Helicopter airfoil sanding

I have just purchased two helicopter kits from Apogee and was wondering what the easiest way to sand a good airfoil would be.

 

[shockwaveriderz]

the hd blades should actually be twisted, but if you just want a clark Y shape:

https://www.charlesriverrc.org/articles/construction/markdrela_airfoilshaping.pdf

https://www.wooshrocketry.org/Tips/Kevin Wickart/tips2.htm

 

[rbeckey]

I just built an Apogee Heliroc. I used a sanding block and eyeballed it. In my opinion, the blades look pretty good. I guess I'll find out in the spring.

I used airplane dope and tissue on my blades. They are very fragile and I at least want it to survive the trip to the range.

 

[powderburner]

At the extremely low Reynold's numbers where modroc helicopter blades operate, I have not found any overwhelming research or evidence whether airfoils do you any good at all. It looks like you may do just as well with a thin flat plate. After all, you are down in the speed/size range of insects . . .

 

[shockwaveriderz]

powder: quit posting techincal information will you please? heheheheh

 

[kkooch]

Originally posted by powderburner
At the extremely low Reynold's numbers where modroc helicopter blades operate, I have not found any overwhelming research or evidence whether airfoils do you any good at all. It looks like you may do just as well with a thin flat plate. After all, you are down in the speed/size range of insects . . .

Keep building those flat plates..

Kooch

 

[Micromeister]

Originally posted by rbeckey
I just built an Apogee Heliroc. I used a sanding block and eyeballed it. In my opinion, the blades look pretty good. I guess I'll find out in the spring.

I used airplane dope and tissue on my blades. They are very fragile and I at least want it to survive the trip to the range.

Rbeckey:
Mass is one of the biggest enemies of Helicopter models, actually autogyro's. Adding a single coat of clear airplane dope isn't a bad idea as in the dry film state it is almost weighless, tissuing however will add unnecessary weight which will certainly lower your models duration potential. Stricktly up to you but in all the years i've flown H/D I don't remember very many broken rotors.. loads of snapped fins, but the rotors are usually laying flat against the model body, very little chance of damage. Are the rotors fragile? well thats true in that the rotors are very thin, but you'd be surprised at their strength. I have a few 13mm & 18mm H/D models I use as qualified flight and demo models that have been flying, recovered by youngsters, and so on for quite a few years now. All have a single coat of clear dope on the rotors. All those energetic recovery crews have knocked off a fin or two but to date never even chipped a rotor
Look at the date on the pic below, this model still flys today at least 4 or 5 times a year on 1/2A and A motors.

Sanding airfoils is a Gas!, Lots of fun if you like rootcanals . I use a 12" x 24" x 1/2" plate glass plate, tape the rotor or wing to the glass with D/F low tack masking tape. After marking the high spot location I use a 1" x 1" x 12" balsa sanding block with various grit sandpapers 80 to 600 to shape than finish sand the airfoils and cambers if used. The work goes fairly fast, but remember to sand in one direction only, much like sharpening a knife blade.

Low reynolds numbers or not airfoiling helps bring the rotors up to speed much quicker than flat plates.

 

[illini]

Originally posted by powderburner
At the extremely low Reynold's numbers where modroc helicopter blades operate, I have not found any overwhelming research or evidence whether airfoils do you any good at all. It looks like you may do just as well with a thin flat plate. After all, you are down in the speed/size range of insects . . .

Very good point. I would tend to think flat plates with flaps on the outer part of the rotor blade would be more effective at generating lift than airfoils at those Reynolds numbers. Can't say airfoils hurt, though. Another key is probably balance. Make sure all rotor blades are the same mass...something that would be more difficult to ensure if airfoiling.

 

[Zack Lau]

Airplanes designed for slow flight often have airfoils that resemble curved plates

https://ourworld.compuserve.com/homepages/JHopkinson/SlowFlyer.HTM

Airfoil GOE 417A This appears to be an airfoil optimized for wind turbines
https://www.aae.uiuc.edu/m-selig/uiuc_lsat/lsat_5bulletin.html

There are two techniques for obtaining similar airfoils.
One is to wrap wet balsa around PVC pipe--I learned this from Bob Kaplow's Rotacrock
Another is to slit the balsa lightly, crack it, and then glue it while using a underneath dowel to set the angle/height.

I've found that balsa will often vary in density across the sheet--the most consistent blades are obtained from a long strip of balsa cut into segments, not a 3x12 inch sheet cut into 1 inch strips. You may want to build both kits at once and try to sort out the blades into heavy/light stacks. But, you can also get lucky--the first Apogee Heliroc I built had blades that balanced pretty good out of the sheet Tim sent me.

 

[shockwaveriderz]

very good info Zack....yes thin curved flat plates seem to be optimal at the very low Rn that HD models operate at...

I have used the same wet balsa technique and taken it a step further....I take 3/4" or 1" diameer wood dowels and wrap them in a thin teflon sheet....I then take balsa blades that I have allowed to soak for DAYS and then I can wrap then around the wood dowel, this provides both undercamber and twist..I then hold all of this in place with heat shrink tape and place the assembly in my kitchen oven for 4 hrs.... I have also used finishing resin with jap tissue on these blades for added strength and they hold their shape very well.....

Its still an experimental technique but it looks promising....


This will result in a twisted cambered blade(obviously you should sand any airfoil into the blade prior to all of this)....

 

[raw9jr]

Powder,

While I won't claim to fully understand the physics of your post, I do very much like the fact that it supports LESS sanding on the helirocs! Anyt other tedious labor saving physics you have up your sleeve is very welcome!

Bob

 

[powderburner]

I have already recited about as much as I remember about Reynold's numbers without digging out an old textbook and 're-learning'

And I too am a huge fan of less (or no) work, especially at home if I can get away with it.

Reader's digest version: Reynold's number (Rn) is a reference index that tells you whether you are dealing with a 300-foot-long SST (Rn=10^7, 10^8, etc), a 30-foot-long transonic aircraft, a 3-foot long model airplane (Rn=10^2? =10^3?), or a 3-inch-long model rocket boost glider wing (Rn=10?).
Air behaves ***very*** differently in each speed regime.
Rn is influenced by the length of the surface exposed to airflow, by the velocity of the object, and by a bunch of other stuff. If you drop from a model airplane wing chord of 6 inches down to a model rocket helicopter blade chord of 1/2 inch, that is an order-of-magnitude change in length. If you drop from a model airplane speed of say 25 to 50 feet per second (15 to 30 mph) down to a model rocket helicopter blade local velocity of say 2 to 5 feet per second, that is an order-of-magnitude change in speed. Put those together and you have a two-orders-of-magnitude change, which generally threatens to put you into a different area of aerodynamics behavior.

Much of our boost-glide, rocket-glide, and heliroc performance is literally down in the speed regime with the larger insects. They have flat, uncambered wings. Many insects have relatively 'rough' wing surfaces due to the veins and hairs that make up their structure, leading me to suspect that rough, unfinished balsa does not cause much of a performance loss in our rocket wings and rotors. Model airplanes are an entirely different affair.