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Patrick

Tue Jul 15 2014, 01:05AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok so for a first start, lets agree to a modded heli blade. With a twist, long length, short chord.
Ash, do you have access to and knowledge of the CAD, and its ways?

what diamerter should we start with, ill need to start thinking about fabrication.

EDIT: on multirotor props they do taper the tip greatly, i wonder if that increases disc loading.
link:

im thinking of a 0010 or 0012 airfoil.

Ash Small

Tue Jul 15 2014, 09:36AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think we should be aiming for a more conventional helicopter blade. These are designed for a lower disc loading than the props you are currently using, which are much closer to airplane props, which are designed for high disc loading.

Helicopter props are designed for low disc loading and low drag, which, unsurprisingly, is exactly what we want.

I'd go for the thinnest section that has sufficient strength. Something like a NACA 0010 sounds like it's 'in the ballpark'. I did link to some typical heli blade cross sections above somewhere.

Are you using a foam core and carbon for the ducts that you are currently using?

A thinner section with thinner width will have lower drag, so you can spin it faster. slower props with greater pitch may run into stall problems, I think.

EDIT: Just remembered that carbon only has tensile strength, no compressive strength. The lower surface of the blade will be in tension, the upper surface will be in compression. A bit of thought will be required here.

EDIT EDIT: Actually, I've just been googling, and apparently it is strong in both tension and compression, but weak in 'bending'. I need to read a bit more. I am aware that it does have limitations, though.

EDIT EDIT EDIT: There seems to be a lot of contradictory opinion on google.

"Carbon fiber is very strong when stretched or bent, but weak when compressed or exposed to high shock (e.g. a carbon fiber bar is extremely difficult to bend, but will crack easily if hit with a hammer)." Apparently from Wikipedia

There does seem to be some concensus that it is twice as strong in tension than in compression, though, which is pretty much what I thought in the first place.

BigBad

Tue Jul 15 2014, 06:29PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Actually for a heli blade i would have thought that along most of its length it's more or less only in tension, due to the centripetal forces.

Dr. Slack

Tue Jul 15 2014, 06:35PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

for making measurements, why not just buy a heli blade?

The root hinges on a heli blade seem to suggest that it's not designed to need to take any bending moment at all, but operates in centripetal tension

Patrick

Tue Jul 15 2014, 08:53PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

yes compression and striking are poor conditions for CF to survive. (a problem for the US F-22 when a 20mm russian API explosive round comes in versus titanium for example.)

to mitigate this, and improve safety, i could use balsa or PU foam core, then 5.7 osy CF, with 4 oz S2 FG. torsional straength still worries me though. (fiber glass, especially S2, is very survivable, even though its the bastard child of CF, which is high tech and glamorous.)

i was thinking of a slightly wider chord and steeper pitch in the root, then a flat-bottom narrow chord, low pitch at teh tip end.

EDIT: i just wish there was a simple freeware program or app that would given and aproximation for NACA profiles.
heres the ugly math, but its looks well expalianed :

Ash Small

Wed Jul 16 2014, 02:13PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, yes, there are obviously centripetal forces

I was just pointing out the (possibly) less obvious forces. For every action there is an equal and opposite reaction (Newton's third law), so there is an upward force acting on the blade that is equal to the downward force acting on the accelerated air. which is equal to the mass of the craft while hovering, and is greater when ascending. This force is obviously divided by the number of blades.

This results in a 'bending moment'(?) acting on the prop that puts the underside in tension and the topside in compression, which needs to be taken into consideration when designing the prop. I assume Patrick is somewhat familiar with the directional properties of carbon, and how to best lay up carbon to achieve the required directional strength. (for example, carbon masts on yachts tend to use unidirectional carbon and utilise a 'vertical' cylinder, with unidirectional carbon running in alternating spirals around this, to deal with the bending and compressive forces acting on the mast. Various other 'weaves' are available, including diagonal and crowsfoot, to name two, depending on the various forces to which the part is subjected. As Patrick points out, a 'composite' including 'S' glass, and maybe even kevlar may be appropiate in some cases. The centripetal forces can be dealt with using longitudinal unidirectional carbon, but the other forces, bending vertically due to 'lift' and bending 'backwards' due to drag 'may' require a bit more thought.

Patrick wrote ...

i was thinking of a slightly wider chord and steeper pitch in the root, then a flat-bottom narrow chord, low pitch at teh tip end.

Sounds reasonable, but I wouldn't use as much pitch near the root as the maths suggests is required, for the previously mentioned reasons of stall and drag. A wider chord at the root would add strength here, where it is most needed, but there isn't a lot of lift generated at the root anyway.

EDIT: Balsa way well prove easier to work with than foam core in this application

Patrick

Wed Jul 16 2014, 06:29PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking of a planform, but i dont know if the flight forces will twist from my low drag - low pitch tip to being higher pitch in rotation. I dont think any of us have a strobe light and high speed camera ?

Ash Small

Wed Jul 16 2014, 08:19PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

im thinking of a planform, but i dont know if the flight forces will twist from my low drag - low pitch tip to being higher pitch in rotation. I dont think any of us have a strobe light and high speed camera ?

If we can calculate the forces involved, surely we can apply this force to the blade and see how much it twists, or even calculate how much it will twist?

This stuff is pretty basic mechanics (forces on a beam, etc. The first subject covered in most mechanics syllabi)

EDIT: We should factor in a safety margin of three, anyway.

Patrick

Wed Jul 16 2014, 09:57PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

as long as you can help me through it, my integral and differential calculus is always questionalble, and in this link :

if we have all the pieces we can start on some functions to apply over surfaces and lengths.

i need force diagrams, of the airfoil section, which show the forces, as i vary the length from tip to root.

Ash Small

Thu Jul 17 2014, 01:09PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm still reading through your link, but if you look at the sections of heli blade I posted on the previous page, they are symmetrical, without the airofoil shape and rely on the pitch of the prop to accelerate the column of air downwards, much the same as ailerons on the trailing edge of a wing.

It is this pitch angle that varies from root to tip on a prop, to give the air the same velocity from root to tip even though the velocity of the prop varies from root to tip.

You are probably aware of this, but it's not clear from your comments.

While an aerofoil section may have advantages, the thickness of the section (naca profile) would need to taper from root to tip in order to maintain the same lift from root to tip, which I don't think would be practical as it would adversely affect drag.

The props you have are concave on the lower surface and convex on the upper surface so they do resemble a naca profile more than a conventional heli pfofile.

I think we need to consider which shape has the least drag, as we need to reduce drag as much as possible in order to optimise the advantages of low disc loading.

I'd assume that the conventional helicopter blade probably has the greatest strength and least drag, compared to the 'conventional' prop design that is optimised for much higher disc loading.

I think we should be aiming to copy a conventional helicopter blade section rather than going for a naca profile as it will increase strength and therefore reduce drag, by enabling the use of a thinner section, I think.

We do need to minimize drag in order to maximize the advantages of low disc loading.

I'll continue reading the link later.

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