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Thruster88

Smoke and the propeller

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We flew to a job today in the bosses beautiful cessna 180, about two hours in light smoke. Picture shows the prop blade out near the tip, even a tiny nick in the paint caused these trails to form. It is interesting to me how the turbulence or whatever is happening fanned out so much. Anyone got an explanation? 

20200110_123940.jpg
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The "nicks" affect the boundary layer airflow. The separation gets greater as the flow gets further from the leading edge. Probably tar in the smoke leaves the trails. It will come off with a bit of eucalyptus oil. Nev

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To visualise airflow NACA researchers had to build expensive wind tunnels and introduce smoke streams; these days we just go a for a fly.

Edited by Old Koreelah
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30 minutes ago, spacesailor said:

AND I GAVE- UP SMOKING,

Now its come back to me.

spacesailor

supposedly the equivalent of 36 fags/day - does this mean I will have to go on a quite smoking plan when the fires are all over & done with?

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The boundary layer at the leading edge is very thin and a tiny protrusion can trip the laminar flow into turbulent flow. The turbulent flow spreads and it makes those tracks.

There is quite an effect on performance, since turbulent flow is about ten times the drag of laminar flow.

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23 minutes ago, Bruce Tuncks said:

The boundary layer at the leading edge is very thin and a tiny protrusion can trip the laminar flow into turbulent flow. The turbulent flow spreads and it makes those tracks.

There is quite an effect on performance, since turbulent flow is about ten times the drag of laminar flow.

So I guess we should try to keep our props glassy smooth like sailplane wings.

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Metal props have greater efficiency than wooden do due to finer sections and better accuracy . Laminar flow wings are  (More) efficient too but more critical and have more rapid stall break away. VG's do help sometimes  to maintain  lift at higher angles of attack by energising the boundary layer and reducing flow separation. Nev

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All wings ( and a propeller is a rotating wing) are laminar flow at the start. The so-called laminar flow airfoils are designed to have more laminar flow than the others. As Facthunter alludes to, laminar flow has the bad characteristic of separating from the surface and making the drag worse than ever.  So some airfoils work with turbulators which deliberately trip the laminar flow into turbulent flow. If you put on the turbulator too far forward, you give away a bit of low-drag area and if you put it too far back, into the separated flow part, then it does nothing. 

You can refer to experts and get conflicting opinions, or you can do a lot of expensive testing.

Once, when I was young and silly, I tried to find if turbulators could improve a Mosquito flapped glider.  The test-rig consisted of an under-wing set of pitots sited at the trailing edge and connected to an ASI via a plenum chamber. The theory is that the lower the airspeed measured by this "wake rake" then the higher the drag.

Then each flight, you can try a new turbulator stuck on in front of the wake rake.

I never found any worthwhile improvement and most of the time the turbulator made things worse.

Now that I'm old and silly, I am wondering about turbulating a Libelle.

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Normally laminar flow aerofoil sections are thicker further back, the thickest section of the aerofoil is a third or further back. The usual clark Y and a lot of NACA sections it's around a quarter chord.

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 That's true, but even1/2 way back seems to be about. I doubt it 's advantage is very marked at about 85 to 120 Knots. The disadvantage would probably outweigh the possible advantages. Not good in a plane you 3 point in normal ops and it requires a more accurate end result on the wing build than is often the case. Fabric is pretty much out of the question. Nev

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Back to laminar flow wings: I've talked to pilots of aircraft with them, including Moonies and P51 replicas. None have been too concerned about their stall behaviour. Either they stay away from that airspeed or their wing hasn't got full laminar flow, as an earlier post said of the P51 wing.

 

I wonder whether our props have a laminar flow cross section, or is prop stall a possibility with fixed pitch?

 

 

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Nobody's going to say bad things about their particular aircraft.  It's still worthwhile to treat such planes with a bit of respect. If it drops a wing on you you aren't going to like it when you are landing. Anytime you come in a bit faster on a tri gear you have to watch out for it wheelbarrowing. Some people put smaller section nosewheel tyres on their Comanches  for this reason.. Never noticed the same problem on a Mooney.   Nev

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