What's new
Van's Air Force

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

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

N

noelf

Well Known Member
So, during my Phase I flight testing, I went through all kinds of permutations of weight and CG configurations. Now, after looking over some of the numbers and results, I got to thinking about guard-band testing, and what I did in my prior life as an engineer designing / developing computer systems.

I can only guess that Van's published W/B numbers include some guard-band knowledge, and that his real numbers are in excess of what we get to see. 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.

I realize that as the builder, we get to declare what we want on the official paperwork, I was just curious if anyone went beyond Van's numbers just to satisfy their own knowledge quest. I suspect that some might be tempted to guard-band the gross weight number, and some may investigate some excess in the forward & aft CG margins or limits.

I know that I am not qualified to know how far is too far, so my testing was done to the limits Van published for my -6A. Just wondering what others have done...
 
Here's my take on this, as someone who does flight testing on type certificated aircraft.

Presumably, you are mentally and physically prepared for things to go bad during your flight testing. If you are doing risky testing, you are likely wearing parachute and helmet, and are prepared to bail out if necessary. This increases your odds of survival if you find a major problem.

In real world flights, there is some risk that you might inadvertently exceed the edges of the claimed operational envelope (i.e airspeed limits, g limits, CG limits, etc). If there is some major problem lurking just beyond the edge of the envelope, you might stumble upon it when you are not mentally or physically prepared to deal with it. I.e. you are caught by surprise because you have never seen this issue before, and you are likely not wearing a parachute, etc. If you stumble upon a major problem you may not survive.

So, in my opinion, it is wise to do the flight testing to a bit beyond the planned operational envelope. This way you ensure there is some small margin beyond the operational envelope to cover small excursions that may occur in service, etc.

But, if you have a very standard RV, built according to the plans, with a conventional engine, well known propeller, with no major mods, the service history suggests that there is lots of margin on all aspects of Van's recommended envelope. There is very little risk of there being a major problem lurking just outside the edges of the envelope.

So, if your aircraft is very standard, there is probably little point to doing much testing outside the edges of the planned envelope. If your aircraft differs from the standard in some significant way, you probably should test a bit past the claimed envelope.

Note: despite all I said above, I do not recommend that anyone test to a higher g than the recommended structural envelope.
 
Guard-Banding examples...

Example 1: if I were to sell a widget, and the specifications stated that the device would function as described in the user provided voltage range of +10VDC to +14 VDC, I, as an engineer, would design the widget to operate correctly in the voltage range of +9VDC to +15VDC. The extra margin of +/ - 1V is the "guard-band".

Example 2: If I were to design a tower and guarantee the structure to withstand a sustained wind speed of 120 mph, I would want to design the structure to 150 mph. The "extra" undocumented 30 mph is the guard-band.

The downside of a guardband-ed design is that there is undocumented performance over and above what is published. In the engineering and marketing world, that unrealized performance can affect competitive pricing, and warranty cost vs a perceived robust design.

In the example of testing my RV-6A to max gross weight, I can weigh the aircraft, add in all the fuel weight, my weight, and any other "stuff" to get to max gross. If I weigh the aircraft at this point, how accurate are the scales, what is the air temperature and did it cause the fuel to expand or become denser than when I calibrated the fuel tanks, and thus affect the fuel weight calculation. What is my real, true weight?

If I were to guard-band the gross weight scenario, I may want to physically add (as an example) 7.5 lbs over and above what I originally calculated. Then flight test this configuration. This would guarantee that as long as I stayed at, or under, the original claimed gross weight, I would have a controllable aircraft.
 
Kevin, my plane is a pretty standard RV-6A. No funny mods, no claims of gross weight other than what Van stated in my builders documentation. I am happy with "only" testing to, and within, the numbers that Van provided.

I was just looking to see if it were prudent, or common practice, to test "just" beyond what Van published, and what numbers other have used.
 
noelf said:
If I weigh the aircraft at this point, how accurate are the scales, what is the air temperature and did it cause the fuel to expand or become denser than when I calibrated the fuel tanks, and thus affect the fuel weight calculation. What is my real, true weight?
Click to expand...

would the weight of the airplane differ with temperature, or only the volume of the fuel as it heats up? I did not know weight is affected noticeably by temperature.
 
Danny7 said:
would the weight of the airplane differ with temperature, or only the volume of the fuel as it heats up? I did not know weight is affected noticeably by temperature.
Click to expand...

I'd say no - unless you change the total number of fuel molecules in the tank, the weight should be the same, regardless of their density (temperature).

Paul
 
Fuel density does change with changes in temperature. I just do not know how to quantify the change. When I fill up the tanks in my -6A in the cool of the morning, just letting the aircraft sit out in the sun, rising air temperature for several hours heats up the wing skins and fuel. Fuel then can start to stream (well, stream may be a bit of an exaggeration) out of the fuel vents.

At this point, if I look in the fuel filler opening, I have the same "volume" of fuel as I did when I filled the tanks earlier in the cool morning. Now, the warmer fuel that is still in the tank occupies the same volume as before, but there is less fuel in the tank (because I can see it puddled on the ramp). That quantity of less fuel equates to some amount of "undocumented" weight that just evaporates. I just don't know to what extent it affects my max gross W/B calculations.
 
The Super -6's have already done some "guard-banding"...

...for you, running 30 or more MPH above Van's VNE. Another single place Rocket exceeds 300 MPH with Van's wings. I personally have been aboard a Super -6 exceeding 240 MPH. An -8 also exceeded the 9G ultimate and became unglued with two fatalities.

Go do some rolls and loops and forget about guard-banding.

Best,
 
noelf said:
Fuel density does change with changes in temperature. I just do not know how to quantify the change. When I fill up the tanks in my -6A in the cool of the morning, just letting the aircraft sit out in the sun, rising air temperature for several hours heats up the wing skins and fuel. Fuel then can start to stream (well, stream may be a bit of an exaggeration) out of the fuel vents.

At this point, if I look in the fuel filler opening, I have the same "volume" of fuel as I did when I filled the tanks earlier in the cool morning. Now, the warmer fuel that is still in the tank occupies the same volume as before, but there is less fuel in the tank (because I can see it puddled on the ramp). That quantity of less fuel equates to some amount of "undocumented" weight that just evaporates. I just don't know to what extent it affects my max gross W/B calculations.
Click to expand...

If you had room in your tanks for the fuel to expand, that is all it would do. there would be no weight decrease.

the weight decrease you are thinking of is strictly related to the fact that some of the fuel is removed from the airplane i.e. it pumped itself overboard due to expansion of volume

edit- it should have no effect on your max weight and balance calculations, since weight is being removed, although i would guess it is a very small amount
 
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).
 
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.

:)
 
How much research on other planes "beyond recommended limits"

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.
 
noelf said:
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).
Click to expand...

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

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

noelf said:
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
Click to expand...
 
Fuel Weight vs Temperature

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
 
Source is the Canadian Flight Supplement and it covers the weights for JetA, JetB, Avgas and a few oil weights.

Weights for one US Gallon of JetA is:
7.32lbs at -40f
7.19lbs at -4f
7.09lbs at 32f
7.00lbs at 59f
6.91lbs at 86f
 
Nice to know...

...However, in the past month's 98 deg temps, the measly weight savings with the heat, doesn't make my Air Tractor exactly 'jump' off the runway when I'm loaded with 450 gallons of 8.33lbs per gallon.:D

Thanks,
 
...to go back to guard-banding... as an example here in the UK we do not have an "experimental" category. We have to build RVs much closer to the Vans design, and if we stray, have to seek a "mod" - a laborious process in time, paperwork and (small) cost. In practice it serves to deter unecessary costly / heavy / unsafe modifications, most of which detract from the excellent Vans design.

The 1st of a new RV series is examined closely, and flight tested, including "guard banding" to say VNE +10% (I think). Some of the flight testing is done by qualified test pilots. All subsequent builds are just tested for 5-10hrs to some of the limits, and can be "test flown" by relatively inexperienced pilots (often a builder with a few hundred hours including relevant, but not necessarily RV, experience).

Andy
RV-8 G-HILZ
RV8tors
 
You must log in or register to reply here.
Back
Top