Weight Questions

[f1rocket]

These are all really good points. My best bet to lighten my airplane is to hit the gym!

[Dprestonsr]

When We Painted Our 8, Which Was Already Primed, We Gained 10 #.
Doug P.

[RV7ator]

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

[n2prise]

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

[chuck]

Originally Posted by RV7ator

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

[f1rocket]

Originally Posted by chuck

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.

[MistyFac]

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?

[chuck]

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/techni...ht/weight1.pdf

Originally Posted by MistyFac

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

[gmcjetpilot]

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.




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)

[RudiGreyling]

Hi gmcjetpilot,

nice table of camparisons, well done!