Trailing edge profile

[Chris Walby]

I am busy shaping a wing (balsa slab) 38" wing span electric model form a plans and am slightly surprised at the TE profile (of what appears to be the wing and aileron).

The wing section is only 10mm, but the TE is indicated as 5mm with slight chamfers.

Should I leave it as is (is there a reason for leaving it this blunt?) or take the profile to a finer point?

[brokenenglish]

Build as per plan. A solid balsa wing needs thickness for strength and warp resistance.

Edited By brokenenglish on 03/10/2017 07:59:52

[Chris Walby]

Hi,

Digging around on the web seems to reveal that blunt TE have little if no negative effect and possibly improved lift!

I was not intending to thin the wing section, but as the aileron forms 90% of the TE was thinking of tapering it off, but as you suggest I'll stick to the plan

Another new thing learnt! From the web:

The trailing edge may be sharp, flat or rounded.

The (incorrect) Kutta-Zhukovsky theory of lift has led to a common (incorrect) idea that the generation of lift requires a sharp trailing edge as expressed on Wikipedia:

• The Kutta condition gives some insight into why airfoils always have sharp trailing edges, even though this is undesirable from structural and manufacturing viewpoints. An aircraft with a wing with a smoothly rounded trailing edge would generate little or no lift.

Accordingly, the standard NACA airfoils all have sharp trailing edge.

In practice, trailing edges are smoothly rounded but do not destroy lift. Navier-Stokes simulations with a rounded trailing edge show good agreement with experimental observations and are explained by the New Theory.

Flatback Airfoils

Airfoils with a flat cut-off at the trailing edge are called flatback airfoils.

Flatback Airfoil Wind Tunnel Experiments, Sandia Lab, 2008, reports experiments with a fatback airfoils with 0.5%, 8.75% and 17.5% trailing edge thickness-to-chord ratio, with about the same drag and lift for 0.5% and 8.75% and doubled drag for 17.5%.

Wind Turbine Blade

Blunt edge is reported to give better lift than sharp, because a blunt edge gives positive lift on the upper surface near the trailing edge, while a sharp edge has negative lift.

[Don Fry]

Chris, thank you for your efforts. You have done some good work there and improved my knowledge base.

One point, however. I read somewhere that a square exit to the airfoil gives a constant breakaway of the airflow from the airfoil. And that constancy provides steady lift and drag. Rounded edges, because of manufacturing variation provide more variable breakaway and hence variation in lift and drag.

The pilot is not going to like flying surfaces which are not constant.

And while efficiency might like rounded profiles, especially with extreme stuff like turbines, when it a bloke with a sanding block, a whisky at hand, and a mark one measuring system doing the profiling, square is easier to execute.

[John Lee]

If square edges are good enough for the fastest aircraft ever built..........

[Simon Chaddock]

Be careful!.

That paper on flat back airfoils is referring to sections used in wind turbines that have a thickness/cord ratio of 35%! Such a section is almost meaningless in conventional sized models.

In my own experience a thin (less than 1 mm) trailing edge (providing it maintains the wing section) improves the glide ratio by a small but noticeable amount..

[Mike T]

Take a look at the Lidl glider, chaps! Puts it all into perspective!

[Martin McIntosh]

You will find that a square trailing edge gives more response to the control surface, like it or not. I used to fly F3A. My models had a rounded rudder TE but my buddy`s very similar ones were squared off and I found his rather too sensitive for my style of flying. Look at today`s F3A ones and they have very squared off rudders to get the response for the current schedules, but of course we have expo now.

[Robin Colbourne]

Rounded trailing edges can cause flutter (high speed oscillation) and loss of control surfaces. Any play in control linkages will exacerbate the problem.

[Mark Kettle 1]

 

The full size Avro Vulcan has a squared blunt TE.

 

Also the Eurofighter TE. I've measured it at 5/8" .

**LINK**

Tail section viewed from port side. Note blunt trailing edges of wing elevons.

 

Below this model at 44" span has a trailing edge that measures 8mm TE squared off and fly's and glides very well. 

I think the thickness of the edge has to be in proportion to the chord width, however with the TE being thicker it does allow it to be stiff and give precise control of the model. 

 

Glide speed with 8 mm TE. This section of video is flown in a 8 mph wing and with no ballast in the model, the model weighs in at 42 oz with a 10 oz a square foot wing loading. 

Edited By Mark Kettle 1 on 04/10/2017 08:22:29

[Mark Kettle 1]

Chris what do you think?

[Nigel R]

I've always tried to take airfoil type trailing edges down to a square edge of 1/16 to 1/8.

Or on a solid 1/4" sheet balsa surfaces, just leave them square.

All very low tech, but worked just fine for me.

As a bonus, as mentioned it is formed by hand with a sanding block measure by eye, with an ale within easy reach.