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Why Do We Blunt The Leading Edges Of Our Wings?


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"Back in the day" ie: 1960s - before the dawn of proportional control - most aerobatic models had rounded, but blunt leading edges. This gave a pretty benign stall, but more importantly, an almost constant speed aircraft!

You couldn't throttle back coming down out of loops with "reed" equipment, and this design feature made the loops pretty much constant speed despite being stuck at full throttle!

One of my current models is a "KingPin" from 1963. In its day, it would have been considered a "hot" aerobatic model. Attempting to modulate the throttle during loops makes them look quite untidy (using modern radio gear). Leave it at full throttle, and it goes around nice and smoothly!

Another interesting feature is that most models of that era had a nose-leg that was shorter than the main gear, giving the model a nose-down attitude on the ground. This leads to a fairly abrupt take-off, once the elevator has enough bite to overcome the nose down attitude, and also makes the model stick firmly on landing, without bouncing.

Landings and take-offs were scored back then, and I cannot understand why they are no longer scored in modern aerobatic contests! Anyone care to enlighten me?

--

Pete

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It's all related to how the air separates from the wing (i.e. a stall) a sharp corner will have the airflow breakaway sooner as the angle of attack increases causing turbulence loss of lift and drag on the upper surface.

Some full size planes have a sharp section near the wing root, this is to ensure the wing stalls at the root first and not the wing tip, avoiding a tip stall https://en.wikipedia.org/wiki/Stall_strips

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Thank you for the replies gentlemen, especially Frank's explanation of the aerodynamics involved.

Decades ago, when I lived in Devon, I regularly attended the Exeter Club's end of season auction which was held at about this time of the year. It attracted aeromodellers from a wide area and there were sometimes over 400 lots. I remember a superbly built Flair Taube appearing every year. This was covered in yellow transparent film which showed off the sharp leading edge to advantage. If the builder had blunted the leading edge I would have put in a bid for it.

I was left to wonder whether successive owners found it such a pig to fly that they put it in the auction every year!

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The Taube wing section may have been scale, typically in WW1 they used a thin under cambered section, so at moderate angles of attack the airflow is still smooth over the top surface, unless of course the builder had left this with a square section. My Mick Reeves Camel has a similar section and flies quite nice (tricky to steer on take off though!)

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Posted by Pete Collins on 03/11/2020 10:04:49:

The leading edges of the flying flea were intended to be left sharp. It also used a somewhat weird aerofoil with lots of reflex. Of course Henri Mignet freely admitted that he was no aerodynamicist so, maybe the flea flew despite its wing section rather than because of it!

That's interesting. My best friend Michael was building Bob Wright's model Flying Flea while he was terminally ill in 2002. He had a PPL but had never built a model before and refused to accept my advice to build something more traditional. I was bequeathed all of the bits and will complete the model one day if I live long enough.

IIRC the plan tells you to leave the leading edge sharp.

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Posted by Peter Christy on 01/11/2020 12:04:11:

Landings and take-offs were scored back then, and I cannot understand why they are no longer scored in modern aerobatic contests! Anyone care to enlighten me?

Pete

The short answer is that they still are, at least in F3A anyway (can't speak for IMAC as I don't know). Both take off and landing are scored in both the Clubman and Intermediate classes, with the K (difficulty factor) reducing from a factor of 2 for Clubman to 1 for Intermediate when you move up reducing their importance on the overall score.

Take off and landings aren't judged in the schedules above that i.e. Masters and FAI (P and/or F), I don't know exactly why but I would assume that by the time you get proficient enough to compete at Masters level a good take off and landing is considered a "given" and also by dispensing with them it saves considerable "competition time".

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I suppose the reason is that for speeds below the speed of sound the drop shape (cut trough a wing profile - it will be some kind of drop shape) is the shape with the lowest drag.

If you go supersonic this changes. Then the shape with the lowest drag is pointy and not round. (look at the noses of supersonic planes...)

Well the chance of going supersonic with a model are hopefully zero. Bad enough to hit the ground at "normal" speeds.... smiley

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As has been said, a blunt LE tends to have a softer stall. Not always, but usually. We also fly our models, generally speaking, over a wide range of speeds and AOA, so a blunt LE can be a bit more forgiving, where a sharp LE might be a bit of a one trick pony.

Not least, however, is that we tend to bash and ding and handle our wings. We chuck them in the car and in a shed. A sharp LE, unless made from very hard material, would soon become notched and bent like the blade of a badly abused knife.

Sometimes, practicality is as good an answer as anything else.

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Philip Lewis: Thanks for the update!

Although it is along time since I took an interest in FAI aerobatics (though I have had a go at a UK Classic event), I seem to recall being told at the Nats that landings and take-offs were no longer scored! This amazed me, as these are two manoeuvres that are compulsory to any flight - regardless of class!

Also pulling off a smooth, straight take-off and a nicely judged 3-pointer (if you have a tail-dragger) is immensely satisfying!

Still better let the thread get back on topic...! wink

--

Pete

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Posted by Capt Kremen on 01/11/2020 17:00:17:

For a common example of a sharp stall strip, look at the inward wing, leading edge of a 'RAF' origin D H Chipmunk. (The Canadian Chippies don't have it !)

Here you are Capt Kremmen. This is an ex-RAFGSA Lycoming Chipmunk,G-BCCX at RAF Odiham, clearly showing the strip on the leading edge that you describe, from just outboard of the wing root to inline with the undercarriage leg.

ians chipmunk.jpg

Photo Credit: Nick Killick

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Hi Mal, G-BCCX has a 180hp Lycoming.  The RAFGSA re-engined some Chipmunks with Lycomings to give better performance as glider tugs. 

There's a picture of the engine installation in this article on G-AOTF, which we had at Odiham before G-BCCX.  Pilot Magazine Article on Chipmunk Mk.23 ex-Crop Sprayer

Correction to the Chipmunk picture I posted. The photo was by Darren Harbar darrenharbar.co.uk

Edited By Robin Colbourne on 03/11/2020 20:21:59

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