Optimizing fairings for local airflow

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Will Aldridge

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In another thread Pops mentioned that he faired the gear legs and cabane on one of his aircraft but did not realize any speed increase.

I've been thinking about this for awhile and wondering why that might be. I'm hoping one of you with a flying aircraft might be willing to test my hypothesis, which is;

The corkscrew shaped prop slipstream requires that the fairings be set at different AoA's at different locations in order to actually reduce drag.

55CKC.png

You can see in the above pic the way the air is spiraling around the fuselage.

The way I would like to test that hypothesis is to mount multiple AoA gauges as seen in this video:

to different locations of an aircraft. For example mount at least 6 of them to the lift strut to see if the AoA changes from inside the prop arc to out in the clean air, with a gopro filming it.

If my theory is correct an aircraft (cub style) with optimized fairings would look like the builder got really sloppy(with twisted fairings), but would actually be a couple mph faster for no additional weight than a aircraft that has regular fairings.

It's important to note that there are 2 schools of thought on the subject of drag reduction.

1. The fast glass crowd which generally seem to believe that if your plane can't cruise at least 200 kts why the hell do you even own it?

2. The low and slow crowd that go experiment with and believe that drag reduction is critical to increasing range and climb rate.

After having spoken to and read the online comments of quite a few in the second camp I find myself firmly in that camp as well. So if you're of the opinion that drag reduction on a 100 mph plane is pointless I'm going to ignore you, but even so if I'm right the F1 rockets could potentially benefit from this on the gear leg fairings.
 

Sockmonkey

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IIRC they did an experiment with a small radial engine plane where they put a very short duct (less then a foot long) around the prop so stators could be mounted just behind the prop to straighten the airflow.
 

dog

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Your theory would work at one speed and then only if the angle of attack was absolutely steady,like in a wind tunnel or better yet on paper,and then would never overcome the complexity and weight.
And the twisted shape you propose would want to shed a whole series of vortexes
that would increase drag considerably,and or create its own interferance drag.
 

Vigilant1

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FWIW, the corkscrew condensation trails behind a prop don't tell us anything about the angle of the propwash relative to the aircraft centerline. They show us only the trace of where the prop tips were in the rearward moving column of air behind the prop.
The corkscrew shape we see from the prop tip contrails would look just like that if the column of air behind the prop moved straight back.
 
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Will Aldridge

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Your theory would work at one speed and then only if the angle of attack was absolutely steady,like in a wind tunnel or better yet on paper,and then would never overcome the complexity and weight.
And the twisted shape you propose would want to shed a whole series of vortexes
that would increase drag considerably,and or create its own interferance drag.
I don't see how it's really any different than the way things are already done. The wing and tail incidence are set to be most efficient at a certain airspeed, likewise the AoA of fairings is ideally set to be 0 at cruise speed. It may well be that the performance gains are too small to justify the expense of doing the testing and fabrication on production aircraft, but I did propose a way to investigate it, and a fairly cheap and easy way at that.
 

Will Aldridge

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FWIW, the corkscrew condensation trails behind a prop don't tell us anything about the angle of the propwash relative to the aircraft centerline. They show us only the trace of where the prop tips were in the rearward moving column of air behind the prop.
The corkscrew shape we see would look just like that if the column of air behind the prop moved straight back.
I guess the FAA has it wrong then since the attached image came from an FAA handbook. KDk2d.png

The prop slipstream spirals around the fuselage and impacts one side of the vertical tail (1 factor of the left turning tendencies inherent in prop driven aircraft).
 

Voidhawk9

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The easiest solution is to put the prop on the back and keep the airframe out of the wash entirely. 🤓

The wash becomes less significant at higher speeds. For an RC bird like the one linked, it is very slow and the wash is likely quite significant. At Kitfox speeds and higher, while non-zero, it will not be significant enough to expend too much effort on correcting for, IMHO.
If it were me, I'd pick a fairing profile that has good drag characteristics across a range of AoA, which most thickness/chord shapes will do fine with anyway.


Note that textbook illustrations may be exagerated for clarity.
 

Vigilant1

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It is true that the slipstream of the prop spirals around the fuselage. What I wrote is also true: The corkscrew contrails formed by the the prop tips of that Corsair are unrelated to this corkscrew motion of the air. We'd see (virtually) identical corkscrew contrails if the prop slipstream moved directly aft with zero rotation.
 

TiPi

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FYI, I removed the wheel pants on my Tecnam and the cruise speed stayed the same, around 100kts at 1,500'. I gained 3.5kg of useful load by not carrying the wheel pants and mounting brackets.
 

Will Aldridge

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Well just for grins I drew a Helix on my cad model of the path of my prop tip (72" diamter) traveling 100 mph at 2800 rpm. I imagine the relative wind would straighten the airflow out to some degree but still looks like some potential for optimization although looking at that image I would guess that it would be more effective on an aircraft with a smaller fuselage relative to the prop. 20210418_173534.jpg
 

Vigilant1

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Regarding the fairings, one question worth considering is how much misalignment there is between the slipstream of the prop and the relative wind over the other parts of the plane. Just how much swirling is there? There's probably a good NACA study on this. The OP proposes measuring the AoA at the fairings under various conditions. One hint that the propeller slipstream is fairly well aligned with the longitudinal axis is that we know that real-world propeller efficiencies at a prop's design speed often exceed 75%. That includes the parasite drag of the prop and the induced drag from making thrust. We wouldn't see efficiencies this good if the thrust vector was highly misaligned with the flight path.
 
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Vigilant1

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Well just for grins I drew a Helix on my cad model of the path of my prop tip (72" diamter) traveling 100 mph at 2800 rpm. I imagine the relative wind would straighten the airflow out to some degree but still looks like some potential for optimization although looking at that image I would guess that it would be more effective on an aircraft with a smaller fuselage relative to the prop.
Can you explain how the helix formed by the prop tip over time is related to the angle at which the prop blast hits the gear legs? I think on reflection you'll find that they are not related.
 

rv7charlie

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RE the helix image: does the prop not move the air aft, at all? ;-)
Add in the speed of the air moving aft off the prop, relative to the airframe, and see what it looks like.
 

Starjumper7

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FWIW, the corkscrew condensation trails behind a prop don't tell us anything about the angle of the propwash relative to the aircraft centerline. They show us only the trace of where the prop tips were in the rearward moving column of air behind the prop.
The corkscrew shape we see from the prop tip contrails would look just like that if the column of air behind the prop moved straight back.
Although that seemed really clear and well explained to me, someone will have to try explaining that to Will again in some other way, because it looks like he didn't get it yet.
 

Vigilant1

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The paper here ( https://www.icas.org/ICAS_ARCHIVE/ICAS2016/data/papers/2016_0060_paper.pdf ) indicates that losses due to the swirl of a prop range from 3 percent (low loading/thrust) to 9 percent (high loading/thrust). These losses should be a (cosine?) function of the angle between the swirling prop slipstream and the flight path.

ETA: Cocktail napkin estimate: If the losses due to the off-axis propeller air movement ("swirl") are about 5% (see above), then the rotating air from the prop is less than 3 degrees offset from the axis of the prop hub. If I've got that right, then at a fairing or anywhere else the difference in air direction due to prop swirl is pretty small (e.g.. less than differences in relative wind angle due to level flight AoA differences at low vs high speed, etc).
All the above is definitely subject to correction. Free body diagrams aren't my strong suit.
 
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Tiger Tim

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I guess the FAA has it wrong then since the attached image came from an FAA handbook.
IIRC wrong isn’t quite the correct thing to call the FAA but they do recklessly oversimplify with that diagram.

I suppose it’s sort of akin to circulation theory around an airfoil. The air doesn’t really flow backwards under the wing, it’s just a way to visualize what’s going on without the relative wind getting in the way.
 

rv7charlie

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Will,

Another clue, related to what V1 just mentioned, is to consider how much right rudder you need at the beginning of the takeoff roll, and how the need basically disappears in cruise.
 

poormansairforce

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Mounted the AoA vane on the right gear leg and flew a data collection flight. Landed, swapped the camera for the laser and made my line on the fuse/cowl. It agrees with the left and it looks like I was "wrong" with the prior position of the fairings on both sides. There is some speed hiding in that "wrongness"

...Need to set it free!
Toobuilder did some experimenting with this but I'll let you wade through this thread and find all his posts.

I don't think to prop swirl was significant.
 
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Jay Kempf

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The track of the prop tip is misleading you in terms of visualizing the flow. The prop induces a slow roll of the flow field behind the prop. To see the real flow you would need to plot the velocity vectors in the flow field which are much more axial than helical.
 
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