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Surprise W&B !?

L

ljpappas

Well Known Member
I finally finished my RV7-A after 2 years and was surprised by the W&B results. Anyone else find this to be the case? RV7-A, painted, freshly O/H IO360-A1B6 angle valve, 74 inch Hartzell C/S prop, full Oregon Aero interior, All Dynon HDX and A/P, etc and a size "D" Oxy bottle behind the pax seat, PC680 battery behind the pilot seat.

I knew it would be heavy, and it is a bit porky at 1203 lbs because of the engine and prop. (Angle valve A1B6 has the counterweighted crank and weighs about 33 lbs heavier than the parallel valve engine). I was worrying that it would be too nose heavy, concerned about the original style nose gear being overloaded, and that the CG would be too far forward. I was completely wrong! No matter how I load it, I never get a cg that is forward of 55% of allowed, (about 83 inches aft of datum). Of course it moves further aft with fuel burn or with baggage in the back.

However, this one is very fast with the big engine, 10:1 pistons, flowed, polished and ported heads! I also can't get it aft of the limit even with my 205 lb butt, 10 gallons of fuel and 100 lbs of luggage.

Does anyone have a 7A that has a naturally occurring forward CG?

N535D RV7-A Pappas.jpg
 
I built an early -7a. First flew in '03. It had a stock angle valve and a CS prop. Like yours, aft CG was an absolute non-issue.
196KTAS on 8.9gph. I always thought the angle valve is the Goldilocks engine for the -7/7A.
 
Walt, Yes, I looked that chart over several times during and after the build. After doing the W&B I still don't come to a place where I can load the plane with an overweight nose wheel. But it was one of the reasons that I installed the Anti-Splat Nose Job 2. I just didn't know how the W&B would turn out. Anything aft of about 82 1/2 inches at gross weight fits the chart within Van's parameters. Still I check W&B every flight using the HDX W&B Feature.
 
196 kts is one FAST 7A. Best I can do with my -7 is about 170 kts at 8.6 gph. IO-360M1B, MT 3 blade.
 
He's probably around 250hp with the hot rod motor. Honestly, I would never fly at that speed, VNE is at your doorstep.
 
Any time I hear “surprise” at a W&B result, I immediately question the weigh or measuring process. One easy miss is the actual position of the tire CL. Was it measured on THIS airplane or was the value taken from the drawings (or worse, copied from another builder’s result)? The closer the contact points are (closer on a nose wheel than a TD) the bigger an error becomes in the actual CG.
 
The airplane was weighed on my FBO’s 3 certified scales with the help of 2 of my friends who are both A&P’s. I’m confident in the weighing process and result.
 
I guess my basic question is: Do most 7-A’s have a more neutral to aft empty weight CG, as opposed to a neutral to forward empty CG? What say you all?
 
Too soon to tell after only 2 flights on a fresh overhaul but this plane seems to comfortably make 170-175 knots. My 185hp RV-8 was more like 160-170 knots.
 
Mine (recent build, primer on all metal, new style nosegear, non-angle-valve IO-360, glass panel) weighed in at 1159lb/79.9" CG before painting, and 1188lb/80.4" CG after painting. So it ended up disappointingly heavy. It flies great though and fast enough for me.

Total coincidence, but at 1188 lb empty, this means I'm at exactly max gross (1800 lb) with full tanks, two 180lb people, and zero baggage. So this makes weight/balance calculations pretty straightforward.
 
Jpm757 said:
196 kts is one FAST 7A. Best I can do with my -7 is about 170 kts at 8.6 gph. IO-360M1B, MT 3 blade.
Click to expand...

Walt said:
He's probably around 250hp with the hot rod motor. Honestly, I would never fly at that speed, VNE is at your doorstep.
Click to expand...
Even at 250 HP (!) that theoretically should yield about an 11% increase in speed, or 189 knots assuming 170 with a stock 180 HP.
 
I had LyCon do the jugs and heads on N535D. They gave me an estimated hp that is significantly less than 250 and in line with what vans recommends as Max hp for a 7. Closer to 210 to 215.
 
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Dang, just caught back up to this thread and need to clarify: 196MPH on 8.9 gph
I sold the -7A 6 years ago. I have now 50 hours on the Rocket and have Knots on the brain….
The jest of my comment was aimed at how satisfied I was with the heavier nose with the bigger engine and less about the performance.
sorry for the confusion….
I feel silly…
 
ljpappas said:
The airplane was weighed on my FBO’s 3 certified scales with the help of 2 of my friends who are both A&P’s. I’m confident in the weighing process and result.
Click to expand...
Were the arms measured for your specific aircraft or where the arms used that are in the manual? The ones in the manual were way off from what I and many others measured. Many people forget to measure and this could effect things a lot.

Mine is also nose heavy and nothing I can do gets the CG too far aft.

andoman said:
Dang, just caught back up to this thread and need to clarify: 196MPH on 8.9 gph
I sold the -7A 6 years ago. I have now 50 hours on the Rocket and have Knots on the brain….
The jest of my comment was aimed at how satisfied I was with the heavier nose with the bigger engine and less about the performance.
sorry for the confusion….
I feel silly…
Click to expand...
haha thanks for the clarification. My BS meter was pegged there for a minute. :ROFLMAO:
 
jcarne said:
Were the arms measured for your specific aircraft or where the arms used that are in the manual? The ones in the manual were way off from what I and many others measured.
Click to expand...
This was exactly my point in post #10. I use a plumb bob and tape measure on all my airplanes and have found several that were off from the nominal value in the drawing. One of my wheels is 1 inch further aft than the other on my Rocket, for instance. If I used book values for the datum, my CG value would be off.
 
Toobuilder said:
This was exactly my point in post #10. I use a plumb bob and tape measure on all my airplanes and have found several that were off from the nominal value in the drawing. One of my wheels is 1 inch further aft than the other on my Rocket, for instance. If I used book values for the datum, my CG value would be off.
Click to expand...
Yep, I got you Toolbuilder. My nose gear was waaaay off. Mains were off a bit too.
 
Toobuilder said:
Any time I hear “surprise” at a W&B result, I immediately question the weigh or measuring process. One easy miss is the actual position of the tire CL. Was it measured on THIS airplane or was the value taken from the drawings (or worse, copied from another builder’s result)? The closer the contact points are (closer on a nose wheel than a TD) the bigger an error becomes in the actual CG.
Click to expand...
Just for fun I changed the nose wheel arm on customers 7A 1" to check the result: CG went from 79.68 to 79.95, (weight on wheel was 309lbs).
So a 1" error on the nose has negligible effect on CG.

Moving the arm on the mains has a greater effect, with the same 1" move CG went from 79.68 to 80.4, so the longer the arm the greater the effect on CG.
Makes sense.
 
Interesting. I did not make the actual measurements from the datum. I used the Van’s published arms. I’ll measure them myself and see if they are accurate for this aircraft. Of course, how do we know the exact location Vans used as the arm data point for baggage, pilot, pax, and fuel?
 
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PhatRV said:
Is the 196KTAS with sub 9gph due mostly to the angle valve?
Click to expand...
andoman said:
I built an early -7a. First flew in '03. It had a stock angle valve and a CS prop. Like yours, aft CG was an absolute non-issue.
196KTAS on 8.9gph. I always thought the angle valve is the Goldilocks engine for the -7/7A.
Click to expand...
Is overpowering an airframe until the cruise airspeed is near or above redline a safe thing to do?
 
ljpappas said:
…Of course, how do we know the exact location Vans uses as for baggage, pilot, pax, and fuel?
Click to expand...
Some of those are “good enough”, but that assumes the baseline empty CG is “dead on”. Compounding errors with an inaccurate baseline, less than perfect baggage loading and then the “extras” that inevitably creep in is where things can get out of hand. Since you CAN get the baseline “perfect”, you SHOULD.
 
You really don't want to be flying an RV at or above the redline/VNE. Inadvertently get the nose pointed down in a moment of inattention or rough air and you could be into a situation where you tear the airframe apart. Others have done it with fatal results. Not just with RV's of course, in any plane. Just stay away from VNE. If you just really NEED to go faster, you NEED a different airplane. 160 to 175 knots is an excellent cross country machine for covering 1/4 of the country in a day easily. For even more funh, try flying at 120 knots and see if you can get down to 4 gallons per hour! That would give you a lot of flying hours around your airport for not very much fuel cost. Taking off and landing from the same airport on the same day is really what most of us do with our aircraft anyway. Not as glamorous as talking about the flight to 48 states and the Bahamas, but it is what we really do the most with any private pilots license. Still, I love it!
 
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I think we can put this to bed. That poster clarified it was MPH not Knots.
There are lots of threads about VNE
 
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