RC India

Is hot wire cutting of an undercambered airfoil a good idea?
The aircraft has to lift a payload of 600 or more grams. Wingspan 1200mm, chord 200mm, fuselage length 840 mm
Airfoils considered S1223, E423, Clark Y, NACA 2412
Or should i go for standard balsa construction?
Will Depron with Carbon fiber spar be able to lift such a weight? (payload weight of 600g + plane weight of around 600 grams.)
 :help:

I forgot to mention that I intend to cut HD Thermocol. If it is not stron enough, I might buy Pink foam (Pretty Expensive :P)

whats your source of pink foam??  is it EPP??

No its not EPP, its the regular pink foam that is used by many rc modellers. Its available in Bangalore in JC Road.
600 or so per sheet of 2 inch thickness.

Is hot wire cutting of an undercambered airfoil a good idea?
No. Not at all.
Here's Why
https://youtu.be/WG_-eue5T5w

Full thread - https://www.rcgroups.com/forums/showthread.php?2463580-Large-Slow-Flying-Plane


Swapnil N.

Hi...

I have been into hot wire cutting in the recent years. You can use it for sure, but you need to know the technique "Gravity feed hot wire cutting". I use pink Fomular board. But cutting alone is not sufficient for 1200mm wing span. Either you got for CF spar or plywood spar. Else the foam board will split easily. Also you need to cover the wing with tape or do epoxy else the surface will be easily damaged. See some of the sample images in below link.

http://www.rcindia.org/tools-materials-and-building-techniques/diy-hot-wire-foam-slot-knife-scoop-cutter/

Regards,
Samayaraj

Some of my sample works using hot wire cutter

Nice one. But is it undercambered airfoil? :headscratch:

No. Its not a under cambered wing. Its, MH45 profile. But you can make your own aerofoil tracer over which how wire is moved and finally you will the desired aerofoil.

Is number of wings included in requirement? Except a biplane, my belief is that a 600g of auw is already too much for 1200mm wingspan to fly properly, let alone a payload. Care to explain more about your requirement please.

Sent from my ZUK Z1 using Tapatalk

@topalle,
Hi Srinivas.
Here are some thoughts about a 1200gm model with a 1200x200mm wing
(With apologies for mixing metric and imperial units)

1. Wing 48x8", or 2.67sqft area (2400 sqcm)
2. AUW 1200gms, or about 2.67lbs
3. Hence wing loading 16oz/sqft. Should fly like 20-25 glow powered model
4. Stall speed about 15mph/ 22fps/ 7m/s
5. Hence cruise speed say minimum 10m/s (~20mph)
6. A 2400sqcm wing flying at 10m/s will need Coeff of Lift of about 0.85
7. 8" chord at 20mph will have Reynolds Number of about 140,000
8. In 100-200k range of Re, the Clark Y will provide CL of 0.85 at about 5deg AOA, where its drag will be minimum
9. Motor will have to be of around 250watts.

So a model of this size is feasible at 1200gms AUW, using the easy to build flat bottomed Clark Y.
The problem lies elsewhere.
The electronics will weigh about 300gms, leaving 900 for airframe and payload.
600gms payload leaves only 300gms for bare airframe.
So the question is whether a strong enough airframe can be built within this weight.
(I have a 48x8" wood spar vinyl covered Clark Y wing of 2mm depron. It weighs 300gms!)

Not helpful at all  :banghead:.
However a 600gm airframe + 300gm electronics + 300gms payload should not be difficult...
Regards

Actually we are building for SAE India competition.
We are going for a boom design, with balsa fuselage and balsa ribs + cf and balsa spar wings. So, our empty weight (without payload) is coming to 600 grams. Eppler 423 Airfoil.
Building a foam plane is increasing our weight.

The aircarft doesn't have to do a lot. Hand launch, One circle around the flying field, and back to land.

According to the calculations (Spreadsheet attached)
I Have updated the design 1300mm wingspan 200mm chord.
Balsa Built up Wing with Carbon Fiber tubes for spars.

https://drive.google.com/open?id=0BykTeWO1ob_fZy1SLVVMeTAzUVk

We have also build Armin style wing with 5mm depron which has an airfoil close to Clark Y and weighs 220 or so grams.

@topalle,
Good that you are using some CF. Will save weight.
Empty weight 600gms includes electronics?

A few suggestions.

Eppler423 polars are very bad below Re 200k. At 11m/s you'll be below 150k. If you've already built the wing and it's too late to change the section, you need to consider a bit higher airspeed.

For air density and viscosity please use values suited to Indian conditions, ie, over 20 deg C.

Thanks for sharing your project details. I often get queries from SAE participants, but so far they always withdrew when the discussion became technical.
Better check that talking to me doesn't disqualify you! Let me know if I should stop further comments  ;D

No, It doesn't. We are only Prohibited from Getting assistance in building the plane / using ready made parts.

And we haven't built the wing yet.

Yes Empty weight includes 1250 mAH 3 Cell Lipo, and a suitable motor, whose weight we estimated to be 80 grams.

At Re 100k, S1223 is far better than E423.
At Re 200k, it is slightly better...

Meanwhile I'm going to look at whether a highly cambered section is required at all.
Are we going to fly at CL 1.5-2?
Or at 0.6-1? (In level flight / cruise)

S1223 is really difficult to construct. We have a laser cutter, but the trailing edge is very thin

For CL=0.8, an easy to build Clark Y is enough.
Mounted at +1 or +2 degrees to the flat bottom, which I guess will give 4-5deg AOA.

I haven't looked into it yet, but prima facie it seems a high CL Max aerofoil would be needed only if it has to be flown SLOWLY at HIGH AOA.

See PM for my Whatsapp number.

You can actually laser cut s1223. I have laser cut s1221 which is similar to s1223 on 3mm balsa

By the way my 60in wingspan 10in chord FOAM wing weighs 154 grams. So I think foam will weigh less than balsa. (I have not used balsa till date though) Also you may build spare wings if one breaks you can change on the spot.