Wing Lift

[Keith Evans 3]

If we have a given amount of lift from a flat plate operating at its optimum angle of attack , How much more lift will be generated by a) putting a radius on the leading edge then b) curving the plate and then c) adding a symmetrical section to this curve ? We will be adjusting the angle of attack to optimise the lift at each stage and keeping the speed of the laminar airflow constant .Just to make it simple ! ! ! Consider a constant parallel chord wing with simple tips ,and a non excessive aspect ratio and with no other aids such as flaps . I've got no idea what the figures may turn out to be .The thought just crept upon me after a few nice drinks while I was relaxing on holidays ! ! ! My guess might be that from the flat plate to the curved section we might see an increase of say 20% What are your thoughts ?

[cymaz]

 

 

 

THIS might be something to get stuck into...personally I’m glad to get my plane down in one piece

Edited By cymaz on 10/03/2018 12:12:40

Edited By cymaz on 10/03/2018 12:13:06

[Geoff S]

I have one of the cheap HK glue-and-go foam board FW190s. It flies amazingly well considering its crude wing form and is great fun. One of its less endearing characteristics is it's quicker than expected descent when landing - it suddenly loses lift without completely stalling. It took me by surprise the first time but I got used to it. As an experiment I glued some half round balsa from my store onto the l/e and it certainly improved it. So in answer to your A) a radius on the l/e helps but I have no numbers.

Geoff

[Denis Watkins]

I have an book about aerodynamics from the 1940s which accounts the virtues of the flat plate wing

In modelling 12" to 24" wing spans, the flat plate is rigid enough to succeed well

Simple statement increase lift = increased drag therefore more thrust required

Adding a slice of curved drinking straw to the leading edge of a 16" flat plate wing does increase low speed lift

But you do reach a point where you need more battery, and more motor, increasing the weight, requiring more thrust.

The flat plate wing begins to twist more easily as wingspan increases, and now benefits from a curve

Jedelski shows this, with high lift from reduced wing components

At 5ft wingspan, we need a top and bottom surface for lift and rigidity.

[Biggles' Elder Brother - Moderator]

One way of looking at this:

The simple 2D lift equation tells us that the lift force we get from a wing is:

L = 0.5 x d x V2 x A x Cl

Where

L is the lift force

d is the density of the air - for a given day dictated largely by altitude

V2 is the velocity of the wing through the air squared (sorry can't do superscripts on here!)

A is the wing area

Cl is the co-efficient of lift of the wing.

So, for the same shape and size wing, flying at the same altitude, on the same day, at the same speed then how much lift we get effectively just comes down to the co-efficient of lift of that wing section. Put simply increase Cl by 10% and you'll get 10% more lift.

Of course Cl varies with angle of attack (AoA). So really we need to compare at a like-for-like situation. Let's take the maximum value as our comparison point. We don't have to chose this, we could for example compare them at AoA equals 5 degrees for all of them. But we'll go with max value for now because that effectively determines the maximum lift the wing can generate before it stalls - an interesting point on the curve!

For a flat plate the max Cl is around 0.6 and occurs at an AoA of around 10 degrees

For flat bottomed aerofoil like a Clark-Y max Cl is typically 1.4 - 1.5 and at an AoA of about 14 degrees or so.

For a more high performance aerofoil like a NACA4412 Cl max is maybe 1.7 at an AoA of 12 degrees or so.

From just these three examples we can draw a few "conclusions". While a flat plate will work as a wing is not actually a very good one! A curved aerofoil of the same size, even a very simple one like Clark-Y, can generate about twice as much lift under the same conditions. But notice that after that dramatic improvement, further improvement is harder to get, the NACA4412 is producing only about 12% more lift than the simple aerofoil. Now the NACA4412 is far from being the last word in performance sections, but even the best are not that much better.

We also need to note that this better aerofoil performance with regard to lift does not come free. The stall on the NACA4412 is much sharper than the very gentle stall of the Clark-Y - this is generally true if thinner wing sections. Also it happens at a lower AoA. Finally the drag is higher for the NACA4412. So this extra lift comes at a price! If you are designing a glider for example, simply maximising Cl will not necessarily give you the best overall performance as this is a balanced compromise between lift and drag.

Just some ramblings but I hope they go some way to answering your question Keith. I'm sure other will have contributions to add as well.

BEB

[ron evans]

Wrong thread

Edited By ron evans on 10/03/2018 18:39:15

[PatMc]

Posted by ron evans on 10/03/2018 18:37:35:

Wrong thread

Edited By ron evans on 10/03/2018 18:39:15

They were probably using a flat plate section with blunt LE.

[Keith Evans 3]

HI all thanks for the responses . Thanks B.E.B that was what I was looking for .Thanks CYMAZ , that was one heck of a tome.

[Nigel R]

Also worth noting, for indoor type models with very low airspeed and small wings, the difference between a flat plate and a "proper" section becomes rather much smaller. And gets to the point where it isn't worth bothering with an airfoil.

So the further part of the answer is, "but it depends on your wing chord and speed".