[Geoff]

So I went flying today to test my newly-installed gear leg fairings and upper intersection fairings. Much to my surprise, the engine made a noise I've never heard before in this plane. At higher power settings (above about 19" MP) the engine was making a pulsing sound. It sounded *exactly* like flying a twin with the props out of sync. Varying the MP between 19" and 24" sounded exactly like moving the prop control slightly on one engine of a twin.

I can only guess that this is some sort of exhaust pulse. However, I didn't make any changes between this flight and the previous flights except to install the gear leg and upper intersection fairings. I'm just not sure it sounds reasonable that these fairings would alter the airflow along the bottom of the fuselage enough to cause an exhaust pulse where none existed before. By the way, there is no floor or other vibration -- just the pulsing sound.

I have some turndowns that Larry Vetterman gave me. I've flown when them on and off and didn't notice much difference. They're presently off, but I plan to put them back on and see if the pulse thing goes away. I'm using the standard Vetterman exhaust, TMX IO-360, vertical induction.

Any other ideas?

Thanks.

 

[WilburD2]

Only way to know..

If it is the fairings is to take them back off and see if the pulsing goes away. If you were flying earlier with the downturns on and took them off to put the fairings on that might also explain things..

 

[RV8N]

My canopy skirt made some serious buzzing the first time I flew in cold weather. Being in Houston, I had quite a few hours on the -8 before the weather got cold. The next time I flew, I heard a slight buzzing on climb-out. When I pushed the nose over for cruise, there was a horrible racket. By pushing rudder peddles to make the plane skid, I was able to locate the origin of the noise.

Just a thought...

Karl

 

[rv9av8tr]

Exhaust leak?

It would be worth pulling your cowls to make sure you don't have an exhaust pipe crack. I experienced that in flight on my Long-EZ once. It also showed itself in a change in EGT on the cracked pipe. It's worth ruling out!!

 

[Geoff]

If you were flying earlier with the downturns on and took them off to put the fairings on that might also explain things..

Nope, they were all separate events:

No downturns, no fairings = no problem.
Downturns, no fairings = no problem.
No downturns, no fairings = no problem.
No downturns, fairings = problem.

The only thing I did before the flight with the issue was to add the fairings. Nothing else changed. You're right, though -- I'll just take them back off and try it again.

My question was more theoretical in nature: Could intersection fairings effect exhaust pulses, and can exhaust pulses sound like a twin with the props out of sync?

Thanks.

 

[airguy]

The "prop synch beat" noise thing makes me wonder if you're setting up a standing wave in the wings that is very slightly different frequency between left and right, resulting in the beat between them. Perhaps without the fairings on the resonant frequency or excitation pulse was simply not right? If this is the case you should be able to change the resonant frequency easily by adding/removing a small amount of weight from the wingtip nav light or landing light and see if it changes.

 

[AlexPeterson]

SNIP

My question was more theoretical in nature: Could intersection fairings effect exhaust pulses, and can exhaust pulses sound like a twin with the props out of sync?

Thanks.

My understanding is that the pulses that hit the bottom of the plane are more due to propeller action than exhaust. Someone who knows a bit about exhausts on RV's extended the pipes all the way to the spar and found that the drumming on the floor remained the same.

 

[Geoff]

Not engine related

So I went flying today to test my newly-installed gear leg fairings and upper intersection fairings. Much to my surprise, the engine made a noise I've never heard before in this plane. At higher power settings (above about 19" MP) the engine was making a pulsing sound. It sounded *exactly* like flying a twin with the props out of sync. Varying the MP between 19" and 24" sounded exactly like moving the prop control slightly on one engine of a twin.

I can only guess that this is some sort of exhaust pulse. However, I didn't make any changes between this flight and the previous flights except to install the gear leg and upper intersection fairings. I'm just not sure it sounds reasonable that these fairings would alter the airflow along the bottom of the fuselage enough to cause an exhaust pulse where none existed before. By the way, there is no floor or other vibration -- just the pulsing sound.

I have some turndowns that Larry Vetterman gave me. I've flown when them on and off and didn't notice much difference. They're presently off, but I plan to put them back on and see if the pulse thing goes away. I'm using the standard Vetterman exhaust, TMX IO-360, vertical induction.

Any other ideas?

Thanks.

Well, it was the fairings. Even at low power settings, I could replicate the beat-noise by diving to get over 150kt indicated. It wasn't as pronounced as in level flight with the engine at high power, but it was there. Rudder application also affected it -- the noise decreased when skidding in one direction and increased when skidding in the other direction.

So I removed the fairings and flew again. Noise gone.

I was flying with the gear leg fairings, upper intersection fairings, and wheel pants -- but no lower intersection fairings. Perhaps the gear leg fairing was somehow twisting or vibrating. It seemed pretty solid (immobile) on the flat gear leg of the -8, but perhaps not at 150kt. I'll have to finish glassing the lowers and then try again.

Thanks to those who responded. I guess I still don't completely trust the engine yet (with only 55 hours on it), so I'm a little quick to assume that strange noises are engine related -- especially when the noise changes when varying the engine controls.

 

[Geoff]

Update -- need the lowers

Well, it was the fairings. Even at low power settings, I could replicate the beat-noise by diving to get over 150kt indicated. It wasn't as pronounced as in level flight with the engine at high power, but it was there. Rudder application also affected it -- the noise decreased when skidding in one direction and increased when skidding in the other direction.

So I removed the fairings and flew again. Noise gone.

I was flying with the gear leg fairings, upper intersection fairings, and wheel pants -- but no lower intersection fairings. Perhaps the gear leg fairing was somehow twisting or vibrating. It seemed pretty solid (immobile) on the flat gear leg of the -8, but perhaps not at 150kt. I'll have to finish glassing the lowers and then try again.

Thanks to those who responded. I guess I still don't completely trust the engine yet (with only 55 hours on it), so I'm a little quick to assume that strange noises are engine related -- especially when the noise changes when varying the engine controls.

Well, I glassed the lower intersection fairings to the wheel pants and tried again. Presto -- no noises or vibrations. Problem solved.

I guess the moral of the story is that even with the flat gear leg of the RV-8, the gear leg fairing needs support at both the upper and lower ends to keep it from doing strange things at high airspeeds.