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

[noelf]

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...

[Jerry Cochran]

Please explain to a non-engineer what the term "guard-band" means...

[Kevin Horton]

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.

[noelf]

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.

[noelf]

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.

[Danny7]

Originally Posted by noelf

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?

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.

[Ironflight]

Originally Posted by Danny7

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.

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

[noelf]

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.

[pierre smith]

...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,

[Danny7]

Originally Posted by noelf

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.

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