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  #1  
Old 08-24-2017, 07:23 AM
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WingedFrog WingedFrog is offline
 
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Default cg question about RV-12 is

When the Rotax 912is was introduced around 2013 there were discussions on this forum about it's introduction on the RV-12. Beside the fact that many arguments were made and doubts raised about fuel economy that have since been proven wrong (i.e. see the results of the VANs flight to Oshkosh) there is one thing that is still not clear in my mind. One issue mentioned against a quick adaptation to the RV-12 was the higher weight of the engine that could hardly be dealt with quickly given the tight situation created by the position of the seats in front of the wing spars. It looks like this issue has been solved (if over several years) by Vans, however when I look at the changes, many tend to put more weight forward: relocation of coolant radiator, new fuel tank, stronger front landing gear. Some gains have been made by using a lighter battery but overall I would like to understand better how Vans managed these weight changes to get it right. May be Scott could chime on this?
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  #2  
Old 08-24-2017, 08:35 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by WingedFrog View Post
I would like to understand better how Vans managed these weight changes to get it right.
Short answer.......... Engineering.

Long answer - a whole bunch of different things...

Addressing a couple specific ones you mentioned.....

The new fuel tank has little to no negative influence in CG change because it is aft of the CG.

Use of the Earthx battery has a huge CG influence. Without that change, a new battery location would probably have to have been engineered.
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  #3  
Old 08-24-2017, 10:21 AM
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WingedFrog WingedFrog is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post
Short answer.......... Engineering.

Long answer - a whole bunch of different things...

Addressing a couple specific ones you mentioned.....

The new fuel tank has little to no negative influence in CG change because it is aft of the CG.

Use of the Earthx battery has a huge CG influence. Without that change, a new battery location would probably have to have been engineered.
Thanks Scott, I understand that the RV-12is was a complete project going beyond just adapting to the 912is.
I beg to disagree with your point about the fuel tank: even if the cg of the new fuel tank is behind the global cg, the old fuel tank had its cg clearly more aft than the new one which should result in a small penalty.
As the new battery was key and seems also more advanced than the old one will it be possible to use the new battery in the RV-12? (my own battery is getting close to end of life cycle)
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Kit # 120395 N124BX
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  #4  
Old 08-24-2017, 10:39 AM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by WingedFrog View Post
Thanks Scott, I understand that the RV-12is was a complete project going beyond just adapting to the 912is.
I beg to disagree with your point about the fuel tank: even if the cg of the new fuel tank is behind the global cg, the old fuel tank had its cg clearly more aft than the new one which should result in a small penalty.
As the new battery was key and seems also more advanced than the old one will it be possible to use the new battery in the RV-12? (my own battery is getting close to end of life cycle)
We are fully aware of the influences that result from having the same amount of fuel but carrying it slightly further fwd
Carrying the fuel further fwd, closer to the CG is in part what allowed increasing the max. baggage limit to 75 lbs (there is a smaller moment change between full and minimum fuel load)
There are other influences related to the fuel tank making it not cause much change in the empty C.G.(The tank it self is slightly heavier). Another is that the high pressure fuel pump pack is rather heavy (just one of the things that makes the iS engine heavier than the ULS), and it is mounted aft of the baggage bulkhead.

Earthx battery will be tested on the ULS airplane and if all goes well will likely be standard in the Gen 2 kit for ULS installations.
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  #5  
Old 08-24-2017, 10:40 AM
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joedallas joedallas is offline
 
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Default Before you discard Scotts answer

Before you discard Scotts answer

Remember the tank is forward of the old tank location by just a few inches
The tank is aft of the C.G.
When the tank is empty the tank don't weight very much
The only difference is the difference of the empty tank ARM

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Quote:
Originally Posted by WingedFrog View Post
Thanks Scott, I understand that the RV-12is was a complete project going beyond just adapting to the 912is.
I beg to disagree with your point about the fuel tank: even if the cg of the new fuel tank is behind the global cg, the old fuel tank had its cg clearly more aft than the new one which should result in a small penalty.
As the new battery was key and seems also more advanced than the old one will it be possible to use the new battery in the RV-12? (my own battery is getting close to end of life cycle)
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  #6  
Old 08-24-2017, 12:17 PM
Mich48041 Mich48041 is offline
 
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The aircraft must be within the CG range whether the fuel tank is empty or full. If the new tank has any fuel in it at all, Its moment will be aft of that of an empty old tank. Even when both tanks are empty, considering that the new high pressure fuel pumps are aft of the baggage bulkhead, there will not be much difference in the CG of the two tanks.
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Old 08-24-2017, 02:39 PM
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joedallas joedallas is offline
 
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Joe
You are correct.

With the Tank forward this will keep the aircraft closer to the center of the envelope ( full or empty )

The new tank location will never have a moment as far back as the old tank.

The difference in full or empty will not be as much as the old design was.

This is a much better situation then it was.

Thumbs up on this design.

Joe Dallas






Quote:
Originally Posted by Mich48041 View Post
The aircraft must be within the CG range whether the fuel tank is empty or full. If the new tank has any fuel in it at all, Its moment will be aft of that of an empty old tank. Even when both tanks are empty, considering that the new high pressure fuel pumps are aft of the baggage bulkhead, there will not be much difference in the CG of the two tanks.
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  #8  
Old 08-24-2017, 07:52 PM
Ernie Ernie is offline
 
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Am I correct in assuming the engineering/testing of the new RV-12 plane with the 912-ULS engine is not completed at this time?
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  #9  
Old 08-24-2017, 08:25 PM
Jim T Jim T is offline
 
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Quote:
Originally Posted by rvbuilder2002 View Post

Earthx battery will be tested on the ULS airplane and if all goes well will likely be standard in the Gen 2 kit for ULS installations.
This part of Scott's post really caught my eye. As a builder of one of the old outdated RV-12 kits, I'll be anxious to see what the "Gen 2 kit for ULS installation" will consist of.

Jim
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  #10  
Old 08-24-2017, 09:20 PM
RFSchaller RFSchaller is offline
 
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I'm not familiar with the earthx battery. Is it a Li ion battery? How much weight does it save? How expensive is it? Is it a drop n replacement for the Concorde lead acid battery?

Rich
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