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Weight Questions

chuck

Well Known Member
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
 
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

f1rocket said:
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.

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
 
ddurakovich said:
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.

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.
 
Test

Rather than quess or quote text, how about...

Find an RV model that you might like to own.
Do anything to have the owner take you for a ride, better still fly it solo.
Do a specific flight profile starting with full tanks.
Repeat the profile with 50 pounds less fuel.

Well? Bet you hardly notice any difference.

My choice went towards building an airplane with the instrumentation, power, and creature comforts that would be appreciated every flight. A few pounds one way or the other gets my Rhett Butler reaction (Frankly, my dear...) :) Keeping it light was on my mind (and I did a few real labor intensive things that saved a few ounces on those anal nights), but, ultimately, saving weight is not very rewarding. I've never walked up to a beautiful...anything... and raptured over how light it looked.

John
 
Sounds familiar...

John,

It sounds like you have seen those commercials about the delicious hand-made milk shakes at Steak & Shake. "My that looks efficient" was the statement by the "waitress" regarding how they are made at the fast food restaurant chains.

I opted for the features and creature comforts for cross country. I have heard that Abby Erdmann makes her interiors lighter than some other vendors. With the Hartzell C/S prop up front, I am going to need some weight in the back. I did put in the Odyssey battery per the plans to save weight on my RV-9A. I should be at the airport in the next 4 to 6 weeks if all goes well.

Jerry K. Thorne
East Ridge, TN
www.n2prise.org
 
Alternative Flight Test

RV7ator said:
Find an RV model that you might like to own.
Do anything to have the owner take you for a ride, better still fly it solo.
Do a specific flight profile starting with full tanks.
Repeat the profile with 50 pounds less fuel.

Well? Bet you hardly notice any difference.
John

Try this flight test:

Grab an RV4 with a wood prop and fly it around, takeoffs, 3pt landings, wheel landings, slow flight, acro. Now throw 50lb's of sand in the back of the luggage compartment and do the same thing.

Bet you notice a difference...

Of course your example is an extreme of putting the weight at the CG where it has minimal effect, and my example is the other extreme putting as far aft as possible making the plane much less stable.

I'm more interested in adding lightness because an RV4 at 1050 + 170lb pilot + 200 lb fuel leaves 80lb for stuff and passenger... It sure would be nice to knock that 1050 down under 1000. It gets even worse if you want to do acro. Take that 1050 with a 185lb pilot+chute and you'll find you can only do acro on a half tank. Don't even think about bringing someone up with you.

Chuck
 
chuck said:
Grab an RV4 with a wood prop and fly it around, takeoffs, 3pt landings, wheel landings, slow flight, acro. Now throw 50lb's of sand in the back of the luggage compartment and do the same thing.

Bet you notice a difference...

There no doubt that HANDLING is affected by weight, especially on a RV-4 or RV-3. However, cross country performance and top speed are not significanty affected. So it depends on what you fly and how you fly it as to whether the issue of weight is important or not. All of Van's designs have weight limitations for aerobatics, so if that is your forte, then weight definitely matters.

As builders, we only have time to obsess over a couple of issues. Before you obsess over weight, make sure it will make a difference in your use of your airplane, that's all I'm syaing.
 
More power or lighter weight

I'm curious about the relationship between horsepower & weight in the RV-8. Can one replace the missing 20 hp (caused by installing an O-320 instead of an O-360) by lightening the aircraft by 20-50 lbs (for example a lighter engine, wood prop, Grove gear, polished vs painted, minimal instruments) or does the extra 20 hp make a more significant difference in performance?
 
Here is info on HP versus performance for an RV4

http://www.vansaircraft.com/public/rv-4per.htm

and this link for technical info

http://www.nar-associates.com/technical-flying/weight/weight1.pdf

MistyFac said:
I'm curious about the relationship between horsepower & weight in the RV-8. Can one replace the missing 20 hp (caused by installing an O-320 instead of an O-360) by lightening the aircraft by 20-50 lbs (for example a lighter engine, wood prop, Grove gear, polished vs painted, minimal instruments) or does the extra 20 hp make a more significant difference in performance?

Yes and no. Without getting an aero engineering book out and lots of physics here are a few simple things to remember:

To increase speed -> reduce drag (first) then increase power.
To increase climb -> increase power and reduce weight.
To decrease turn radius -> reduce weight add VG's

A constant speed prop will improve speed or climb depending on what FP prop you compare it to.

I think what most people who talk about keeping it light allude to is that the plane is more fun to fly. If you don't do acro, ACM or very short feild landings, and instead are a XCountry person, it doesn't matter so much other than trading one feature for another.

There are no easy gains to be had in any of these areas (i.e. Van did a good job). There are only crumbs to pick up, but if you pick up lots of crumbs you will notice. A 20hp change in power results in 8mph for a typical RV. That's not much for switching from a 320 to a 360.

But again the goal isn't to make a racing plane, it's to make a plane that meets your mission and to have the information to make an informed decision.

Chuck
 
Weight of Rv's

Related to the topic, I made a little database of empty weights for RV's completed in the last year or so. Total non-scientific, but somewhat interesting. No surprises, avg empty wt are heavier than Van's specified range/ recommendation. Although some builders have achieved lower weights than Van's spec. There is no accounting for paint, fairings, and interior.


The avg wt for all (115) RV's = 1075lbs. The RV-6A was the highest over average. The lightest Avg was a RV-4 @ 94lbs under.

BUILDER EMPTY WEIGHT
Model; count; MIN; MAX; AVG; Delta-from-avg
RV-4..; 11; ....874; 1107; 980; -94
RV-6..; 28; ....980; 1244; 1058; -17
RV-6A; 22; ....990; 1195; 1105; 30
RV-7..; 15; ...1011; 1141; 1073; -2
RV-7A; 10; ...1020; 1134; 1092; 18
RV-8..; 15; ...1035; 1178; 1102; 27
RV-8A; 07; ...1032; 1165; 1101; 26
RV-9A; 07; ...1034; 1168; 1086; 12
AVG = 1074.6 lbs
The actual builder weight Vs. Van's "Spec" empty weight. Positive Delta means over Van's spec, Neg # indicates a lighter weight than Van's spec. The lightest weight was a RV-7A, 57 lbs below Van's Min spec. ;) The fattest was a RV-6 @ 226lbs over max (recommended) empty weight. :eek:




VAN'S DATA vs. BUILDER EMPTY WEIGHT
----------Vans--------Vans-----
Model; ...Min; Delta; Max; Delta
RV-4...; ..905;..-31;..913;..194
*RV-6.; ..965;..15;..1018;..226
*RV-6A; ..985;..05;..1038;..157
RV-7...; 1061;..-50;..1114;..27
RV-7A.; 1077;..-57;..1130;..04
RV-8...; 1067;..-32;..1120;..58
RV-8A.; 1067;..-35;..1120;..45
RV-9A.; 1028;.. 06;..1075;..93
DELTA= VANS (MAX)/(MIN) to Actual (MAX)/(MIN) Wt. Difference
DELTA (Pos) = Builders Min/Max greater thanVANS Spec.
* Van does not publish range so I used 53lb range, like RV-7/-8
Light RV-4: 905-31= 874lb; Fat RV-6: 1018+226= 1244lb



The last table breaks weight down by eng or prop or both. The Heavy wt was an IO360/CS prop at 1244lbs (RV-6). The light wt had a 320/fixed, 874lbs(RV-4). No shock here. However the diff between max (RV-8) & min (RV-4) is 370lbs, albeit diff models, the airfoil is the same NACA23013.5 w/ wing area of 110 vs 116 sq ft. A light RV-4 is a kick. Someone asked Mr. Van what his favorite RV was, RV-4,-6,-7,-8. His reply RV-3. Go figure. Several auto conversions, Subie, Rotary not included were on the high side, equiv to IO-360 (200HP)/CS prop. It is obvious eng/prop choice affects weight. However it will give you an idea how you stack up. Most popular combo of group: O-320/Fix or O-360/CS, RV-6 (for the 115 RV's I looked at).

EMPTY WEIGHT BY PROP, ENG or ENG/PROP
Total; Eng/Prop; MIN; MAX; AVG
60; FIX;..........874; 1170; 1041
55; CS;.........1010; 1244; 1111
41; 320;..........874; 1168; 1034
60; 360;..........986; 1189; 1084
14; IO360;.....1103; 1244; 1151
32; 320/Fix;......874; 1106; 1019
28; 360/Fix;.....986; 1170; 1066
09; 320/CS;....1010; 1168; 1084
32; 360/CS;....1026; 1189; 1101
14; IO360/CS;.1103; 1244; 1151
Fix or CS= no breakdown of brand, blade material, most popular=Sensenich&Hartzell
320= 150 or 160HP Carb or injected.
360= 180 HP Carb or Injected
IO360= (200HP)



My 2cents, less weight= less lift req, less lift = less drag, less drag = less thrust req, less fuel burn. This also means more useful load, more range, lower stall speed and better climb.

(Subjective) I like the feel of a lighter RV. I have flown RV's with both heavy and light empty weights, the lighter one has a better feel on the controls. If you have flown differnt RV's you know what I mean. If you only do S&L cruz-ing OK, weight is less an issue, but if you like the aerobatics, dog-fight and short field capability, weight affects these charicteristics.

If you want full tilt IFR Bells-N-Whistle, 200HP, CS prop, full interior & paint, it will weigh more and you will sacrafice something. Nothing wrong with the choice of making a Caddy but don't expect it to be a sportscar to. No need to justify choices, just be aware of the trade offs. Van wrote many RVator articles in the last 10-15 years that address the topic of weight and RV's.

I think the paint weight of less than 10lbs is the bottom range, I have read of 20lbs or more (30lbs), plus you have to add all the internal priming/painting. As someone suggested, ask yourself do you really need it? Can you add it later if you find you can't live without it? What-ever you do, it adds weight, cost and build time.

Cheers RV-4, RV-7(project) :D
 
Last edited:
f1rocket said:
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.

I've had a number of people respond to me privately saying, in essence, I'm full of "stuff" :eek: with regards to my comment above, so I'll elaborate.

I have to admit that the actual flight testing to prove the above statements were performed at a speed that favored the Rocket, although that was the original intent of the test to begin with. The RV-6 was flown at WOT (wide open throttle) and both the speed and fuel flow were noted. The Rocket was then flown at the same airspeed and the fuel flow noted, and then at the same fuel flow, and the speed was noted. In both cases, the Rocket was either faster or more efficient.

Now, a couple of caveats. Sure, the RV can get down to less fuel burn than the Rocket because it has fewer cylinders, but at a speed penalty. The test favored the Rocket in that the speeds and fuel burn are right in the area of maximum efficiency for the IO-540, and WOT is not the most efficient spot for the O-360, so all these favor the Rocket.

With regards to drag and weight, the Rocket and RV are very close to each other in both regards. So obviously when you strap 260 HP to the front and the rest is about the same, you're going to gain something.

In closing, I'm not suggesting that the Rocket is faster or more efficient at ALL settings, just at the one at which the actual flight test was performed.
 
Plenty of opinions in these forums...

chuck said:
One of the things that I find over and over again is that people say to build RV's light.
I'm curious as to your experience and opinions.

Chuck

I'm not an aeronautical engineer or a professional pilot or anything else glamorous. Just a guy who has built and flown an RV-4. But I would think that any time you are building something that has to fight against gravity, you'd want it as light as possible.

My RV-4 came in at 876 lbs. I haven't painted it. I have a wood prop. I don't have any fancy doo-dads in it. I don't have nice upholstry. I don't have a CD player or an XM radio. I tried to build it as light as I could.

Does it fly any better than any other RV-4? I couldn't tell you. But I'm very happy with it. That's the beauty of building your own airplane. You make it the way you want it. Everyone thinks their way is the best way. I think going light is the best way. Others want flying easy chairs with all the bells and whistles and they think that's the best. All this acedemic debate is interesting but ultimately it's your airplane, these other guys won't be flying it. It's all good. Just build the airplane you want.
 
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