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First Condition Inspection Questions

AndyRV7

Well Known Member
I took my plane (and just picked it back up) to its 1st inspection. I enjoyed the experience in spite of the fact that it was a lot of work! I was also very impressed with my mechanics (2). Very professional and organized. What you would hope for in someone that is working on your plane! If anyone is in the NJ area and looking for a mechanic, shoot me a PM.

Anyway, I have 2 questions. One, is there a reason the floor panels are held in place by 2 diferent size screws? I believe they are all size 8 round head screws but there were 2 different length screws in my plane. There was no apparent rhyme or reason at least in some cases as to why a particular length would be chosen. I mapped every screw when I took them out because I didn't want to mix them when I put them back. This was a very tedious process.

Two, I had what I thought was a missing or backfiring under low power as I would approach the pattern. It was a random cadence and not terribly loud. After the inspection, the situation is worse. I now have a pronounced drumming coming from my belly skin under my feet I think. I tried to research this but couldn't find more than a single similar thread. The suggestions were that the skin (or firewall in this pilot's case) was flexing and causing the noise. Can anyone shed more light on this phenomenon? The was no resolution in the other thread so I would be interested to know if that is a consensus answer or if there are other things that might cause this problem. Also, is there a fix or if not, some way to fly around the problem. It is unsettling to say the least.

Thanks! Andy
 
The two different length screws that are installed in the floor panels that cover the spar and controls. The shorter screws are installed in location that would cause fowling with the control linkage if they protruded very far beyond the nut plate.

The backfiring noise you hear at idle during approach is called after firing. If is caused by unburned fuel being ignited in the exhaust pipes and is perfectly normal.
 
The two different length screws that are installed in the floor panels that cover the spar and controls. The shorter screws are installed in location that would cause fowling with the control linkage if they protruded very far beyond the nut plate.

I presumed this would be the issue. I was just surprised at some of the locations of the shorter screws. I am going to pull my plans out and try to create a map for myself for next time.

The backfiring noise you hear at idle during approach is called after firing. If is caused by unburned fuel being ignited in the exhaust pipes and is perfectly normal.

I'm not at idle. I am probably 2300 rpm and 13". I just want to make sure we are speaking about the same thing. I have had two different issues. One if this problem and the other was a clear rumbling backfire as I moved the mixture and prop forward on the downwind leg. Once this issue resolves itself after a handful of seconds, I never have it again until the next landing. Idle power doesn't cause it to flare up again over the numbers or wherever.
 
you might check...

...to see if the rubber strips that seal the gap between your wings and the fuselage are tight. i had some strange sounds coming from under the airplane when mine got loose, especially in the pattern in turns. sounded just like the engine misfiring, except it wasn't!
 
...to see if the rubber strips that seal the gap between your wings and the fuselage are tight. i had some strange sounds coming from under the airplane when mine got loose, especially in the pattern in turns. sounded just like the engine misfiring, except it wasn't!

Interesting idea. Going to check that out.
 
My planes have also had unpredictable screw lengths. I don't have any good solutions other than to make all screws as short as possible.

It also wonders me why there are #6 and #8 screws in the plane, as the #6 are insignificantly lighter. If I were king of the mechanical universe (ha!), all screws would be #8 and of the same length.
 
After firing

After firing is pretty common on engines with straight pipe exhaust systems with low back pressure. In very simplistic terms, the engine is an air pump and when the conditions are right, excessive fuel is pulled through the engine when its being driven (wind milling) and not driving (creating thrust). Its pretty typical in RVs with CS props and manifests itself when the throttle is closed rapidly or the RPM is rapidly increased, both conditions can cause the engine to be "driven" by the prop. Due to the low back pressure in the exhaust system, the fuel air mixture, which is somewhat rich and doesn't burn completely, passes through the cylinder rapidly, and it "after fires" in the hot exhaust pipe. You can avoid this by not rapidly increasing prop RPM at low power settings or making gradual power decreases so that the airspeed does not drive the prop excessively. This situation is most likely during pattern entry. Its seems most prevalent on carbed engines, but I have seen it on one injected engine. Who doesn't like to enter that pattern and use the prop to slow you down? Fixed pitch props are not as susceptible to this because its hard to "drive" them at low power settings, but it is possible. Rapidly down shift a Harley with straight pipes and it will do the same thing.
 
I pretty sure that the plans call for AN515-8R8 screws for all the screws in the RV7 floorboards, as well as all the other panel screws in the interior.

If you had different length screws, it was probably because the builder/prior owner used the other screws for some reason only known to that person.
 
After firing is pretty common on engines with straight pipe exhaust systems with low back pressure. In very simplistic terms, the engine is an air pump and when the conditions are right, excessive fuel is pulled through the engine when its being driven (wind milling) and not driving (creating thrust). Its pretty typical in RVs with CS props and manifests itself when the throttle is closed rapidly or the RPM is rapidly increased, both conditions can cause the engine to be "driven" by the prop. Due to the low back pressure in the exhaust system, the fuel air mixture, which is somewhat rich and doesn't burn completely, passes through the cylinder rapidly, and it "after fires" in the hot exhaust pipe. You can avoid this by not rapidly increasing prop RPM at low power settings or making gradual power decreases so that the airspeed does not drive the prop excessively. This situation is most likely during pattern entry. Its seems most prevalent on carbed engines, but I have seen it on one injected engine. Who doesn't like to enter that pattern and use the prop to slow you down? Fixed pitch props are not as susceptible to this because its hard to "drive" them at low power settings, but it is possible. Rapidly down shift a Harley with straight pipes and it will do the same thing.

This is the best explanation of the problem that I have found. Thanks! And yes, I am carbed, CSP'ed, and straight piped. I've learned to control is to a large degree with my approach but every now and then, you end up having to make more aggressive power changes and it's out of your hands. Thanks!
 
My planes have also had unpredictable screw lengths. I don't have any good solutions other than to make all screws as short as possible.

It also wonders me why there are #6 and #8 screws in the plane, as the #6 are insignificantly lighter. If I were king of the mechanical universe (ha!), all screws would be #8 and of the same length.

I immediately came to the same opinion during the inspection disassembly.
 
I pretty sure that the plans call for AN515-8R8 screws for all the screws in the RV7 floorboards, as well as all the other panel screws in the interior.

If you had different length screws, it was probably because the builder/prior owner used the other screws for some reason only known to that person.

I am close to agreeing with you. I went through my preview plans last night and determined that in fact the aft bulkhead U&L, the baggage floor components, and the side covers that house the flap linkage, all use the AN515-8R8. I have yet to find the seatpan screw size on the plans. But if you are correct, that will save a lot of time going forward. I still have an A/P to install and I need to add the pitch servo and wiring too.

As it is, the 8R8's are 1/2" 8/32's. The other screws used seem to be about 1/16-1/8 longer...maybe a 5/8"?The odd thing to me was that the screw sizes were the same on both sides of the plane by hole location. Oh well. Learning more each day.

Thanks everyone for the help.
 
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Popping in the exhaust

What you describe is heard on nearly every landing on the S2B Pitts when you pull the power when turning base for a landing. On the Pitts it is caused by air being sucked in at the exhaust systems slip joints. This provides oxygen to make the popping/after fire of the very rich unburned exhaust gases. If you have a leaking exhaust gasket it can get/will get worse sucking still more air into the exhaust and after fire gets more pronounced.

So check the exhaust gaskets (no white residue around the flanges), if they are not leaking your good to go (the P51 does the same thing)!
 
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