Veetail88

Veetail88

Well Known Member
With only a few hours left in Phase 1, I'll be spending the weekend pushing out to the weight and balance limits.

It's imperative that I'm able to take my brother in the airplane, and neither of us is "dainty", so that means I have got to push pretty much all the way to the aft and max gross limits with ballast now.

I understand that the pitch forces will become very light and that will make landings a bit more tricky, and I know that the design has been tested and deemed "safe" at the published limits, but I'm wondering. Have most folks gone all the way into the corner of the envelope during testing? Does anyone consider the airplane unsafe there? Does the airplane scare anyone at that point?

Aside from the above, I would appreciate any input or tips on this. I'll go out there a chunk at a time, fly it to altitude and do some stalls and slow flight before landing it at each step.

Currently I've got 140 lbs in the back seat and the thing is extremely pleasant, but I suspect that's going to change quite a bit.

(Image removed by OP for inaccuracy)
 
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With aft CG, I have experienced a reversal in stick pressure in the flare. With aft CG and trim so there is forward trim pressure on the stick on final, as I flare the forward trim pressure on the stick decreases and actually reverses just before touchdown. In other words, I am holding back pressure on the stick on short final and just before touchdown, without change in elevator, I must decrease my back pressure to 0 and then go to push on the stick to maintain angle of attack. There is at least one thread here on VAF about this. It suprised me first several times I experienced this stick pressure reversal.
Search"RV-8 stick force gradient when landing".
 
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So far I've done about 240 in the rear with no baggage. You basically land with cruise trim. As you slow the nose will come right up. I do wheel landings in this config with plenty of speed and it works just fine. As indicated above be ready to PUSH on the stick if needed to keep the speeds up and put the main gear on the runway. It actually lands fairly easy with weight in the back, but 3 point is a bit more tough as the nose will come up quickly when you slow.
 
Jesse,
I've flown with max rear cg without any issues. As noted, it does require getting use to both for 3 point and wheel landings. For testing to the max limit cases of plastic pint size water works well for ballast. They stack up nicely in the rear seat and baggage area.
 
I've seen no issues at/close to aft limit.

However, I would not 100% trust all the figures are right, nor translate between different RV-8s. So build in some testing as above both at aeros aft limit and full aft limit.

I'm sure you'll NB easy to overstress, be wary of rough surfaces (tailwheel discussions with rudder). In general 3 point easier aft CG, wheeler fwd IMO.
 
Just as a follow up data point for future thread searchers, as of this past weekend I've flown the 8 ballasted to a CG of 85.13". 79% aft.

As noted by all who have come before, the pitch stick force becomes 0 at this point and is quite a bit different than I've been used to. With some practice, landings are ok but strange. The tendency to overcompensate in the flare happens even though you expect it and you can't rely at all on stick feed back.

After a bit more practice here I'll load it to 85% aft and I think I'll quit there. As I haven't yet experienced the feedback reversal near the stall, I know my numbers are close enough and that I still have a bit to go to get to full aft CG. At this point I don't want to push it there. I'll just be sure to add ballast to the front baggage compartment to avoid going past that 85% mark.

My take away from this is that I'm glad I postponed pushing these limits to the end of the flight test phase. I've been able to develop a feel for landing the airplane prior to receiving screwed up stick inputs at this critical phase of flight.
 
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gather time

At what point in the build were you thinking of giving your brother a ride?
I know it is tempting, after pink slip to jump right in and start giving rides in your pride and joy. Might think of logging some more hours at that point with some water bottles in the back.... just to be sure the handling is something you can handle. After all the build effort... it is worth it to wait and protect your investment.... and keep the brothers wife happy too !
 
Yeah, I'm planning on logging quite a few more landings with water bottles before putting my brother's welfare on the line. :D

I currently have 4 of these in the stack with 2 more on the rear baggage floor.

1 more will be stuck in where a passenger's left leg would go and that will bring me to 85.82", fully 1" forward of Van's limit, which is not anywhere I want to go.

280im83.jpg
 
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Just wanted to post a follow up on this thread.

Having researched a bit and hearing from some of you what your empty CG was, things weren't adding up and I became skeptical of my numbers.

Speaking with a fellow 8 owner at last night's local EAA chapter meeting, he'd agreed to send me his weight and balance information.

Today, comparing our numbers, a very DANGEROUS error surfaced.

The spreadsheet I've been using is one I'd gotten from another builder some time ago and modified it for my airplane. The numbers I'd used for the right and left tires and the tailwheel were incorrect! The mains were off by a bit, but the tailwheel arm was off by 11"!

I'm quite embarrassed about this and apologize for having posted an image of that sheet. I've deleted it from the original posting so future browsers cannot use them as well. Also, I hope no one has taken that info and plugged it into their numbers.

It turns out, the last condition I'd flown the airplane in was actually 97% aft in the range! It is a really good thing that I had that nagging going on in my head as my next move was going to be adding another 35 lbs in the back seat which would have moved me well aft of the limit and I'm thankful I'll not experience how the airplane handles there!

You can call me lucky, but I'm pretty sure the Big Guy was watching out for me!

Two good things though, 1, I can consider that part of my flight testing to be complete, and 2, with careful fuel level management and a few pounds in the front baggage compartment, I can indeed take my brother flying, and that's very important to me!
 
Jesse, glad you caught the mistake. Hope all goes well and you really enjoy the little fighter plane...
 
RV-8 CG

Jesse: Good pick! Glad you found this error before trying any CG location farther aft, and that you can now fly with your brother. My -8 flew first 12/4/15 and is located near you, for now at KETB. Only 10 hours flying so far. Jeff Point helped me prepare in his RV-6. Hope to see your plane soon. Have fun with your bro. Bill
 
Veetail88 said:
...The spreadsheet I've been using is one I'd gotten from another builder some time ago and modified it for my airplane. The numbers I'd used for the right and left tires and the tailwheel were incorrect! The mains were off by a bit, but the tailwheel arm was off by 11"!..
Click to expand...

That's a good catch, and illustrates a potential pitfall with the considerable "spreadsheet sharing" that goes on here. Swapping spreadsheets is a fine way to find a format you like or entertain yourself while building, but when it comes to the actual weight and balance of your airplane, you really need to break out the plumb bob and tape measure. Though it looks like a fat finger mistake with the spreadsheet you used, these airplanes are custom built and may be a bit off. If nothing else, physical measurement is a way to double check your math.

Personally, I take the measurements every time I weigh the MY airplane - just to make sure I didn't screw it up last time.
 
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