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!

Comments: Post a Comment

<< Home

This page is powered by Blogger. Isn't yours?