RV-6 Bottom Skin Concave in Flight

[petercavallo]

Flying my RV6 in formation, it was noted that the Fuselage skins aft of the trailing edge were sucked in at least 1/2" in flight, creating a scalloped effect. Anyone have any expirence with this, seems like it could slow you down. Is this pressure from lift, or negitive pressure within the fuselage?
Peter
RV6 Slider

 

[terrye]

Buckled Skins

There were some pics a while back of an RV-7 doing a pullup/roll that clearly showed buckled lower fuselage skins in the two bays aft of the baggage compartment. Check the archives.

I am fitting a Van's AHARS mounting bracket aft of the baggage compartment and after I saw the pictures, I duplicated the longitudinal bracket so I have one on each side in addition to the belcrank mounting bracket in the center. I also added a leftover piece of J stringer on the centerline of the lower skin on the second bay aft of the baggage compartment.

These brackets/stringers are easy to fit even on a completed airplane.

 

[Mel]

I duplicated the longitudinal bracket so I have one on each side in addition to the belcrank mounting bracket in the center. I also added a leftover piece of J stringer on the centerline of the lower skin on the second bay aft of the baggage compartment.

These brackets/stringers are easy to fit even on a completed airplane.

When adding the stiffeners be sure to "attach" them to the bulkheads fore and aft. Otherwise they will cause the skin to crack at the end of the stiffener due to vibration.

I've seen it happen time and again.

 

[terrye]

Stiffeners

Thanks Mel, for the reminder. The Van's AHARS longitudinals are attached to the fore and aft bulkheads and the opposite hand part I made is too. The J stiffener I added also has clips to attach it to the fore and aft bulkhead.

 

[Vlad]

Here is the thread Jamie Painter created a while ago with pictures, tips and tricks.

 

[petercavallo]

Caved in Belly skins at speed

It appears the effect increases with speed, the current idea is to clean the belly area inside and prep for X30 bonding foam. On a level fuselage I plan to pour the foam in and coat the bottom. The x-30 bonds well and will act as a total stiffiner to each bay, preventing any caving in (and speed loss due to it).
Any Comments, I need to get my speeds calibrated first to see if this makes an improvement before doing it.
Peter

 

[elippse]

Flying my RV6 in formation, it was noted that the Fuselage skins aft of the trailing edge were sucked in at least 1/2" in flight, creating a scalloped effect. Anyone have any expirence with this, seems like it could slow you down. Is this pressure from lift, or negitive pressure within the fuselage?
Peter
RV6 Slider

You say "sucked in" but it is really pushed in. What that means is that the pressure outside of the skin is higher than inside. So why is the pressure inside lower than outside, and why is it worse when pulling Gs? There is lift over the top of the canopy, a low pressure, and the lift must increase when you increase the load on the wings. Most likely you have a leak at the lower pressure region of the canopy. My plane shows about 0.1"Hg. That's about 7 psf. That's also why if you use cabin air as your static, your IAS and altitude read on the high side. That might change when your cabin air inlets are open.

 

[scsmith]

not sure X-30 is a good idea

I have found that most 2-part mixed urethane foams will absorb water over time and get heavier. I think it can absorb water out of the atm.
I would worry about that.
Also, it could eventually be a corrosion problem against the skin - it would probabily have to get wet (with liquid) to get enough water to do that, but it could.

It appears the effect increases with speed, the current idea is to clean the belly area inside and prep for X30 bonding foam. On a level fuselage I plan to pour the foam in and coat the bottom. The x-30 bonds well and will act as a total stiffiner to each bay, preventing any caving in (and speed loss due to it).
Any Comments, I need to get my speeds calibrated first to see if this makes an improvement before doing it.
Peter

 

[koda2]

Common area to oil can

When I first started repairing my 6A quickbuild fuselage, I noticed a pretty significant "oil can" effect on the same area. It has been several years but IIRC the movement was enough to make the familiar "oil can" noise and stick in one position or another.

At that time I think Sam Buchanan had just posted adding a stringer in the same area and I made the same kind of addition.

I have yet to fly the plane so I can't tell you how it will work out but I am pretty sure I wouldn't have been happy if I had left it alone.

Fixed it pretty much like everybody else did.

Dave A.
6A build