Prop efficiancy

[birdy]

We all know that the cleaner the airflow to the prop, the more efficiant it will be at converting dead dinosours into moven air, and that we gyro nuts are startn way behind the 8ball with our props follown us, but theres sum little things we can do to improve the airflow.
Simple things like streamlining anythn up wind of the prop to minimise turbulance, and keepn everythn as narrow as possable to disturb as little of the flow as we can.
And i accidently found anatha thing that, as it turned out, makes a big difference, at any AS.
Exhaust gas.
I knew that exhaust gas flown through the prop made the prop noiseyer and this effected the efficiancy, but i didnt think it had as much effect as i found yesterday.
Id been putn round for bout 4 hours getn sum moos together and pulled up for fuel and other things.
But wen i restarted the 912 it sounded like a charff cutter.
One side of my [ home made] low back pressure exhaust systm had finaly said enuf is enuf, and cracked through. [ a 18" pice of flex exhaust pipe] Not wantn to spend the next 4 hours with 2 loose ends there waitn to fall through the prop, i took them off, leaveing the 2 right side header pipes [ bout 4" long] blown exhaust streight upwind of the prop.
After pushn me ear plugs in that extra bit, i took off and resumed the job.
It wasnt till id leveled off at ten foot that i noticed i needed bout 150-200 more erpm to stay level, but assumed it was coz of the now full 70l tank, and it was a bit warmer. I also noticed there seemed to be alot more prop noise, but dissmissed that as my imagionation.
A coupla hours passed and, at low AS, i noticed i needed to hold left pressure constantly to stop it diven rite. And to convince me i wasnt imagioning things, my stick arm was startn to feel the pinch too, and me rite tit muscle was burning from constantly haven to hold left pressure.
Then it dawned on me.
The machine was crabn in yaw all the time, and instead of correcting with the rudder, id been countering with the stick. [ no yaw string ment im abit slow to register these things]
But why was it always yawing rite??
By the time id realised sumthn was amiss, i was bout to yard up, and couldnt afford to land just yet, [ no ground crew] so i had a quick look behind me to make sure the engine was still pointn the rite way, and that me rudder wasnt hangn off, and in the mayhem that followed for the next half hour, i kinda forgot bout it.
Till the 30 min trip home.
Back to the S/L, and the yaw was still there.
"Cant be anythn drastic" i thought, coz it woulda failed for sure in that last half hour, but it was no wores or better.
Got to thinkn bout it, and the penny droped. [ bing]
The exhaust gas, wen the systm is intact, passes through the prop close to the hub flange, where theres hardly any air pushn happening, but with that pice of flex tube missn, theres bout 2 squ foot of the props disc be'n fed hot disturbed air.
The rite side of the prop disc is getn dirty air, so its not pushn as hard as the left, so the machine was constantly be'n pushed rite by the uneaven thrust of the prop, also maken it less efficiant. [and noisier]

Wacked a new bit of flex onit this arvo, went for a blap, and the old girl is better n new. Streight as an arrow.

 

[C. Beaty]

It could also be hot air (literally, not figuratively).

Hot air is less dense and an engine’s exhaust is less dense than the surrounding air.

Force = mass x acceleration so you don’t get as much force where there’s a large volume of exhaust gas.

With exhaust gas running through the center of the propeller disc where there’s not much thrust being produced anyhow, hot air is less pervasive.

 

[birdy]

Thats why i said;
theres bout 2 squ foot of the props disc be'n fed hot disturbed air.
CB.
Iv since realised that my home made exhaust systm is pretty good at not disturbing the inflow of air to the prop.
The exhaust gas, wen the systm is intact, passes through the prop close to the hub flange, where theres hardly any air pushn happening,