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cg question about RV-12 is

WingedFrog

Well Known Member
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?
 
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.
 
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)
 
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.
 
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

My View





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)
 
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.
 
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






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.
 
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?
 
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 :D RV-12 kits, I'll be anxious to see what the "Gen 2 kit for ULS installation" will consist of.

Jim
 
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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This part of Scott's post really caught my eye. As a builder of one of the old outdated :D RV-12 kits, I'll be anxious to see what the "Gen 2 kit for ULS installation" will consist of.

Jim

By Gen 2 kit I meant the updated RV-12iS kit. From this point on all new fuselage kit deliveries (once they begin in October) will be the new version fuselage, but builders will still be able to opt for using the ULS engine if they choose. Even if they use the ULS engine the standard kit will still include the Earthx battery.

The actual installation of the ULS engine will largely be the same as the iS... it will use the same cowl with the fwd mounted heat exchangers, etc.
 
By Gen 2 kit I meant the updated RV-12iS kit. From this point on all new fuselage kit deliveries (once they begin in October) will be the new version fuselage, but builders will still be able to opt for using the ULS engine if they choose. Even if they use the ULS engine the standard kit will still include the Earthx battery.

The actual installation of the ULS engine will largely be the same as the iS... it will use the same cowl with the fwd mounted heat exchangers, etc.

OK got it Scott. Makes sense. Thanks.

Jim
 
EarthX battery on current RV-12

Having done a good "scratch and sniff" on the RV-12iS at Oshkosh, I came away very impressed with the refinements.
Also, having hefted the normal battery and an EarthX battery, I came away lusting after the potential weight savings if I could plunk one of those into my now-"classic" RV-12. Since the batteries are not physically identical (EarthX battery's terminals are in different positions than the 680) a bit of massage of the battery holder would be needed. However, CG-wise, the lighter battery forward of the CG seems like it it would have the effect of lessening the max luggage capacity aft of the CG. Ten cuss words if my analysis is correct.
Comments, Scott?
 
W&B

I don't think you can answer this question without knowing the optional equipment in the front of the CG and the weight of the pilot and copilot
Do a W&B to get your answer ( deduct the weight and arm difference from the old battery )

Also remember the arm of the baggage is somewhat under your control.

Note:
When the fore-aft center of gravity (CG) is out of range serious aircraft control problems occur. The fore-aft CG affects longitudinal stability of the aircraft, with the stability increasing as the CG moves forward, and stability decreasing as the CG moves aft. With a forward CG position, although the stability of the aircraft increases, the elevator control authority is reduced. This can cause a serious condition during the landing flare.

From

https://en.wikipedia.org/wiki/Center_of_gravity_of_an_aircraft

A good read

My View
Joe Dallas






Having done a good "scratch and sniff" on the RV-12iS at Oshkosh, I came away very impressed with the refinements.
Also, having hefted the normal battery and an EarthX battery, I came away lusting after the potential weight savings if I could plunk one of those into my now-"classic" RV-12. Since the batteries are not physically identical (EarthX battery's terminals are in different positions than the 680) a bit of massage of the battery holder would be needed. However, CG-wise, the lighter battery forward of the CG seems like it it would have the effect of lessening the max luggage capacity aft of the CG. Ten cuss words if my analysis is correct.
Comments, Scott?
 
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