Help with CG calculations RV9

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sibriggs
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Help with CG calculations RV9

Post by sibriggs »

I'm building an RV9 with a IO320 and an 18 lbs composite prop. In order to keep the CG in the empty weight design range, I ordered a 2" longer engine mount. I put together some calculations based on the data provided in the RV9 plans to determine a target CG.

I guest-a-mated the datum locations for the engine and prop based on the drawings and a note from Ken Scott that the prop on the demo RV9 was located 54 " forward of the wing leading edge. I can't confirm the actual locations because my RV is not that far along yet.

If anyone has the time I'd really appreciate another set of eyes looking over my work for errors. I sent a note to Vans, but find they're quite busy and don't have time to look the info over.

Here's the email with the info I sent to Vans:

Ken Scott to Me
10:30 AM (19 minutes ago)
Ok… look for a shipping call tag in your email soon. I’m afraid our engineers are pretty tied up and aren’t able to do individual consulting, so we’ll have to trust your numbers.

From: Stephen Briggs [mailto:sibriggs@gmail.com]
Sent: Tuesday, February 16, 2016 8:12 PM
To: Ken Scott
Subject: Engine Mount

Hi Ken,

I have the Sensenich composite prop I bought through Vans, not the metal prop. I think based on the weight difference I'll stay with the longer mount.

These are my numbers, they are based on my engine weight at 291 lbs, moving it forward from location 28" to 26", using the numbers shown for the demo RV9 in the plans document (pg 14.4)

Removing the Sensenich Metal Prop off the demo RV9 at datum location 16" at 31 lbs and adding my Ground Adj FP (I bought from Vans) at location 14" at 18 lbs.

Below at my calculations based on the numbers in the plans (pg 14.4);
Weight Location Moment
Aircraft 1057 77,68 82107
Engine -291 28 -8120
Engine +291 26 +7540
Prop -31 16 -496
Prop +18 14 +252
Totals 1044 77.8 (cg) 81281

These calculations didn't include additional cowling length or the added weigh of metal in the engine mount.

Would it be possible for one of the engineers to look these over and see if I have made any bad assumptions or calculations?

If it checks out OK then perhaps you can email me a shipping label I'll round up a box and wooden base for to return it for honing

Thanks for checking into this for me and the help you've provided.
AP-IA - ATP,CFII,Multi,SES,Gliders

Built a Vari-Eze in the 80's, now RV9

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bruceh
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Re: Help with CG calculations RV9

Post by bruceh »

I have an IO-320 E2D and the Sensenich Ground adjustable composite prop. I ordered the standard engine mount for the IO-320. I did get the wider IO-360 lower cowl, which is Van's recommendation for FI. My FI is Airflow Performance. Everything fits up just fine. No need to mess with offsetting the Filtered Airbox, or any interference with the mixture arm.

I certified my Max gross at 1800, instead of Van's recommended 1750. Here are my weight numbers. Mains are 410/408 and nose is 249 lbs.
Image

I'm well within my CG at any real loading. Full fuel 216 pounds, max baggage 100 pounds and 417 pounds of pilot/copilot are under the max gross. The worst case aft CG is with minimum fuel, max pilot/copilot and max baggage. It is still within the CG range.
I don't think you need to get a longer engine mount. You want to have a light nose wheel. Van's limits the nose wheel weight limit at 325 pounds.

When in doubt, don't overthink things and just follow the plans! I'll send you my W&B numbers if you'd like.
Bruce Hill
RV-9A N5771H flying over 1100 hours!
Build Log at http://www.overthehills.com/RV-9A-Project
Blog at https://flyingoverthehills.wordpress.com/
EAA Tech Counselor, A&P

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captain_john
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Re: Help with CG calculations RV9

Post by captain_john »

Bruce,

Steve has a 9. No nosewheel...

Nevertheless, if the 9 loads up like the 7 does, you want to keep adding forward cg.

Empty, i am right on the forward limit on my 7. I put everything as far forward as I could.

I can carry a full 100 pounds in my baggage area with two beef eating Americans on board.

I am not sure how the 9 compares to the 7. Can I guess that they might be similar?

:? CJ
RV-7
Garmin G3X with VP-X & a TMX-IO-360 with G3i
It's all over but the flying! 800+ hours in only 3 years!

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bullojm1
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Re: Help with CG calculations RV9

Post by bullojm1 »

CJ brings up a good point to the similarities between the -9 and -7. They basically share the same exact fuselage (except for the seat ribs and some other parts). What I don't know is how the RV-9's wing differs from the RV-7 in terms of weight, and how it affects the CG. Bill Repucci (He is one of the owners of http://www.eicommander.com) has a RV-9 with a FP prop and O-360 and I don't recall him having any CG issues. You might want to get in touch with him through his website. He would definitely be able to steer you right. As CJ said, make sure you keep everything as far forward as possible. Also, a heavy battery on the firewall is you friend - don't even think of a LiFePo lightweight battery!
Mike Bullock
http://www.rvplane.com
RV-7 | Superior IO-360 | Whirlwind 200RV
Garmin GTN650 | GRT Dual Sport SX EFIS
Status: FLYING!

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