Does anyone guard-band their Phase I W/B numbers??

[noelf]

Sometimes, it is a curse to be an engineer. My goal for some of the flight test was to test at max gross weight, performing an accurate W&B calculation prior to each of these flights. Not knowing how much weight was being lost (or gained) due to fuel density changes in the fixed volume of the fuel tanks caused me to think back on guard-band testing.

Since my goal was max gross weight, I filled the tanks and added ballast to get there. Losing fuel on the ramp prior to engine start (temperature increase = less dense fuel = expansion of the fuel load in the same given tank volume = fuel puddled on the ramp) results in an aircraft weight that is less that my expected take-off weight of max gross. The fact that it is less than gross is nice for Phase II, I just want it to be at gross for the test flights.

The engineer in me wants to know. Guard-banding, specifically, consciously loading to "just over" max gross is the way to do this (not addressing exceeding forward / aft loading here).

[Danny7]

if you are an engineer and really want to get some numbers on it, fill up your tanks, put some tubes that drain from the vents to a bucket, let the airplane sit however long it takes to warm up what you think is a normal amount of time. measure the volume of gas in your catch bucket, calculate the weight lost, throw another 10 lbs of ballast or whatever it comes out to and go flying your test flight.

[thinkn9a]

There are certainly documented examples of those that been tested and documented with Gross weight significantly (more than 5-10 lbs, or fuel and clothing tolerance) beyond Van's recommended max Gross. There are some extreme cases as in the folks that have made multi-continent trips with heavy fuel loads.

I'm sure a number of folks on the forum could provide logbook data of tests. Believe most were on weight... CG probably not so much. e.g. I know when I did forward CG, I really did not want to push further considering issues with nose gear. (do a search on "Enterprise" and "gross"... scroll to last entry, where there is a discussion of two sets of V speeds and weights... up to 2000 lb for a 9A)

Take a look.. do some research and thinking, then figure out what you and your experimental experiment would like to end up at. Make appropriate preps, calcs, and test as the experts on here will tell you if you wander off the beaten path.

[Bullseye]

Originally Posted by noelf

Sometimes, it is a curse to be an engineer. My goal for some of the flight test was to test at max gross weight, performing an accurate W&B calculation prior to each of these flights.

[snip]

The engineer in me wants to know. Guard-banding, specifically, consciously loading to "just over" max gross is the way to do this (not addressing exceeding forward / aft loading here).

Hey Noel,

We do this in the Flight Test world, although only after being cleared by EVERY group within our engineering department. Since we try to certify a certain envelope, we have to test on both sides of that line (see 14 CFR 23.21(b), which basically allows weights of +5/-10% and CGs of +/-7% travel to "count" as the line you are trying to certify).

(Side note: AC 23-8B points out that this is a spread, and not a tolerance, ie., you can't have all your points at -7% of the intended CG. You have to test on both sides of the line.)

Given this, engineering gives Flight Test a slightly larger envelope to assist in certification of the production envelope that ends up being published. (Sometimes, these larger envelopes come with some pretty hefty restrictions, such as a maximum sink rate on touchdown, etc.)

I have to assume (I know assuming is bad) that Van's tested both sides of each weight and CG limit, but I would not go there myself without first being cleared by Van's.

[noelf]

Thanks for the feedback AZ and the reference to the CFR.

And...will your "project" be on display at the upcomming Winston-Salem Airshow?

[Bullseye]

Yes, sir.

http://www.wsairshow.com/performers/

(Second one down on the right.)

[Flyfalcons]

I have a feeling Van's did plenty of testing to come up with the limits they did. I don't see a need to go past those limits when I test my airplane.

[APACHE 56]

[quote=noelf;507091] I am wondering if we, as experimental aircraft builders, also "need / should" perform some level of "just beyond the envelope" testing to ensure that we also have an aircraft that safely meets Van's published W/B numbers.
QUOTE]

Noel,
I can't tell from your post if you are contemplating some "envelope expansion" testing or are really unsure of the validity of Vans numbers.

There are margins in all Vans numbers. The published numbers are not ?butt plucks? but well thought out limits. In ?Right Stuff? school (Class 79, USNTPS) we were taught to fly to our comfort limit then apply a ?what the average fleet pilot can do? margin. I?m pretty confident that Van and his crew have worried all their numbers to a fault. There are over 7000 RV?s flying and they don?t seem to be falling out of the air due to published limitation issues.

I concede you are free to expand any envelope you wish. As this series of posts and many others have said, ?exceeding Vans limits has its burdens.? If your natural inquisitiveness won?t let you rest until you ?know? then the first thing a prudent person would do is contact Van. You can ignore what they tell you but at least you know what the ?Oracle of Aurora? knows. Next you have to be aware of just how much effort goes into expanding an envelope. Determining just how far you intend to go is critical. Are you going to go until the aircraft become unstable? Or maybe you just add 10%. Do you have some critical need to go 10% over? You see where I?m going with this I?m sure. The game is to build an unassailable case that compels you to do the test. Once that?s done the flight test planning begins followed by the safety mitigation finally flight test prep. Tedious and boring. Curiosity isn?t a good enough reason to bust your butt and add another statistic to our already unfavorable accident rate.

Don

[David Z]

For those curious about how avgas weight changes in relation to temperature, here you go. One US gallon weighs:
6.41lbs at -40f
6.26lbs at -4f
6.12lbs at 32f
6.01lbs at 59f
5.90lbs at 86f

[thomaswimmer]

Do you have a source for that?
What about Jet-A1?

Originally Posted by David Z

For those curious about how avgas weight changes in relation to temperature, here you go. One US gallon weighs:
6.41lbs at -40f
6.26lbs at -4f
6.12lbs at 32f
6.01lbs at 59f
5.90lbs at 86f