W/B Gross for RVs higher than Van?s specs?

RVswede · #1 11-01-2005, 12:38 PM

Hi all!

Thanx for a GREAT community, the fellowship of the RV has no comparison, (thanx Doug for a great site)

Now to my question, Why do RVs differ in gross weight? (read on past the obvious)

I found out on Dan Checkoways site (great site Dan) in the WB section that the gross weight differ a LOT, not so strange you might say (diff engine, equipment etc) what I do not understand is that some aircraft have 1950 lbs in gross weight, 150 lbs higher than Van?s specs.

What am I Missing (I am looking at RV 7 both models)????

Blue skies everyone

John Burman
Sweden
RV wannabe, 120 hours in PA-28 spamcans
Will start soon, but not as soon as I want to

dan · #2 11-01-2005, 12:45 PM

You, as the manufacturer, can set max gross weight at whatever you want it to be. Make it a million kilograms if you want.

I selected 1950 lbs for my RV-7. Van's specs out max gross at 1800 lbs. Why did I use a higher number?

I wanted to be LEGAL if I got into some wacky circumstance where I wanted to fly with me, a 300-pound passenger, full fuel, and bags. It wouldn't be the smartest thing to do, but at least I would be legal and within the "published limits" for my aircraft.

Why didn't I just pick 2500 lbs instead? Because at some point, the likelihood is that somebody else will own this plane. I don't want to open the door up to that person getting into too much trouble just because the "manufacturer" said it was "ok to load it like that."

So 1950 lbs seemed like a good balance between "I'll be covered" and "the next guy won't get into too much trouble."

)_( Dan
RV-7 N714D
http://www.rvproject.com

RVswede · #3 11-01-2005, 12:51 PM

Thanx for the QUICK answer, as stated before, the RV community rules

Thanx a lot Dan,

John Burman
Sweden

Kahuna · #4 11-01-2005, 12:52 PM

Oh No, here we go.
Now you have done it Dan! Couldnt leave well enough alone could ya!

Bob Axsom · #5 11-01-2005, 01:06 PM

You establish the W&B numbers for your creation and do the testing and record the data to justify them. Each one is treated as an individual experimental airplane rather than a type certified airplane that you assemble at home. The gross on my RV-6A nameplate and W&B sheets is 1800 lbs which allows full 55 gallons of fuel and two standard 170 LB. occupants. If I want to add another wing and strut brace everything and call the gross 2500 lbs I think that is my call but I do need to get an air worthiness inspection sign-off. I think but I do not know for sure that the spirit of experimental development prevails with the corresponding lattitude for the airplane. There are a significant number of people that are designing and building aircraft from scratch and the motivation to do that should not be squashed because many of us choose to start our project from materials and assemblies that are available as a "kit" for building an airplane. My airplane has a 1.5 foot longer wingspan than Van's specs, it carries 17 additional gallons of fuel and it has a lot of other one of a kind modifications.

Bob Axsom

jcoloccia · #6 11-01-2005, 01:23 PM

I was just wondering about this the other day (not the W&B thing, but how Vans arrived at the g-loading numbers). Anyone know what g-limits are at the 1800 gross weight?

Here's why I'm curious. For the RV-7 +6,-3 G's @ 1600 lbs would make +4.4, -2.2 (utility category...I think I got those numbers right) seem perfectly reasonable @1950 lbs. Maybe Vans built in a little extra safety margin to keep us all alive, and that's totally OK with me

I wonder, though, what the limiting structure is with the gross weight. Maybe the horizontal stab or landing loads?

I'm not trying play engineer. I'm really just curious

Maybe I'm just not thinking about this right. It still feels like Monday morning for some reason...I'm a little slow...

BillyBob · #7 11-01-2005, 01:57 PM

Has anyone ever seen or made up a Vg graph for various Vans models?

I attended a Wings Weekend this past weekend and in one seminar the instructor made a big point of the fact there is a negative G limit and that it is smaller than the positive G limit (maybe not true for Vans?).

It would be nice to see a typical Vg graph for say a typical Vans RV-6A.

gmcjetpilot · #8 11-01-2005, 06:35 PM

Quote:

Originally Posted by BillyBob

Has anyone ever seen or made up a Vg graph for various Vans models? It would be nice to see a typical Vg graph for say a typical Vans RV-6A.

Billy Bob:
If you have the load factors (limit +6, -3; ultimate +9, -4.5) and limit speeds (Vne, Vno, Va, Vs1) you can make the Vg diagram.

http://www.faatest.com/books/FLT/Cha.../VgDiagram.htm

This data is provided by Van's. The graph is nice but not of too much use in flight; you just need to know the "Numbers" or limitations.

If at higher gross weights your allowable load factor is less, so structural strength is not exceeded.

Your maximum maneuver and structural cruising speed past Van's standard gross is affected. CG of course should be in the recommended range. In my opinion when over standard gross your CG should not be near the limits due to stability.

Needless to say performance, climb, t/o, landing, cruise will be less, but the RV's have excess get-up it is not a big factor. However a over gross RV at high density altitude will be more Cessna 152 than the fire breathing hotrod you are use to, at light weight and sea level conditions.

Quote:

Originally Posted by jcoloccia

Anyone know what g-limits are at the 1800 gross weight?

Here's why I'm curious. For the RV-7 +6,-3 G's @ 1600 lbs would make +4.4, -2.2 (utility category...I think I got those numbers right) seem perfectly reasonable @1950 lbs. Maybe Vans built in a little extra safety margin to keep us all alive, and that's totally OK with me

Not sure where you are going. You are correct 1950 lbs is reasonable, but you can't assume you are good for utility load factors at this weight, normal category, sure.

+3.8g to ?1.5g for the normal operational category;
+4.4g to ?1.8g for the utility category
+6.0g to ?3.0g for the aerobatic category.

Also you are correct that there are margins of safety built into the engineering, which is standard practice. This accounts for things not being perfect, more structural wise than pilot wise. The pilot is always required to operate with in the limitations. However at 1,600 lbs and 6 g's something may bend. Over 9 g's there is a good chance that something important breaks. Respect thy limits ye pilot, dost thee ground comeith up and smite thee mightily.

George

ericwolf · #9 11-02-2005, 07:25 AM

Quote:

Originally Posted by jcoloccia

I wonder, though, what the limiting structure is with the gross weight. Maybe the horizontal stab or landing loads?

I believe that gross weight in RV's is limited by the landing gear. I know that when Jon Johanson got approval from Van's for a higher gross weight (because Australian homebuilt rules are more strict), they said that takeoff was acceptable at the higher gross weight, but not landing. Since he landed with way less fuel, this was acceptable. I suspect that most RVers who takeoff over gross weight will burn enough fuel such that when they land, they are near or within gross.

jcoloccia · #10 11-02-2005, 08:21 AM

Quote:

Originally Posted by ericwolf

I believe that gross weight in RV's is limited by the landing gear. I know that when Jon Johanson got approval from Van's for a higher gross weight (because Australian homebuilt rules are more strict), they said that takeoff was acceptable at the higher gross weight, but not landing. Since he landed with way less fuel, this was acceptable. I suspect that most RVers who takeoff over gross weight will burn enough fuel such that when they land, they are near or within gross.

Maybe my POH will have a Max Ramp Weight, Max TO Weight, and Max Landing Weight, just like the big boys.