Weight Questions
 

One of the things that I find over and over again is that people say to build RV's light. I hear stories that a day, vfr wood prop flys the best of any of the RV's because it weighs 100lb less than a Day/Night/IFR/180/CS RV. If I want to pick the best balance of features/weight I need info on weight. For example I know that I can save 10lbs using an Odyssey battery and spend less for the battery (vs Concord RG) I can also shave 28lbs by using a Whirlwind CS vs A hartzell CS. (yes I know there is more to it than weight...) I could also stand to loose 10 lbs ... at least. Oh yeah there might be 4 lbs of wire that I don't need since my battery doesn't have to go in the back. That's almost 50lb right there.

I'm curious as to other ways to save on weight, or even better how much do things weigh. There are many starters and alternators to choose from, what do they weigh, how much weight do I save switching from a vacuum system to an EFIS, weight diff between slick/emag/lasar/unison, titanium gear legs, how much does paint weigh? Do I want that clear coat? Are there a bunch of little things that really add up e.g. interior, bs rudder peddals/throttle/wire etc) How much do parachutes really weigh, mine is pretty darn heavy.

Obviously weight is not the only factor but I can see that you could end up with an 1100 lb plane in no time.

It seems like a bunch of people have thought through some of this. Do you have numbers (weighed plane before/after painting).

I'm curious as to your experience and opinions.

Chuck

Very good question, Chuck, I'm watching in anticipation.

Chuck,

Why the preoccupation with weight? I say this because my RV-6 weighted 1072 lbs. empty, I weight 240 lbs, and I never faced much of a restriction in the utility of the airplane. I think it is a good thought process to try and keep weight to a minimum. The old adage is 80% of the extra weight is added during the last 20% of the project.

In the long run, I suspect you will be much happier with your airplane if you install the options that fit you for the way you intend to fly. We have some folks here in Indy that really like light airplanes because they mostly fun fly, VFR, with the gang, and they like the performance. Several others, including myself, prefer comfort, utility, and features over stark performance. That's because my mission profile is different, more cross country.

Of course, the REAL solution is to strap 260 HP to the nose. That's why I'm building a Rocket. ;)

Preoccupation with weight...
 

I hesitate to call it a preoccupation. I do have a preoccupation with RV's though...

Anyway the performance of an airplane is related to:

power
weight
drag

Everybody seems to be 'preoccupied' with power IMO. It seems to me that paying attention to weight and drag is atleast as important as power. That's why an RV4 holds the CAFE record not the Rocket :). As far as I can tell if you want to go faster you are wasting your time increasing your HP by 10%, but if you could reduce drag by 10% you'd really have something. Reducing weight will increase climb rate, increase power out glide, and lower stress on the airplane during acro as well as provide a fairly neglible speed/economy improvement. (to name a few).

I'm not a racer or a performance nut, but as an engineer I like to understand the big picture so I can make informed decisions.

If you are a feature guy you might be interested in weight because a savings of 20+lb in propellor could put extended range tanks on your plane of a 0 net gain in weight. 10lb of battery weight savings could go into your autopilot...


Chuck

I'm not a aeronautical engineer, but I do fly with a lot of RVs. Here's what I've seen:

1) Weight is practically irrelevant when looking at performance in the RV. Perhaps because they all perform so well, perhaps because there is so little drag on the airframe to begin with. You'd be hard pressed to tell a difference in speed with a fully loaded RV versus one that is lighter ALL OTHER THINGS BEING EQUAL!

2) My Rocket will cruise at the same speed as a RV with LOWER fuel flow.

3) My Rocket will cruise at least 10 KTS faster on the SAME fuel flow.

4) My Rocket will burn fuel by the truckload if I push it over 24 square.

The point being, horsepower DOES matter, up to a point. Beyond that, you're just sucking fuel for no gain. Now, if you want to swap the weight of one option for the weight of another option, that's great. That's what homebuilding is all about. But my experience has been that the weight of the airplane makes very little difference in cruise speed.

Now handling is another matter. If you want to throw your RV around the sky and have it be very light on the controls, then yes, you want minimal weight expecially given the aerobatic weight limitation of these designs. Cross country flying???? Won't make a hill of beans of difference.

Just my experience.

If you want the lightest, best performing RV, build a RV-3 with a wooden prop. You want a screaming fast, light on the controls aerobatic RV, build a RV-4 with a 360 and CS prop. All the other models are somewhere in between.

Power off glide improvement?
 

If not mistaken during my glider training, best L/D is fixed per design, just the speed at which it ocurrs is based on gross weight.

So, the issue is not so much the distance flown and where you're going to crash, as opposed to how quickly you get there! :eek:

However, I've looked to keep it as light as possible while not sacrificing creature comforts for those cross country trips, and I doubt that I will ever really lose those pounds off of my somewhat 'bulky' frame. And with the price of fuel these days, the additional HP doesn't seem to make a lot of sense for the requisite increase in GPH.

Dave

drag not power
 

When you give an example at equal flow rates you are throwing out power arguments. VERY roughly speaking fuel flow rate = power. I suspect you will find that your Rocket goes faster in these conditions because it has less drag than an RV. Those wings are clipped for a reason. The tradeoff off course is that the Rocket ends up stalling at a higher speed and turning worse.

Same reason why a Lycoming 360 power glass air will smoke an RV, less drag.

Chuck

You are very correct! Distance is not dependent on weight, my bad. But I'll still take a lighter plane travelling slower in an engine out situation. Less energy to dissipate is a good thing!

Chuck

For most of us I suspect the concept of less energy to disipate is probably academic. However, I think the concept of slower is precisely the issue.

Everything improves to some extent with less weight: better climb, faster cruise, better efficiency, etc. But then, lower best L/D speeds, and quite probably the lower stall speeds is what may make the difference between surviving that crash or not.

Now that's something to pay attention to!

Dave

Paint weight
 

One local builder, after hearing all the talk about 30-40 lbs for paint, decided to make it his mission in life to determine how much his 3 color clear-coat-over-color coat paint job weighed. He did about as good a job as you can do at minimizing the variables and carefully weighing before and after. the answer: 9.5-10lbs.

As far as the other stuff -- there isn't much you can do to make the airplane lighter. Van's done a good job of designing a light airframe. All you can do is refuse to make it heavier. Do you REALLY NEED dual brakes? No? 3 lbs never installed. Do you NEED a vacuum system for the way you are going to fly the airplane day in/day out? No? 5-7 lbs never installed.


get ruthless. stay light. 50 lbs really does make a difference. 100 lbs makes a big difference.