what wing chord for an ASW 27

[Raglan]

Hi Chaps,

Ive started a 1/5 scale ASW 27 which has a fairly high aspect ratio. The span will be 3 m with flaps. I want it to be a model that will have moderate to good speed on the slope but also be capable of flat field thermalling, and so the problem of whether to extend the wing chord has arrisen. Extending the wing chord by 10 - 20 % seems to be quite common amongst commercial kit manufacturers, and I'm not looking for pure scale fidelity either, but rather a model that will not dissappoint in the air.
Does anyone have any comments about me extending the wing chord from 160 mm at the root ( which would be scale ) to 180 mm. This would taper to a tip chord extension of 10mm.
I feel that the model, built like this would be fine with E207 14% ( biased towards the slope) or even a more cambered profile such as HQ 3.0/14. Biased towards less speed.

Anyone dissagree ? Thankx in advance.

Paul (Raglan ) Broome.

[Daniel Gut]

Hello Paul

I have been told that the Eppler profiles where not conceived as flap profiles whereas the HQ profiles are excellent flap profiles. I think the thickness also depends on the wingloading that you have in mind for the 27. I have seen two very different approaches here. A collegue calculates the specifications for his scale sailplanes and has performance as his main criteria. His models are light, sturdy have no undercarriage or detailed cockpits etc and therefore do not need thick profiles to carry all the extra weight around. I have seen them fly, they are impressive and have outstanding allround characteristics. Other pilots ,on the other hand, cannot live without all the detail and their models are 20 or 30% heavier and in their case the thicker profiles are justified, because they have the weight to push the additional resistance through the air.

I would personally go for the slightly bigger wing cord and a HQ 2.5 12 profile with 20% Flap depth (makes landing with butterfly setting very easy and you won't need airbrakes). 3m is still not very big for a scale model and my experience with the smaller exact replicas, is that when they get even vaguely close to a pleasant thermalling speed, they start to behave quite badly and require copious quantities of adrenaline on standby to assist in frequent unannounced spin recovery exercises, been there, done that, it wasn't fun.

Regards from Zurich

Daniel

[Sail 'n Soar]

3m is still not very big for a scale model and my experience with the smaller exact replicas, is that when they get even vaguely close to a pleasant thermalling speed, they start to behave quite badly and require copious quantities of adrenaline on standby to assist in frequent unannounced spin recovery exercises, been there, done that, it wasn't fun.


This is most likely a result of scale taper ratios with scale washout and with associated relatively low Reynolds numbers. Healthy amounts of washout will cure most such problems, won't be that noticeable, and won't impact L/D significantly except at higher speeds.

[surfimp]

3m is still not very big for a scale model and my experience with the smaller exact replicas, is that when they get even vaguely close to a pleasant thermalling speed, they start to behave quite badly and require copious quantities of adrenaline on standby to assist in frequent unannounced spin recovery exercises, been there, done that, it wasn't fun.

ROTFLMAO! :D

I saw some of these "unannounced spin recovery exercises" at Los Banos last weekend. Sure wasn't fun for those it happened to! :eek: A good thing to consider for "smaller" scale models of modern ships!

Steve