a full 54 below the higher UK gross of 1550lbs.. We will be at 73% of range 75", which is quite reasonable. Solo I will be at 16% which is a bit forward, so I may well throw 25lbs of something in the baggage compartment for the first flight. That would bring it back to about 30% at half tanks.

If you haven't seen a picture, here she is. Hope you like the colours.

Tuesday, September 05, 2006



I am always surprised how much 'stuff' people want in these aircraft. How you want yours is clearly your decision. My view is that folk pay far too little attention to weight when they build. Less is better, and I think this can not be overemphasised. Van confirms this several times in the performance section of '24 years of the RVator', and indeed the performance specs say the same thing.

I am building a -4, and this really emphasises the issue for two reasons. Because the basic design is the lightest of the RV range, any one item you might add is a bigger percentage of the gross. Also, since the gross is lower than the other RV models, it concentrates the mind since it reduces the baggage capability.

I am still building, but many of the decisions have already been made as to how to keep the weight down. Working from the biggest to the smallest. I have seen conflicting numbers for some of these items, but the shape of the issue is what matters.

1. The XP-320 weighs 11lbs less than the XP-360. Go for the lighter engine. "It produces less power" I hear you say. I will come to that in a bit.
2. The two blade MT prop weighs 12lbs less than the three blade. Most of the advantages are with the two blade!
3. The new electronic gauges Dynon or ACS weigh significantly less than the steam gauges. (Do you need IFR kit? Here in the UK homebuilts are not allowed to fly IFR.)Do you really need backup instruments for most of the electronics.
4. Do you really need two radios?
5. Do you need a vacuum system?
6. Keep the wiring simple. A foot of wire weighs nothing, but pick up a reel.
7. The Odyssey battery saves several pounds and results in reduced wiring runs. (This is a biggy.)

At a much lower level, examples would be:

8. Do you need ALL the floors secured with nutplates. LP4 are lighter.
9. Drill holes and more holes in metal which is doing nothing. (If you have never taken a structures course I guess you should take care here that you know what you are doing.)
10. ...and the list goes on, but you get my point.

How much of the interior finish do you really want? Leather lining and lots of extra metal enclosing everything can look nice, but to my eyes the minimalist approach can also. Active head sets really can handle the noise problem, and are much lighter than masses of sound absorbing material. To do a good job, sound absorbing material has to have mass.

The latest I have heard about is a flap positioning system. These are not Airbus 380! My solution is to have a stripe of paint on the flap which will emerge when say 10 deg are set, and another when say 20 deg are set. A glance to my left before takeoff confirms that I have what I want. That's an awful lot of stuff that cant go wrong; well the paint might fade. It costs and weighs nothing. After its set for takeoff, the only things I can see you want are a) full up (hold the switch up for a few seconds), and b) at the end of the flight....a bit of flap....a bit more....a bit more....etc.

Now perhaps you agree and perhaps you don't. If having masses of 'stuff' is what turns you on, then fine, but understand what you are loosing. There are four issues to my mind:

1. Handling. Light aircraft are far more fun to fly than heavy ones. Think about flying solo .v. two up from the handling viewpoint. Its subjective, you cant measure it, but its there.

2. Baggage capacity. This you can measure in lbs. My goal is to build the aircraft no heavier than 990lbs. Hopefully, considerably lighter. That will be with a carburated XP-320 and a 2 blade MT prop. If I do, then the weight will work out as follows.

Aircraft(Target) 990
Fuel 192
Pilot 154
PAX 130
Bags F 20
Bags R 50

Total 1536
Gross(in UK) 1550

This will mean full fuel, and a ten day trip with the luggage we need, is quite possible. More frequently it will mean for local flying, one up, with half fuel the weight will be down to 1250lbs.

3. If STOL performance is important weight is crucial. Because of my short strip, 1020',it is crucial to me. On touch down the energy you have to dissipate is proportional to M*V*V remember. Well if M goes up the stall speed goes up. Using VANS numbers for solo of 1160 lbs (stall at 48mph) and gross of 1500 lbs (stall at 54 mph), the energy is in the ratio of 1:1.63. Quite why the landing distance does not go up by 63% escapes me. VAN's specs show it going from 300 to 425, but that is still 42%.

4. Reliability. For example the flap positioning system I dont have, wont go wrong!

In fact there is a 5th thing you are loosing each time you add 'stuff' and that is money.

So I have not convinced you yet, and you are still worrying about speed and performance?

Well I ran a few numbers derived from the performance section in the RVator. You could argue about the detail but the shape is right. (Please if anyone calculates very different answers please let me know because I could always have screwed up.)

Putting more power in really gives you very little extra speed. Van calculated that a 150hp RV4 will max out at 200mph. To increase that to 250mph, all things being equal would require 293hp. Nearly double. The problem is the drag increases approximately proportionally to the cube of the speed.

The assumptions I made assumed three identical RV4.
a) A 160hp at a weight of 1360 lbs.
b) A 180hp at a weight of 1460 lbs.
c) A 200hp monster at 200lbs.

My thought was that while the weight does not perhaps increase by 50 lbs with the larger engine, if you add in the heavier associated equipment it will be close. Once you take the viewpoint of the builder into account it is probably an underestimate.

I then assumed 3 different flights.
a) One climbing to 2000' over 20 miles.
b) Another to the same height over 200 miles.
c) The third to 8000' over 200 miles.

To make the calculations simple each aircraft then climbed at 100mph and full power to cruise height. (I guess this implies a C/S prop.) At cruise height 75% power set for that engine, and finally a 200mph descent at 700' fpm to the destination from cruise height.

The time differences over these distances was trivial. (When I work out how to include the spread sheet I will include it. I would be happier if you had my workings in case there is an error. For now I will summarise.)

Over 20 miles the big engine is about 20 seconds faster. Its big advantage is the climb, but that lasts less than a minute at 2400fpm compared with 1920 for the lightweight. Soon they are all sliding down hill at 200mph. Yes VNE is a little faster, but you get my point.

OK you say, so lets climb high and go a reasonable distance. This is where the big engine pays off, but not by much; 4 minutes in just over the hour. The fuel burn went up 13% by the way. Again, a significant part of the journey is the descent where the engine is not the limiting factor.

Not convinced that light and lower power is not the way to go? Well its your aircraft so do it as you want. Just my opinion.

Have fun!

Monday, June 26, 2006


RV-4 Cowls.

If you are building a -4 you need to know about the available engine cowls. It took me a while to understand this issue so I thought I would document it here. There are (at least) two current variants of cowls. I think it is only the lower section that varies but since you will almost certainly order both parts together that's not an issue. If you are using a standard O-320 engine you need the O-320 cowl. If you are using an IO or O-360 or an IO-320 you will need the O-360 cowl. The reason that the IO-320 needs the -360 cowl is because the injection system together with the air box on the bottom is deeper than the carburated O-320. Its not very different but it's enough to matter. There is not much spare room down there!

This became an issue for me because I was quite interested in putting an injected -320 in my -4. However, the cowls don't look quite the same. When you see them side by side you will see that the -360 cowl is deeper and to my mind just takes the edge of the looks of the -4. It's not a big thing but at least if you know the issue you can make your own mind up.

If you decide on 200hp....I have no idea.

Friday, June 23, 2006


Carb Heat

VANS do not include carb heat equipment in their Firewall Forward kits, nor do they implement them in their demonstrator aircraft. When I asked about this, they indicated carb ice had not been a problem for them. My rationalisation of this is that since the VANS designs all include a tight cowl system, the air temperature of the lower cowl keeps the carb body above freezing, and despite the presence of cold humid air rushing through, the ice can not form on a warm surface.

There is an interesting thread discussing this at
Carb Heat

After much consideration I think it is probably unecessary, unles you operate your aircraft in unusual ways, but despite that, I will incorporate an exhaust heat muff to provide warm air for the carb. At least it will give me peace of mind.

Wednesday, June 07, 2006


Corrosion! It has probably already started!

When your beautiful kit arrives for any VANS aircraft, you need to think about when you are going to get the (usually) blue plastic off. It certainly protects the aluminium from minor scratches but it encourages corrosion. Let me explain.

Kaiser makes the aluminium and VANS buy it from them. Each sheet is labelled as to its specification of grade and thickness. You will see this written on the metal in blue ink. When I first started building, I found it very hard to get the ink off with a typical de-greasing agent. The thing you need to know is that it is very easy with water. (Put one drip of soap - not detergent - into a large bucket of warm water and the writing just wipes away with a damp sponge. Remember to dry the metal off afterwards. The fact that it wipes away so easily with water is a clue to an insidious problem which has caught many out. Being water-soluble, the writing is hygroscopic to some extent. This is probably not a problem in the dry heat of Arizona, but in the humidity of the English climate, a kit left in a normally unheated garage will by the end of one or two years show mild corrosion underneath the plastic when you remove it. This has to then be polished out, with red Scotch Bright, before you can etch prime the metal.

Therefore, my advice is:

1) Get the plastic off parts as soon as possible after they arrive if there is writing underneath the plastic. (If its a flat pack you might assemble it first. If it is a QB, I think it is the first job.)
2) Wash off the blue writing with clean water.
3) Etch prime (or alodine) to protect it.

Posted by Picasa
You can just see the blue writing on the ally. No corrosion this time but I had it on the -9A and had to work hard to get rid of it.

I did approach VANS on this issue 6 months ago. The first answer said "....Have not heard of that before but will research it and see what comes up." which was rather disappointing since it was much discussed from time to time on the builder group web sites. As of yesterday the comment has moved on to ".... Steve, I was working on this thru our Kaiser rep and have spoken to him several times about it with no definitive answer from Kaiser. We
have since seen several instances of minor corrosion on the lettering
of the sheets (now that we're looking)...only the lettering...odd."

From our point of view as builders, the solution is easy. Don’t use a water soluble ink! Kaiser might view that differently.

Postscript dated 23 June.
Today I was preparing some 2024-T3 sourced not from VANS but elsewhere, and made I think by Alcan. It was interesting to note that the spec printing was mostly on the plastic, and when I tried to remove it from the aluminium in the few places it occured, it was not water soluable. It wiped away with a touch of MEK!

Friday, May 26, 2006


"2 blades or 3?"

Lots of friends ask what prop are you going to put on the front, and then quickly ask "2 blades or 3?" This lead me to think about the relative advantages of one over the other. I should say first of all I fly out of a short strip and a constant speed prop has a very significant advantage for landing, so I had already made that decision in favour of constant speed.

I am not an aerodynamicist, but after some thought it appears to me that the 3-blade prop has all the significant disadvantages. They are:

- it costs more.
- it is harder (more expensive) to ship.
- there is more of it to get damaged and repair.
- it is less efficient and therefore very slightly slower, though I suspect this is academic.
- it is heavier, therefore increases the empty weight.
- it makes the cowls significantly harder to get on and off since the lower cowl has to come forward to clear the airbox, (and the training gear leg if you still use one.)
- you can never put it out of the way in a crowded hanger.

The one genuine advantage is it is very slightly smoother, though the 2-blade MT is very smooth.
For an (I)O-320 the radius of the appropriate 3-blade MT prop is 1/2 inch less. If you have the tail that high it seems unlikely that this will save you from a prop strike. This difference in radius is hardly going to make any difference in tip speed so the noise difference is minimal. The most frequently quoted reason for 3-blade is that its "sexier". I guess if you are that is the basis of your decision then there is no point in reading this. Get down to the hairdresser :-)

There is one frequently stated reason which I think is bullshit. If it were true I would be very interested, but I can see no reason why it should be. The stated reason is that it acts as a better air brake. I can see no aerodynamic justification for this statement. If someone knows a good reason why it is true I would love to know.

Postscript dated 12 September 06.
I really wanted to run the air brake or disking issue to ground, so after an abortive attempt to find someone who understood these things on RVSQN, asked the same question on Vansairforce. It is an unusually complex subject. What I concluded from the conversation is that for a given diameter the 2-blade should probably provide better disking, however it is more complicated than that. eg How much of the swept area is solid? In practice, it is pretty clear from users of the 3-blade MT on 200hp engines, that for stopping, the 3-blade IS more effective. They also complain that at the top end it appears to limit the top speed. This conversation was useful to me in that it confirmed to me the intelligent choice for those not in doubt about their manhood was 2-blade! :-)

You can see the discussion thread here and draw your own conclusion.

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.

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