Changing cg for heavier firewall forward weight

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slociviccoupe

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wasnt sure where to post this but if have chosen a plane and chosen the powerplant, and the firewalk forward weight is heavier than the design calls for. Could the cg of the plane be changed before construction to compensate for the known firewall forward weight. The plane is pro composites vision and the engine is 2.5 subaru conversion. Power goes up some but not drastically. I have not ordered plans yet so dont even know the firewall forward weight but do knos the auto conversion can and will most likely weigh more than aircraft engine.
Please no debate on auto vs aircraft. Plenty of threads on here and probably read all of them.
Im just doing my homework before i proceed.
The other plane was considering was the kr2s
The corvsir sparked my interest butvthe conversion cost snd low hp was not attractive. That and the old technology cylinder heads and old core parts again made it un desirable to me.
 

TFF

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Yes is the simple answer. How hard is the hard answer. Technically the needed CG does not change; how you achieve the mix of parts does.

You will need to figure out the weight needed in the tail first. Then you will have to decide if it’s reasonable or not. Then you have to decide if it’s strong as is or how much more it needs to be. No one s going to just say glue 5 lbs of lead in the back.

Adding weight or adding length will change how strong it was or should be. That will either take working out with numbers or accepting guesses.
 

slociviccoupe

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Thank you. I didntvthink of added length i thought more of it moving the wing foreward.
Since auto conversion i can move radiator to behind rear seat with belly scoop and duct running to it. That moves some weight.
But i know i have to start with firewall forward weight first and go from there.
 

wsimpso1

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I am sure it is fixable, the biggest question is can you do it without ballast. Weight is the enemy.

This problem is made to order for shifting something heavy aft to compensate. For instance if the plans call for one engine and a battery just aft of the firewall, and the new engine shifts CG forward some, sliding the battery behind the seats might make all the difference. There is always further back in the tailcone, or making it two batteries. Figure out the base CG, and then play with positions of stuff with the new engine until you shove the CG back to where the base engine had it.
 

wsimpso1

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Another solution is a bit more tail length. Making the aft fuselage a little longer will add a touch of weight aft and move the tailplanes aft as well. adjusting the CG while also adding more tail volume, adding damping in pitch and yaw axes, and adding more ability to extract the airplane from a spin. Weight penalty is tiny and you may need a little more tail volume to handle the increased mass moment of inertia that will also result. Are you sure the EJ engine will add to weight?

Oh, the radiator and oil cooler can be hung below the wing in the same manner as Ross did in his RV6 or Russell does on his Glasair for a modest aft shift.

Billski
 

slociviccoupe

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Billski i was hoping for your reply. I dont know what the weight of the ej25 will be. Currently have pile of subaru parts on the floor. Dont have my psru yet either. Planning on the spg-5.
But figured i can get firewall forward put together and built by end of 2020 and start on plane beginning of 2021.
So far prety set on the vision but anything else is possible. Any other 2 seat side by side composites out there. I'd love a lancair or glassir but not feasable . Looking for something low wing, fast, 2 seat and would suit an ej25. So far that seems to be the vision or kr2 super. The vision was built with a direct drive subaru ea81? So guess could figure out weight difference between the two. With the 2.5 hoping should be between 130-150 hp estimated.
 

BJC

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The original Vision was powered by the EA 81 Sub., but versions have been built with Lycomings up to (at least) 160 HP.

Suggest that you contact Pro Composites and ask Scott. Contact info on their web site.

Also, search here for posts by, IIRC, Vision_2012, a Vision builder.


BJC
 

Mad MAC

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Hmm so you are looking at something on the order of 100 lb changing from an EA81 to EJ25. That's going to hit your payload quite hard, even before getting it in balance.

A mix of the following might work:

Spend more money on a lighter prop, cowl and spinner.

Shorten the engine mount

Be really strong on weight control and then use those savings to ballast the tail. Probably the easiest & cheapest, if you can live with plain paint, a bare cockpit & instrument panel).

Moving the wing forward can be a good approach, structural not too hard, improves the stability, dependent on the fuselage shape may have a measurable effect on the drag.
 

BJC

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But figured i can get firewall forward put together and built by end of 2020 and start on plane beginning of 2021.
The Vision, a design that I really like, is also a design that takes lots of hours to complete. You may want to think about not building up the engine until the rest of the aircraft has been completed. Two reasons; the engine won’t be sitting there aging, rusting, etc., and you may change your mind about choice of engine. Just a guess, but you may decide that what you currently have invested in engine parts is not relevant to your final engine choice.

Wrt to relocating the wing; it has been done, with, for example the Glasair II S, which ended up being tail heavy at minimum fuel and with baggage and two aboard. Moving the location of the wing is easy. Knowing exactly where to move it depends on many assumptions about as-built weight distribution. The Glasair II S (Stretched G II) morphed into the Super II via another fuselage stretch, a repositioned wing, and a larger span horizontal stabilizer.

The standard for fast glass via Subaru has been set by a Glasair builder named Russell who posts here regularly.

For other designs, take a look at http://www.kisaircraft.com/

Another design, though not primarily composite, is the Celerity. Beautiful airplane.


BJC
 

Lendo

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S, I believe the Firewall forward limit is 300 lbs, that's everything from memory. If you can't keep it at that do what Billski suggests. I believe Steve Rahm used a 100hp Subaru engine initially. and the weight was OK. It may have been a EJ, not sure.
George
 

wsimpso1

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See rv6ejguy's posts on his turbocharged EJ22 equipped RV6. Ross has a lot of useful stuff to say about weight in these guys.

Be very careful with the Vision on excess resin. Being a wet layup fiberglass bird it can get heavy very easily and that will easily shift CG aft a bunch in a tail-in-back airplane. You will want to make some samples to assure yourself of what your panel weights are before adjusting design. I have seen two examples of conventional tail-in-back airplanes built via open wet layups that came in seriously tail heavy. While that might solve your CG problem and perhaps drive a longer engine mount, it is all wrong from a Weight is the Enemy perspective.

Billski
 

Pops

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I always build the engine mount last. When everything is done I do a W&B with me in the cockpit ( Have to have help :) I find the CG of the engine and firewall forward (Hang it from a hoist) Then work out the length of the engine mount to put the CG where I want. If you are careful in the weights and arms, you can get the CG very close. On the SSSC I got the CG within .10 of where I wanted with the engine mount at 9.5". I had a forward and rearward baggage area.
On the JMR I forgot to include the weight of the baggage area and had to add .75" to the engine mount so I could get 50 lbs in baggage
 

TFF

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There is a shift of culture between car and airplane when it comes to engines. The car world, the engine is the star, the value, the focal point. Not so with an airplane. There is disdain for the engine in the airplane world. Something you have to put up with. The airframe is the star in the airplane world. Performance is a distant second to reliability in airplane engines. Engines blowing up at a car race is part of the spectacle. With an airplane it’s a panic attack.

Getting a good core is important and budget may allow buying up pieces for the time when needed or available, but the engine is the “easy part”. Building an airplane is the challenging part. It’s more like restoring a rusty Midwest car with no aftermarket parts. Got to enjoy being the body man not the engine man to get through it. Of course if you enjoy both, you are set.
 

Riggerrob

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Roy Lopresti moved the wing forward to balance the LoPresti Swift. Tempco built the first Swifts with anemic 65 horsepower engines, but by the time LoPresti finished his version of the Swift, it had more than triple the horsepower (200) and a much heavier engine, so he moved the entire wing forward by a few inches.
A secondary benefit was that the modification slightly lengthened the tail arm, taming a skittish airplane.

The OP's plan to complete the engine first makes sense. Run it a few times, then "pickle" it. Weight the firewall-forward package and do more calculations.
Start by building wings and tail feathers and weigh them to ensure that they are close to design weight. Vacuum-bagging will help reduce excess resin.
Once you have built a few major components, do another set of weight and balance calculations. Poll other Vision builders to get a grasp of the average weight of cowlings.
Leave engine mounts until late in the build process. Maybe even bulge the firewall aft to accommodate magnetos, pumps, or whatever hangs off the back of a Subaru conversion.
 

slociviccoupe

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I appreciate all the replies and ideas.

Have a lot to think on. Few more emails to write and get the plans. Also starting to get ready to purchase my psru.

Hopefully can save weight where needed. Also hopefully some vacuum bagging will help with the layups.
 

Doggzilla

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The weight change is negligible and will be well within existing CG limits.

When calculating CG simply add the weight like you would for any cargo, but use a negative position at the proper distance forward.

You would need to add something like 150lbs to a firewall to put it out of forward CG. It’s a non issue.
 

wsimpso1

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Hmm so you are looking at something on the order of 100 lb changing from an EA81 to EJ25. That's going to hit your payload quite hard, even before getting it in balance.

A mix of the following might work:

Spend more money on a lighter prop, cowl and spinner.

Shorten the engine mount

Be really strong on weight control and then use those savings to ballast the tail. Probably the easiest & cheapest, if you can live with plain paint, a bare cockpit & instrument panel).

Moving the wing forward can be a good approach, structural not too hard, improves the stability, dependent on the fuselage shape may have a measurable effect on the drag.
Every time I hear someone talk about relocating the wing, I wonder if they have fully considered the issues. IIRC, The Vision's seats are nested around the spars of the wing. Depending upon how much the wing must move, this might have nasty tails in forcing new position, both longitudenally and vertically, of the crew. It may also have significant impact upon control column placement and design, flight control routing, and other items. This sort of redesign must not be taken lightly. On top of that, wing weight and any fuel stored in the wing are substantial parts of airplane weight, so it not only moves the Neutral Point, it shifts CG too, and ends up needing to be moved more than you may think it would.

If CG can not be maintained by engine placement and battery relocation, a modest stretch of the aft fuselage is a far less disruptive approach than "moving" the wing.

Billski
 

wsimpso1

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The weight change is negligible and will be well within existing CG limits.

When calculating CG simply add the weight like you would for any cargo, but use a negative position at the proper distance forward.

You would need to add something like 150lbs to a firewall to put it out of forward CG. It’s a non issue.
Sounds like someone has already considered this change and run through the numbers. This is way better than all of us responding to the supposition that the CG will shift too far forward. This should be easy enough for the OP to check with plans data for the Vision and some engine weighing.

Billski
 
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