What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

variabilty in empty weight

bryanrene

Active Member
I am not a builder but have a question about building in lightness. It would seem that all examples of a particular model would have a very similar empty weight of the basic airframe and then the difference in weight would be due to the add on items (engine, prop, avionics, interior, maybe even paint). If you had a baseline airframe weight, you would then be able to estimate weight based on the add on items, or the effect of removing/changing items.

Are there significant weight difference choices in the construction of the airframe?

Trying to wrap my head around the vastly different weights of planes. Is it maybe due to inaccurate initial weighing?
 
Too many variables

While nearly every certified airframe is built to a spec, and pretty much identical, our RV' are not. keep in mind also, an experimental is not required to be weighed on an accurately calibrated scale, and many have been weighed on bathroom scales..and may not reflect a totally accurate weight. Then there is the prime the inside, some, all ,none, varying layers of paint ect. Seemingly simple things like wiring weight (harbor freight wire vs zefftel aircraft wire) can be pounds of difference. The only baseline airframe weight I could see achievable accurately MAY be via VANs for a quickbuild kit, and that is meaningless. Even looking at the W/B data for 99% of experimentals, that data was put together and accepted by an individual using the best information he/she could obtain and seldom followed by the quality control of a certified aircraft. That why I often recomend to a "new owner" of a previously built RV to take the W/B as a baseline. Fortunately, the RV's built to or close to plans, with typical engines and equipment are a pretty safe bet of being in range even if the W/B data is sloppy. The data shared on this site between builders is a wonderful way to find a pretty fair target weight and C/G considerations to understand.
 
30# composite prop vs 65# Hartzell C/S prop
15# Odessey battery vs 3# EarthX lithium
7# cloth seat cushions vs 15# leather cushions
6# floor carpets vs no carpets
2 of everything on the panel vs basic instrumentation
40# ballistic parachute vs not
8# wheel pants vs not
320 vs 360 vs 390 vs whatever
interior panels vs not
plus 100 other builder choices, in the end they all add up
 
30# composite prop vs 65# Hartzell C/S prop
15# Odessey battery vs 3# EarthX lithium
7# cloth seat cushions vs 15# leather cushions
6# floor carpets vs no carpets
2 of everything on the panel vs basic instrumentation
40# ballistic parachute vs not
8# wheel pants vs not
320 vs 360 vs 390 vs whatever
interior panels vs not
plus 100 other builder choices, in the end they all add up

Tires
Battery location, rear needs a couple of lbs cable
Glass v steam gauges
Type of foam in the seat cushions
Landing lights or not
EI is lighter than mags
Light starter and alternator
 
Last edited:
Then there are the older planes which gave the builders the option of putting in lightning features. Ie.. lightning holes, trimming parts, removing exces material wherever possible. Well being seemingly minuscule, adds up. Consider the weight diferance between a standard washer and an L series washer. Tiny right but add up a couple hundred and you have a couple pounds. Same with lock nuts verses castle nuts, or fiber lock nuts compared to steel locknuts. It all adds up. Saving an ounce here let’s you apply an extra ounce there kind of thing.
 
Certified Aircraft

Certified aircraft vary quite a bit too. One fleet type at work weighs ~11000lbs empty, but varies 250lbs from plane to plane. This is identical avionics, engines, props, seats, everything. This aircraft type has most weights about 10x heavier than the RV-7/8s, so it's easy math to draw comparisons. Looking at two identical RV-8s and seeing a 25lb difference would make me wonder too.

Throw in all the variables in experimental, a 100lb difference isn't surprising. Metal prop, angle valve engine, dual lead-acid batteries, IFR panel with lots of redundancy, primer throughout, fancy multi-colour paint job, leather seats, bigger tires, steel gear (for those with aluminum gear options). That will be much heavier than a fixed pitch composite prop on the small engine, basic VFR panel, no priming, polished aluminum exterior, cloth seats, standard tires, and aluminum landing gear.

Just speaking of props, a heavy 3 blade constant speed prop can be upwards of 60lbs, where a 2 blade fixed pitch composite prop can be 10-15lbs. That's up to 50lbs difference in the prop alone when looking at extremes.
 
7# cloth seat cushions vs 15# leather cushions

15 lbs for leather cushions. This made me smile as it is one of my "pet peeves" interior options. A complete, commonly used, leather interior for an RV-7 can easily weigh 55 lbs.

Another big weight variance is paint. A good paint job from a professional aircraft painter can weigh as little as 18 lbs. compared to an auto painter who is typically not concerned about weight can be up to 30-40 lbs.
 
Last edited:
Correction noted - cushions should have been cushion
I was totally floored recently after doing a W&B on a F-1, on how heavy the seat cushions were in that thing.
 
Weight and Stall

A few years ago I went for biennial. Afterwards the DPE, a far retired ATP through me a question:

Why is stall speed greater for an aircraft with a minimally weighted load compared with maximum?
 
A few years ago I went for biennial. Afterwards the DPE, a far retired ATP through me a question:

Why is stall speed greater for an aircraft with a minimally weighted load compared with maximum?

All other things unchanged, this statement is untrue. However, in many airplanes adding load also moves the cg aft. Moving the cg aft means the horizontal stabilizer down-force is reduced, which means the total force required from the wing (weight plus elevator down force) is reduced, which means for any given speed the angle of attack is reduced, which means stall speed is reduced. So if the weight is held constant but cg moves aft, stall speed goes down. If you increase weight but move cg aft, you have to do the actual calculation, stall speed depends on the details.
 
All other things unchanged, this statement is untrue. However, in many airplanes adding load also moves the cg aft. Moving the cg aft means the horizontal stabilizer down-force is reduced, which means the total force required from the wing (weight plus elevator down force) is reduced, which means for any given speed the angle of attack is reduced, which means stall speed is reduced. So if the weight is held constant but cg moves aft, stall speed goes down. If you increase weight but move cg aft, you have to do the actual calculation, stall speed depends on the details.

Also, some planes, including certified ones, run out of elevator authority at forward CG’s and low speed before they reach stall. This sometimes shows up in POH performance tables and charts leading people to believe stall speed may be increasing at light loads.

Skylor
 
Weight Adders

While nearly every certified airframe is built to a spec, and pretty much identical, our RV' are not. keep in mind also, an experimental is not required to be weighed on an accurately calibrated scale, and many have been weighed on bathroom scales..and may not reflect a totally accurate weight. Then there is the prime the inside, some, all ,none, varying layers of paint ect. Seemingly simple things like wiring weight (harbor freight wire vs zefftel aircraft wire) can be pounds of difference. The only baseline airframe weight I could see achievable accurately MAY be via VANs for a quickbuild kit, and that is meaningless. Even looking at the W/B data for 99% of experimentals, that data was put together and accepted by an individual using the best information he/she could obtain and seldom followed by the quality control of a certified aircraft. That why I often recomend to a "new owner" of a previously built RV to take the W/B as a baseline. Fortunately, the RV's built to or close to plans, with typical engines and equipment are a pretty safe bet of being in range even if the W/B data is sloppy. The data shared on this site between builders is a wonderful way to find a pretty fair target weight and C/G considerations to understand.

Primer and paint as mentioned above are a biggie. If you choose to prime interior and structural parts, pay particular attention to keeping the application as thin as possible because the weight can add up fast.

One thing that I haven’t seen mentioned is that many builders add things like extra inspection panels, access panels and additional removable floor sections. These items will also increase weight particularly where extras doublers, nut-plates and fasteners are involved. It may not seem like much, but these do add up.

Bill mentioned wire, and this can be a biggie. My IFR RV-8 with rear mounted battery has 26 lbs in the wires, contactors, connectors, fuse-blocks and switches. I used all aircraft tefzell wire and I carefully calculated wire size vs load and length of run and was careful not to go to oversized wires. I even used 24 AWG instead of 22 AWG for most avionics signal wires, I kept my switch quantity to a minimum, I used lightweight Bussmann fuse-blocks (lighter than breakers), and I did not add extra connectors in places like the wing roots. I also ran about the minimal wire lengths possible and I don’t have large service loops anywhere. Some of the things adding up to that 26 lbs include the large positive battery wire to the aft mounted battery, three RG400 runs to wingtip antennas plus two more to the aft fuselage for the GPS and transponder antennas, wiring for old-fashioned Whellen strobes in the wingtips and tail, and wire for incandescent position lights, landing lights, and taxi lights at the airframe extremities. I wish LED lighting options were available when I was building. I now have led position and strobe lights, but the heavier wire remains.

Knowing what I now know about how much weight can end up in the electrical system, I really cringe when I see panels with dozens of switches and individual circuit breakers (“acres of breakers”) because I know those are adding a lot of weight.

One other area that can potentially add to aircraft weight is the extensive use of steel-braided hose with stainless fittings for fuselage plumbing, vs carefully routed aluminum tubing and aluminum fittings.

Skylor
 
Last edited:
To answer you YES.....

The BOW (basic operating weight) is the airplane, empty of pilot, passenger(s), cargo and fuel.... It does include basic fluids (engine oil, brake fluid).

Will empty basic weight vary *widely* with Amateur built kit planes??? YES.... It will vary with builder adding fancy instrument panel*, fancy paint, interior, bigger engine, constant speed prop (vs light wood fixed prop). What Van publishes is realistic only if you watch weight.

I flew the first RV6 proto type with Mr. Van off his grass strip. This was in the 1980's. It had the bare min day night VFR set up with 160HP engine and fixed wood prop. It was a delight. Super light (also I weighed less and Van is a skinny guy), The many decades since I have flown many RV's, one's I built and other RV's.... KEEP IT LIGHT.

* Full IFR +30 yrs ago, involved vacuum pump, mechanical gyros, larger heavier avionics... Now with "glass" or EFIS you can add a lot of capability with less weight. However becoming totally ELECTRICALLY DEPENDANT (flight instruments, electronic ignition, electronic fuel injection) you need a dual bus system. This complexity adds weight. So a day VFR only fixed pitch prop RV is going to have better performance (takeoff, climb, cruise, payload), have better control feel and stay under aerobatic weight limit with a passenger. Some RV's are so heavy you can not do aerobatics with a passenger.
 
Last edited:
Back
Top