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False floors

LettersFromFlyoverCountry

LettersFromFlyoverCountry

Well Known Member
I'm back on the false-floor kick and intend to use the technique of putting some .063 3/4x3/4 angle onto the floor stringers in the RV-7A. Here's the question I have for those of you who have gone this route: How many pieces of angle should I install and how big are the pieces?

I assume you install nutplates on the angle pieces to screw down the AL "floor", but how did you locate those holes.

Did you attach the angle to the angle on either side of the center cabin cover angle? How did you prevent the fastener (bolt or rivet) from interfering with the cover?

I bought a big 4x4 pieces of aluminum (nothing like a $115 charge for shipping for that puppy!), and I'd like to tackle this this week but am trying to think the technique through a bit.
 
Bob,

Although mine are not 100% done yet, I set the elevation of my sub-floors so that the inboard edge of both the pilot and passenger panels would rest on top of the center tunnel cover. The tricky part was making some joggled strips for the outboard edges to rest on the lower longerons. Since this longeron is not parallel with the rest of the floor, the strips needed to be joggled more at the back end than at the front end.

20100615-06-tn.jpg


If you want to rivet your angles to the middle two floor stiffeners, you could just use countersunk pop rivets to avoid interference with the flanges of the center tunnel cover.

Hope that helps.
 
False Floors

Bob,

They say a picture is worth a 1,000 words so here goes. Here are the 3/4 x 3/4 x .062 angles in place (2 per side):

Felton150.jpg


There are two floor panels, left and right. To find the center of the angle, make a centering Sharpie mark at the end of the centermost angle on each side--on the firewall and on the spar bulkhead--then fit the floor panels and draw a Sharpie line between your marks. The other angles you can measure from the sides. You are going to want to drill all the holes in assembly with the floor panels (start off with #40 drill), cleco as you go. When done, remove everything and install your nutplates. I used #8 screws.

You only need two angles per side, as the photo indicates:

Felton153.jpg


Here is the floor panel already drilled in assembly with the angles:

Felton154.jpg


For the outboard nutplates, use the existing structure. You may want to add a third floor panel forward of the center console. This is optional and easy to do using the existing nutplates from the side panels.
 
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Purpose of false floors

Chad & Seth,

False floors are entirely a user preference and not necessary at all. BUT, here's why I did it on all 3 of the RVs that I built.

1. I added a 3/4" insulation between the floor panels for sound attenuation. A lot of noise comes through the floor from the exhaust pulses.

2. Also, the insulation cuts down significantly on the heat transfer from the floor to your shoes/feet. After a few hours on a long cross country your feet get really hot! (Not an issue with the insulation!)

3. Anecdotal perhaps, but there is evidence that adding the additional structure cuts down on "smoking rivets" just aft of the firewall on the belly.

I originally flew my RV-4 without them, went back and put them in, and the difference was significant! So I put them in my other RVs. It's a real solid feel. :)
 
Got it...kinda what I was thinking it was for. My carpet kit has 3/4" insulation included, cut for the space between the floor angle, and the carpets lay over that.
 
cjensen said:
Got it...kinda what I was thinking it was for. My carpet kit has 3/4" insulation included, cut for the space between the floor angle, and the carpets lay over that.
Click to expand...

Yep - that's what I did, I also glued a little bit of .016 aluminum between the foam and the carpet to get all the noise damping I wanted with minimal weight.

Every annual I lift the carpet out and look around and it seems to work fine.
 
cjensen said:
Not trying to be rude, just curious, but what is the point of this???:confused:
Click to expand...

For me it's a few reasons:
1. Flat surface for carpet to lay on.
2. Fireproofing insulation in the center three spaces, soundproofing insulation in the outer two spaces.
3. The forward flanges of the floor panels will be an attachment point for a "false firewall". This in turn will create a place to trap fireproofing insulation on the back side of the firewall.
4. The added weight I believe will be a good thing since my light engine and prop will likely result in a somewhat aft CG.

That said, the fact that it's a considerable amount of work is a pretty good reason not to do this mod.

Pat,

How did you attach your panels to the lower longeron? Did your floor panels just flex down or did you use a spacer?
 
I went a different route that I will throw out there because it worked very well for me. I also wanted to get rid of vibration, heat, and get that "solid" smooth floor feel.

I took out the two outboard floor stiffening stringers. I cut down the vertical legs of the two center floor stiffener angles to 1/2". I then cut two 1/2" thick foam sheets to fit the area between the center stiffener angles and the outboard floor longerons. I then epoxied the foam sheet to the floor (aircraft quality foam used for composite cores, I weighted it down with some ceramic floor tiles to apply the bonding pressure). I then cut an .016" sheet of aluminum to cover these two areas and epoxied them to the foam sheets the same way. This formed an aluminum sandwich, foam core composite panel as a smooth floor on the pilot and passenger side of the cockpit- very stiff and strong. I used the center section to conduct fuel lines and wire as normal. I covered this center gap with an .032" aluminum sheet cover, screwed down as normal.

This provides a smooth "flush" floor, very ridgid (no vibration, no heat, no noise, no bunching carpet, no heel plates on carpet, no flamable material). Also this is a "standard" type of aircraft construction (aluminum faced, foam core panel -can you say Grumman Yankee).

I recall that back several years ago that someone posted a similar floor construction (probably on the Matroncis list) but actually used nomex honeycomb as the core material (ala F-18 and similar). He was talking about marketing a kit (pre-cut honeycomb cores and appropriate epoxy) - apparently never did. Too bad, this makes a great smooth and strong floor.
 
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mcencula said:
Pat,

How did you attach your panels to the lower longeron? Did your floor panels just flex down or did you use a spacer?
Click to expand...

I just used a shim of some sort to make the panel level. In a couple of places where nutplates were impossible, I was able to drill and tap the longeron. Worked well.
 
What I will try

I'm posting a link from someone else...hope they don't mind!
http://www.4sierratango.com/Fuselage9.htm

I'm going to try this..I like the fact that Balsa is lightweight and easy to work with. I particularly like the "insulating" quality of wood, and reducing the vibration. I ordered my balsa this week from the company listed...66 bucks shipping included for 4 pieces.. 3/4 x 8 inch x 36 inch pieces. Seems like a lightweight, safe and easy solution. Haven't decided if I will epoxy them in, or find a removable solution..Take a look at this guy's page, and see what you think. YMMV.
 
cjensen said:
Not trying to be rude, just curious, but what is the point of this???:confused:
Click to expand...

Listen the podcast I made with Tom Berge last year. He noticed when he was flying that the pitch and sound in the cabin would change and he couldn't figure out why. Then he realized that when a passenger moved his/her feet, that's what caused it.

Adding the floor allows you to put sounproofing underneath. It knocks down the vibration and quiets things down, lowers the heat on the floor.

People who have flown both say there's no comparison between the two.
 
I have a "rumble" under my feet while flying. It's a bit of a distraction. Also a bit uncomfortable. I wish I had known about this before (it's a common thing for RV's apparently) I would have done a similar mod that you guys are doing. Right now I have to move my feet to a "sweet" spot that doesn't vibrate. The passenger side is fine, just the LH center, right under the pilots pedals/feet. If I move my heels left and right it's not bad. I may have to do something with this. My carpet has a nice thick pad that does help, but not perfectly.
 
Brian, You will not get the same rigidity and dampening effect (IMHO) without fully bonding the core material (foam or balsa) to the aluminum face sheets. It is the intimate contact (laminating) that gives composite panels their strength characteristics. It is the bonding that transmits the bending load of the skins to the relatively uncompressable core that does the trick.

You will pick up some desirable insulation effects and smooth floors, but you'll miss the "full monty" without bonding. You will essentially be building a partially de-laminated panel.
 
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Double floor

Bob,
My floor is similar to Pat's, although I have a hybrid type of 6A cabin. The angles were riveted and nutplates and c/s screws were used. Spacing was arbitrary. I also cut some lightening holes in the far ends where there would be no weight on the skin.

doublefloor.jpg


The floor was to cover soundproofing. After Dan Horton's work on fire retardant materials, I am probably going to switch to something different.

I also raised the rudder pedal bars slightly to accomodate the loss of pedal height.

Dave A.
 
False sandwich floor

I put a false floor in my Super Six and glad that I did. My plane seems as quiet or quieter than stock RV sixes I've ridden in. I put some black foam that is fire resistant between the two layers. The main reason that I put in a false floor in, is because when I took a ride in the original Super Six, my heels were vibrating on the floor. Another advantage is I was able to run fuel lines, battery cables, and wireing under the floor.

Steve "The Builders Coach"
 
To Gary

Ok Gary..you sold me..! Just to make sure I've got this right, the boards are painted or sealed as I understand it..does it matter what sealer is used, if anyone knows..?..Also, all the "cracks' between the floor angles and the boards are filled with epoxy/flox mixture, correct?
 
Balsa

Although I see the general point of doing this I think it could be done with a thicker foam with a lot less work. The thick foam will provide you with the flat floor, sound proofing and ease of carpeting you're looking for.

I highly recommend not using balsa unless you seal it with epoxy. Balsa is extremely expansive. Take a small piece and drip some water on it. You can see it grow.
 
Brian, I used a water and fuel resistant foam (last-a-foam from ACS). I am not sure about the characteristics of balsa regarding absorbancy as mentioned by Steve. You would minimize the concern by sealing the sides of the balsa with epoxy.

It is not necessary to fill or bond the edges at the longerons. It is the flat sheet with laminated opposing faces that build the panel stiffness.

I bonded the core down first. Allowed it to set, then bonded the face sheet down, just as the guys at 4sierratango did. I kept the thickness minimal (1/2") so as to minimize issues with changing floor heights to rudder pedals and interference with installing wing spare bolts on my Six. Good luck, I think you will find it a very worthwhile mod.
 
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What about fiberglass?

Did any of you guys consider or see anyone else fiberglass over balsa instead of aluminum?

I was thinking to use 1/2" balsa with a 1/4" of foam against the bottom skin and then 2 ply of fiberglass on top with the carpet on top of that.
The whole thing would be removable.

Thoughts?
 
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