Seat modification question

A friend of mine is considering modifying the seat in an RV-6A. I've posted here because this really isn't a specific RV model question but more of a general weight and balance 'guesstimate' question.

He wants to move the top of the seat backwards about 4 inches leaving the seat base unmoved. This will gain him headroom and he claims this will not change W&B at all. I estimate (roughly) that this change will move about 2/3 of the pilot+passenger weight back by 2 inches, resulting in moving the total weight back by something like 1.5 inches. Moving ~340 lbs back by more than an inch seems a fairly significant change to me.

Can anyone firm up my guesstimate, point me to known W&B problems in doing this sort of thing, or maybe even tell my fears are unfounded? I'll strongly suggest he redo the W&B anyway which should catch any change in arm.

Pedal position?

Before he considers that drastic a change, have him check the the pedal supports (assuming overhead pedals) are as far forward as possible.

Make it so full rudder has the pedals almost hitting the firewall.

az_gila said:

Before he considers that drastic a change, have him check the the pedal supports (assuming overhead pedals) are as far forward as possible.

Make it so full rudder has the pedals almost hitting the firewall.

Click to expand...

He mentioned not moving the base of the seat so the change would have only a very minimal influence on leg room.


Tilting the seat aft by that much for a person of the stated weight would definitely have some influence on moving the C.G. aft. there are different ways to calculate ho much but the simplest and most accurate is to weight the airplane empty and then again with the person seated in the seat. From that info the actual arm position of the persons C.G. can be extrapolated.

BTW, before doing all of the work for this mod. he should be absolutely sure it will get him the result he desires. The standard seat position already puts a person head right at about the high point of the canopy (depends on the actual position of the seat back). The canopy begins to slope downward as it goes back, so moving a person head further aft can end up having the net change in head room be zero or even negative. The primary benefit in moving the seat further aft is in leg room (and it doesn't sound like he needs that).

What may benefit him the most is doing what Gil suggested and make sure the rudder pedals are as far fwd as possible and then move the base of the seat fwd as appropriate. This would keep his head at the tallest part of the canopy and lower his head slightly because of the seat back having a lower tilt angle. Using as thin of a seat back cushion as possible would help also (most of the seat makers, make them way thicker than they need to be).
The down side to this is that it would move him closer to the stick which could be a problem.

Isn't this the same thing as the "almost 14" mod?

As Ty1295 noted, and with a link to help:
http://antisplataero.com/products/almost-a-rv14-seat-mod

As per the 2 replies above, this is exactly what the Almost an RV14 mod does. They (Antisplat) weighed their test aircraft with the seatback in the original position and in the new reclined position, with the seats occupied, and found the change in cg was not measurable.

In response to RVbuilder2002's comment regarding changing the seatback position having very little effect on legroom, that would certainly seem to be the case logically, but in practice it makes a really significant improvement. I'm 6'3" and prior to doing the mod I felt extremely cramped in the cockpit, and had to wear very thin-soled shoes to keep from having my knees right up under the instrument panel. Since doing the mod, I can wear hikers or any other footwear I want with complete comfort; reclining "opens up" your body position and effectively gives you more legroom. Moving the seatback aft allows your pelvis/hips to move backward in the seat; thus more legroom. No issues with head clearance, as reclining lowers your head as you move backwards. As mentioned, I'm 6'3" and wear a helmet to boot; only hit the canopy with really nasty bumps in turbulence, and that doesn't happen if I have my shoulder harness tight.