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HomeScale✈️AircraftScale HeliScale Helicopter Main Discussion › Dynamics of the 5 bladed head
12-20-2010 06:14 PM  9 years ago
seattle_helo

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Seattle, WA USA

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I'll be the first to admit that the math and true dynamics behind this stuff are all way beyond me. But I can share some interesting empirical data that I've gathered over the years talking with other pilots. I've got a Vario 500 with five 800mm blades and an OF head powered by a PHT3. The head is big so the total rotor span is just under 2M. Many of these birds have been built and flown with a two blade, flybar head. It's always interesting for me to compare engine temps, fuel flow and the like to see the differences. Not to mention collective settings for a given stage of flight. For instance, when I am cruising in FFF I need to keep my collective down at around +1 to +2 otherwise that thing would just ascend out of sight. The OF blades on this machine are just lift monsters and with the combined transitional lift in forward flight you got to keep the collective down to maintain level flight at speed. This, contrasted with the flybar head setup on a similar machine (not the same machine, but similar) where you would still reduce collective but not nearly as much. Also interesting is that my particular 500 is quite heavy at +40lbs and it still likes to float. Surprisingly some of the engine loads aren't dramatically different. The EGT's that I've been able to compare with others are similar in the 550 to 600ºC range. Not a super narrow range but close. The fuel flow, however, is different. I'll burn about 2100mL in ten minutes where as one pilot with a two blade head burned only 1600mL in twelve minutes. Torque load also increases greatly with the five blade head. I'm running a four blade TR on my to keep the bird scale but it too adds to the load substantially.

I think the profile of the blades has a lot to do with all this as well. A semi-symmetrical blade of a given length will produce more lift for us than just using normal 3D symmetrical blades. But the drag is increased as well. I do wish I understood this stuff better.
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12-21-2010 01:02 AM  9 years ago
permagrin2010

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Surrey

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...Surprisingly some of the engine loads aren't dramatically different. The EGT's that I've been able to compare with others are similar in the 550 to 600ºC range. Not a super narrow range but close. The fuel flow, however, is different. I'll burn about 2100mL in ten minutes where as one pilot with a two blade head burned only 1600mL in twelve minutes. Torque load also increases greatly with the five blade head...
Substantiates a lot from my earlier post.

The energy required to lift a 4O lb heli by hand will be the same if it has a 2 blade head or a 5 blade head. It is the same for the rotor. A two blade rotor must generated the same lift to do so as the 5 blade rotor. The difference in energy each will consume will be related more to the head speed then other factors.

One significance difference in energy required to do so with the 5 blade head, compared to the 2 blade head, will be the amount of parasitic drag presented by the 5 blade compared to that of the 2 blade head. This would account for some of the difference in fuel burn. That being said, one could make a better analysis if you could give us the rotor speed the 2 blade head is flying at and also the rotor speed of the 5 blade head.

I am assuming they both are using the same blades...at least of the same length.

I have always found it very easy to make rotor head efficiency, or blade efficiency, tests using electric power. This is because the measurement of watts (power) is very easy to monitor in an electric powered system. And today with systems like those available form Eagle Tree Systems etc. we can now make such analysis so much easier then we did 10 and 15 years ago.
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12-22-2010 10:36 PM  9 years ago
landinggearup

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Beaumont TX

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Yep,data logging is fantastic. It's a shame some don't see the use of it for tuning.

Update with some real numbers.

trex 600, 11:170 gear ratio, 600M motor, 600mm 3G blades, 2650mah 6s2p, shy hunter penta head.

1500 rpm governed w CC ICE 100

EDITED________________________
613 Watts at 1500 0 pitch
765 Watts to hover using ~4degrees pitch.
______________________________

12 degrees will over peak the motor numbers @ 2300 watts and just blow out the tail in the last little bit, but is climbing rapidly (don't need this much)

reduced the curve to ~10 pitch. tail holds and 1885 watts peak.

Again I couldn't be happier so far power wise, but I still have to remount the body and that can change things. I may need to bump up the headspeed a small amount to help with the tail. I'm still playing with headspeeds vs pitch.

I have flown it in forward flight three times and still have some bob when the head is loaded in a turn. I have a SK720 sitting on the shelf waiting for an sbus reciever to set up. Gonna see if it handles the bob any better or is it's an airframe problem.

Les
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12-22-2010 10:40 PM  9 years ago
Frank Bostwick

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Cincinnati Ohio

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Thats good stuff right there!RIP ROMAN
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12-22-2010 10:42 PM  9 years ago
landinggearup

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Beaumont TX

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Also I had someone at work point out that if I drive a bus down the road and it makes X amount of drag, the drag does not square if I drive two busses down the road. It doubles!

The drag increases on the square with speed and linear with count, otherwise you would get much better gas mileage when you were the only car on the road!

Les
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12-23-2010 01:56 AM  9 years ago
spy29

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USA

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>> 2 buses driving down the road...each has the same drag so ..hmmm really? really?
that is only true of the buses where "side by side" the drag is doubled..of course each bus is on its on path and traveling at the same speed.USING ITS OWN POWER. (common sense)

wings on each side of a planes fuselage is SIMILAR >>NOT true for HELICOPTER BLADES

Blades are very much like INLINE wings BUTT,,they operate as ONE SINGLE DISC WHEN SPINNING ,, EACH BLADE CREATING lift and drag PLUS also Catching each others "Wake" or vortices.
and yes they are exponential in drag. NOT DOUBLED
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12-23-2010 04:53 AM  9 years ago
Santiago P

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Dayton

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Please, Do not try this at home, I am a trained proffesional.
Hopefully I got you all laughing; while I am going to TRY to shed a little light here.

I will not use any math, so y’all be gentle with me

The analogy of the biplane is not too far from the case here as almost all biplanes have aero interactions between both wings being too close to each other. Yes, is not the same as one blade flying into the wake of a leading blade, but in a rotor that is very minimal and not the main cause of the additional drag.

An example
You all know in a 10lbs heli, with two blade rotor, each blade is carrying 5lbs
A 10lbs heli with 5 blade rotor, each blade is lifting 2lbs

In the case of the 2 blade rotor, at 5lbs of lift, the blade may see (just an example) 1.5lb of drag force, or 3lbs total drag.
Here is where it gets interesting, in the 5 blade rotor where each blade is only lifting 2 lbs, the drag may still be as high as 1lb/blade (in comparison with the 1st case) even if using shorter, thinner smaller blades at a lower angle of attack. Total of 5lbs of drag force.

Why?

If any of you is familiar with a lift to drag diagram u will see that all airfoils have a minimum amount of drag even at zero lift (for symmetrical blades).Drag is never zero. Also, lift to drag diagrams are published for 2-dimensional cases, that is they don’t account for the span of the wing or blade. When you add the span, you have a more realistic picture, and two forms of drag, the first called induced drag, which is the drag directly related to lift creation (and leads to the formation of those cool swirls the tips make while splitting nitro smoke). The 2nd is called parasite or skin drag, related to the friction of the blade going through the air.

To optimize the multiblade setup you can choose thinner airfoils (less draggy), with smaller chord blades (less skin friction) and even shorter span blades, all to minimize drag at the desire operating point (rpm, power…)

In the end you will still have more drag out of the higher blade count system simply because of the added drag due to higher skin friction (viscous effects)

Note,
Peter Wales test is as accurate as it gets if he used the same exact blade type in each rotor, and just changed the count. It also tells you that many of the blades and airfoils we are using are pretty efficient in creating lift (each blade created less induce drag), and the most significant form of drag he measured was the friction drag on each blade.

Chago
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12-23-2010 05:57 AM  9 years ago
spy29

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USA

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Thank you..
i have posted on a previous post that headspeed and PITCH is not the only thing needed.. and that working on the DRAG ,( induce, parasite or friction ) will be required .,, shortening both blade width and blade length of the 5 blade the match the 2 blade requirements and yes aspect ratio too.

sorry the 2-5 bus reference is BAD because BUSSES do not produce lift. AND HAVE NOTHING TO DO WITH THIS DISCUSSION...ROTORS ARE like 5 wings in front of each other.( not biplane which are on top of each other and staggered too) again not linear.
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12-23-2010 06:03 AM  9 years ago
landinggearup

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Beaumont TX

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Sorry but busses following each other would produce LESS drag. (drafting)

What you are saying is that the busses that follow each other will have 2 times the drag of two side by side(2x vs 2^2x.)

BS!

Otherwise we'd all drive side by side to save gas.
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12-23-2010 06:49 AM  9 years ago
Santiago P

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Dayton

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Guys,

In trying to explain a measurable lift-drag model, one has to assume a steady state condition, just like in a helicopter hovering steady, out of ground effect and no wind, with most of the rotor producing lift. In such condition, the blades see little or no wake from the blade in front, as the lift action and the momentum created by the lift produced moves the unsteady flow downwards. (Think of a propeller moving foward) Such state applies well also in a fwd flight and in a climb.

This is why a straight down decend in no wind is dangerous as the heli loose lift when it flies directly into the wake it created, and therefore recomemded never decend straight down, but always moving foward, sideways or into the wind.

The two buses drafting demonstrates the principle of form drag reduction as their combined shape produces a body with total drag smaller than their individual sums.
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12-23-2010 06:58 AM  9 years ago
landinggearup

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Beaumont TX

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Yep, that's true.

When at 0 pitch on the ground the blades really complain about the wake they fly thru. Sounds a like the 5 bladed garret turboprop in beta.

If multiblade heads and props really increased drag on the square, no one would use them as the losses would be too great.

I guess I'm gonna have to put the two bladed head back on just to take some power numbers off it to post.

Les
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12-23-2010 07:27 AM  9 years ago
landinggearup

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Beaumont TX

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have posted on a previous post that headspeed and PITCH is not the only thing needed.. and that working on the DRAG ,( induce, parasite or friction ) will be required .,, shortening both blade width and blade length of the 5 blade the match the 2 blade requirements and yes aspect ratio too.

-------------------------------------------

I'm thinking about giving up the RC helis as my vision is failing.

Just this morning I thought the heli I was flying had 5, 600mm blades, and a stock 600M motor. I must have halucinated the 6 minutes I flew around. I even think I saw 35 amps hover draw when I reviewed the flight on the computer.

I know this couldn't be true as I've been told that the 5 bladed head would have 6.25 times more drag and that means it would require 15 time more power to fly (you know power required rises on the cube.)

Must be why all the multiblade scalers take thier helis out to be static judged and then put them back up on the table for the rest of the day.

Wish I could keep flying them though as it was COOL!

BTW... If I reduce the span 10% and the cord 10% what's the new aspect ratio?
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12-23-2010 07:37 AM  9 years ago
spy29

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USA

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why dont you show us LES?

heres what you need. AR = s^2 / A = s^2 / (s * c) = s / c

now reduce a 700mm blade length and width 10%
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12-23-2010 05:33 PM  9 years ago
landinggearup

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Beaumont TX

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Since it's a RATIO, it remains the same with a 10% reduction of both dimensions.

You know that!

In the interest of making this thread productive again, I'll stick to posting my results with the helicopter, and drop the rhetoric. I think the numbers speak well enough. If there are specific numbers anyone wants just ask.

I'm on the road for a few days, so no more flights this week.

Les
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12-23-2010 06:20 PM  9 years ago
Peter Wales

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Orlando Fl

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I would like to know

Experiment 1
1) How much current used on 2 blade head, hovering
2) how much current used on 4 blade head, same blades, hovering
3) How much current used on 2 blade head carrying extra load
4) How much current used on 4 blade head carrying same load

Experiment 2
1) How much current used on 2 blade head to hover
2) rpm change used on 4 blade head to use same current to hover
3) Blade change at original rpm to use same current to hover

The last one will have a lot of work involved as it is unlikely you can get the same blade width and profile in shorter lengths. You could always go the extra mile and start sawing pieces off the blades until you get the same current consumption

That information will answer a lot of questions concerning multi blade heads and as it is a 4 blade head it will be easier to get a choice of blades than a 5 blade head
Peter Wales
http://scalehelicopters.org
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12-24-2010 07:46 AM  9 years ago
landinggearup

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Beaumont TX

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I think I could set up most of those tests, even doing the two bladed flight on a four bladed head so as to not change much.

One of the reasons I checked the wattage at 0 pitch was to compare the profile drag with additional blades. It looks like the 150ish watt difference is the real cost to lift the heli to hover in ground effect.

I don't have either a 2 or 4 bladed head, but might pick up a 4 bladed just to run these tests. It's intresting stuff.

Les
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12-24-2010 06:56 PM  9 years ago
martn1sw

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Hungary

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Hi ! I finished my new 10T pinion.

And I build the new swash my 5 blade system.

Martin
My Galery: www.rcpilot.hu
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12-26-2010 06:49 AM  9 years ago
landinggearup

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Beaumont TX

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OK, I bit.

Ordered a 3 and a 4 blade head to match my 5. Should get to a couple of tests the fist couple weeks of January.

I don't want to speend too long on it as I'd like to have my 500E up by the flyin season.

Les
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01-18-2011 02:46 PM  9 years ago
coptercptn

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Mesa AZ. USA

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Landingearup, Which align mototr are you running? (KV) and where did you get the 11 tooth pinion? I am setting up a similar bird (except using the 4 blade tail) I really like the numbers you are seeing! I have an nue setup in the wings but if I can by with the stock setup all the better. (And just installed the 720 skookum)Home of the "Sea Cobra".....
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01-19-2011 06:27 AM  9 years ago
landinggearup

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Beaumont TX

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http://www.readyheli.com/RevCo-Hard...ze_p_33794.html

I am currently flying the 12 tooth as the 11 was on the edge of the rpm range I wanted, but may go back to the 11.

Using the 600M motor.

I have put about 10 more flights w/o body and am still tuning the SK720.

I'm about ready to put it in the body, but have just got the other heads in to test power requirements before I do.

While I had hoped to be done with that by now, I'm just now gettig the heads.

Les
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HomeScale✈️AircraftScale HeliScale Helicopter Main Discussion › Dynamics of the 5 bladed head
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