Spats

[Old Koreelah]

After spending a couple of months building fairings around my fat 600X6 tyres I finally got to do some proper tests today, flying extended squares at 4000' with and without the spats.

 

In the least impressive test, while cruising at 2900rpm fuel consumption dropped 8.95% and speed increased 6%.

 

I never particularly liked the look of those ugly big wheel pants, but they work.

 

 

[sfGnome]

That's a pretty impressive "least impressive"... :)

 

 

[pylon500]

Actually, the old Jodel may suit having 'pants' ah lah Chilton Monoplane?

 

 

Choose your own colour arrangement...

 

 

 

[Old Koreelah]

That's a pretty impressive "least impressive"... :)

The improvements at 2600 rpm were much greater; the opposite of what you'd expect. Just when you think you understand something, reality muddies the water.

 

 

[Old Koreelah]

Actually, the old Jodel may suit having 'pants' ah lah Chilton Monoplane?[ATTACH=full]27943[/ATTACH]

Choose your own colour arrangement...

 

[ATTACH=full]27944[/ATTACH]

Crickey! Nice work Pylon. You could have saved me all those weeks of grovelling around in the dirt under the wing.

The Chilton looks sleek, but those angular intersections are the enemy of smooth airflow.

 

 

 

[pylon500]

Gliders are often having the same debate, fillets or no fillets?

 

Only way to know would be to try....

 

 

[facthunter]

If you fillet it might be too heavy. I will have to sneak into the hangar quietly and hear the gliders debating. A couple I encountered recently were just playing possum. I saw the thread "SPATS" so I thought I would get one here. Nev

 

 

[pylon500]

Just some examples of fillet vs no fillet.

 

Some could be for ease of manufacture or weight saving, some could be related to perceived cross sectional area at a high velocity area...?

 

The Astir with butt intersections;

 

 

The Jantar with small fillets moulded from the fuselage;

 

 

And the Salto, which looks positively organic;

 

 

I have seen somewhere a discussion on how best to create fillets based on airflow in the area, interaction angles of the surfaces in question and the best radius to use based on these factors.

 

No idea where it is now?

 

 

[SDQDI]

Hmm organic works but I reckon that even to a stol farm boy like me that Salto looks divine:drool:

 

 

[Keenaviator]

My understanding is that intersecting angles less than 90 degrees should have a fillet to reduce drag whereas intersecting angles greater than 90 degrees don't need it. Laurie

 

 

[Marty_d]

Crickey! Nice work Pylon. You could have saved me all those weeks of grovelling around in the dirt under the wing.The Chilton looks sleek, but those angular intersections are the enemy of smooth airflow.

 

[ATTACH=full]27946[/ATTACH]

Nice work OK. What sort of engine are you running?

 

 

[Old Koreelah]

Nice work OK. What sort of engine are you running?

Hydraulic J2.2

 

 

[Bob Llewellyn]

My understanding is that intersecting angles less than 90 degrees should have a fillet to reduce drag whereas intersecting angles greater than 90 degrees don't need it. Laurie

As soon as the air next to a surface begins slowing down, it creates an "adverse pressure gradient". Seperation is imminent. If a low wing is rigged with a small positive incidence, such that at cruise the fuselage centreline is aligned with the airflow, an anti-interference fillet has to grow in radius as it moves aft; and normally, the rear edge of the fillet has to turn up - see also Grumman 4-seat lighties, which twist the root of the flap/TE up.

You can also live without fillets if you place a VG at or just ahead of the LE to energise the junction airflow. The cat can be skun many ways; but air dinna like flowing around solid bodies at high speed...

 

 

[Bob Llewellyn]

After spending a couple of months building fairings around my fat 600X6 tyres I finally got to do some proper tests today, flying extended squares at 4000' with and without the spats.In the least impressive test, while cruising at 2900rpm fuel consumption dropped 8.95% and speed increased 6%.

I never particularly liked the look of those ugly big wheel pants, but they work.

There's a NACA report - a series on undercarriage - which looks at spats, leg fairings, pants, and finer points of fillets and geometry - when I reboot my desktop I'll post the number...

 

 

[rgmwa]

What's the theory behind `pressure recovery' spats, or wheel pants as the US guys like to call them?

 

rgmwa

 

 

[Bob Llewellyn]

What's the theory behind `pressure recovery' spats, or wheel pants as the US guys like to call them?rgmwa

Reports 485, 518, and 522 tell almost everything about light aeroplane undercarriage. 518 covers spats and faired struts vs "pants".

The theory behind any form of streamlining comes back to Lanchester (1892); any body of "icthyoid" shape (i.e. fishlike) owes all of its drag to skin friction. Which is to say, if separation (and compressibility, but Lanchester didn't know about that) is avoided, only friction drag remains. All of this stuff about "form drag" is simply a way of combining typical friction plus separation in a way that avoids the computational complexities that CAD so neatly deals with.

 

Incidentally, fish don't use much fairing to avoid interference drag (sharks and whales do...); but air has 14 times the kinematic viscosity of water, so it's much less critical once you're out of the sticky stuff and underwater...

 

Such highly successful small "racing" aeroplanes as the Miles series and early Lockheeds used "pants" as a structurally simple way of fairing the U/C legs, brakes, wheels and tyres all at once. The drag reduction was substantial. The reduction in directional stability was also substantial, but as everyone was used to directionally unstabke fighters, who cared? It's worth looking at the 1930's european development of small racing aeroplanes, e.g. the French Deutch de la Muerthe Trophy racers ~ 20ft wingspan, 240~350hp, averaging ~ 380kph (207 kts) over two races of 1,000 km each in the one day (in 1934).

 

The main drag from typical spats comes from the airflow within, and how it leaves the spat. Putting closing doors on the bottom of a spat reaps dividends, and provided they're not made of Unobtanium, it's safe to land with the gear "up" ...

 

 

[Downunder]

Not to get too far off topic but someone posted a youtube video on this forum showing a bloke holding different aircraft parts over a fan on a desk.

 

The part was connected to a scale and it's drag was read off of the scale.

 

I have been unable to find this video again, if anyone knows?

 

What I found really interesting (besides the simple test setup) was that an aerodynamically designed part was many times more efficient even though it was many times larger than say a wire cable or something quite small...

 

 

[Bob Llewellyn]

Not to get too far off topic but someone posted a youtube video on this forum showing a bloke holding different aircraft parts over a fan on a desk.The part was connected to a scale and it's drag was read off of the scale.

I have been unable to find this video again, if anyone knows?

 

What I found really interesting (besides the simple test setup) was that an aerodynamically designed part was many times more efficient even though it was many times larger than say a wire cable or something quite small...

The system was an "open throat" wind tunnel with no flow straighteners*, which is a form that has done quite a lot of useful work in comparative analysis.

*Though the massive tip vortices from a typical desk fan give me the shudders...

 

This became a major issue for Lanchester - the drag need be no more than the friction drag; but everybody was jumping on "form drag" and ignoring the subdivision of drag sources - and is demonstrated very well in the Cd for a teardrop body of length:diameter 3:1; the Cd is ~3% of the maximum sectional area. Compare this to the ~25% of the Cherokee fuselage. Taylor's Mini-Imp fuselage should have approached 0.05 (5%), or 1/5th as draggy as a Cherokee fuse. Truly we need to think outside the box (and inside the fish? DH Mosquito supposedly had fish input to the fuse...)