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

[RVswede]

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]

You are the manufacturer.

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]

Thanx

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

Thanx a lot Dan,

John Burman
Sweden

 

[Kahuna]

Oh No

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

 

[Bob Axsom]

Basically You as the manufacturer set the gross Weight

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]

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]

Vg Graph

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]

You have all the information

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/Chapter17/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.

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]

Landing Gear

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]

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.

 

[jferraro17]

I had this conversation with a member of this forum a couple of years ago...in order to protect him due to my inacurate recall/interpretaion, I'll leave his name out, BUT:

He explained to me that G loading is dirived lineraly. So by accepting 1800 lbs and a given max G (positive or negative), and then also 1600 lbs and the max aerobatic g limit; you can define the relationship mathmatically, then change the gross weight and come up with the new g limits for any given weight.

It made more sense when this individual explained it TO me...but I hope you get the idea. If I can find the original e-mails when I get home tonight, I'll try to post them.

By using this method, you could set 1900, 1950, 2000 as your gross, and come up with a G limit that should pass the sanity check from your DAR. The one thing he said is not to add or subtract anything to the structure...load transfer, etc (see other threads!)

 

[Alex]

He explained to me that G loading is dirived lineraly. So by accepting 1800 lbs and a given max G (positive or negative), and then also 1600 lbs and the max aerobatic g limit; you can define the relationship mathmatically, then change the gross weight and come up with the new g limits for any given weight.

This is ok for interpolating between known G/weight points, but is invalid for extrapolating beyond the published gross weight. The reason is weight distribution. When you add that extra 200 lbs you want to carry, most likely you're putting it in the fuselage. Think of a suspension bridge with something heavy in the middle. Adding weight in the fuselage is not the same as adding weight uniformly over the span of the wing.

 

[mgomez]

This is ok for interpolating between known G/weight points, but is invalid for extrapolating beyond the published gross weight. The reason is weight distribution. When you add that extra 200 lbs you want to carry, most likely you're putting it in the fuselage. Think of a suspension bridge with something heavy in the middle. Adding weight in the fuselage is not the same as adding weight uniformly over the span of the wing.

And then there's the matter of CG...where can you add 200 lbs to a little airplane without affecting the CG? If the answer is "add some weight ahead and some weight behind," then you've changed:

- the bending moment on the fuselage
- the mass properties of the fuselage, which can affect rolling, spinning, and dynamic stability

I'm not saying it can't be done...clearly people do it and most do it successfully. All I'm saying is "you should know what you're doing before you try this."

I recognize that these are experimental airplanes, that we're free to set the limits as we please, etc. But let's give Van's a little credit: if they thought that it was OK to build a 2000-lb, 4 g RV-7, they would say so. It can only increase sales, right?

 

[jcoloccia]

let's give Van's a little credit: if they thought that it was OK to build a 2000-lb, 4 g RV-7, they would say so. It can only increase sales, right?

That was exactly my point, though. Although 1950 gross seems reasonable, there's obviously a reason he made it 1800 instead. I was curious if anyone knew what that was.

Personally, I'm trying to lose 20lbs....that should get me an extra 1/2 hour of fuel right there

 

[Thermos]

That was exactly my point, though. Although 1950 gross seems reasonable, there's obviously a reason he made it 1800 instead. I was curious if anyone knew what that was.

I don't think you'll get a better answer than "that's how Van's designed it." For whatever reason, they picked 1800 lbs max gross weight for the RV-7/8, and that preserves whatever structural strength margins Van's wanted in the airframe over the whole spectrum of expected loads - takeoff, cruising, yanking, banking, landing, etc.

Can you use that margin? Well, it's an experimental airplane so you can do what you want. But as Martin said, you'd better know what you're doing.

Speaking of loads, I'll be cutting down on the pizza and beer too to get a few extra pounds of gas or baggage in the airplane!

Dave