FP/FF fluctuations - interesting cause and solution

[Bob_Belville]

Thanks to Paul @kortopates and Lynn Mace @AGL Aviation. 

I requested Savvy study a recent flight (KTPF-KMRN) in which it was clear that the FF was jumping around while I was in Lean Find mode heading to LOP cruise. As Paul and I went back and forth about the symptoms we discovered that the FP was behaving similarly and finally that the fluctuations which were steady at 8000' calmed down completely when we descended to 5000'.

Paul suggested that that looked like vapor lock to him. I discussed this with Lynn and he asked about cowl flap position and knowing the design of the PowerFlow Exhaust, he suggested that the fuel line ell fitting on the lower firewall might need to be shielded from the cyls 2&4 exhaust risers. He cut a short length of fire shield and gave me a couple of ss tie wraps. (That's the way he treats me... do it yourself, old man.) See pics, fuel line ell before and after, the exhaust header is just to the left.

Who'd have thunk, I took it up to 4500' for several Lean Find passes and then up to 8500' for a few more plus LOP Mag checks per Savvy SOP. 

Live and learn! Much, much better. 

[Bob_Belville]

BTW, when I came back to report the flight results to Lynn he happened to have a PA28-R Arrow in his shop with the cowl off. Piper has a box over this ell except on the outside, toward the cowl and the cowl has a NACA scoop blowing outside air on that fuel line fitting!

[Marauder]

Bob - was this going on for a while?


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[Bob_Belville]

44 minutes ago, Marauder said:

Bob - was this going on for a while?


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Yes. Paul @kortopates went back and confirmed the pattern that I had not noticed.

[Browncbr1]

Nice, I'll have to cover my fuel line elbow as well.  

[Bob_Belville]

48 minutes ago, Browncbr1 said:

Nice, I'll have to cover my fuel line elbow as well.  

Yeah, that's why I posted in detail with pics. I suppose my plane is hardly unique though the PFS risers may be closer to the fuel fitting than the stock exhaust.

[carusoam]

Nice work, Bob!

elbows have a tendency to collect bubbles, and suffer other odd flow related issues...

Best regards,

-a-

[tony]

That is interesting.  During a hot start in the heat of the summer sometimes I have to use boost pump because my fuel pressure drops.  After I get the bubbles out of the line, she runs fine.  This explains why.  Never happens this time of year.  Thanks Bob, I'll have to try this too. 

[Bob_Belville]

Interestingly, this is what Piper does for what must be good reason on their Arrow. That's a 5 sided aluminum box. The baffle seal material fits to the inside of the cowl and there's a NACA scoop sending air into the "box".