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Cut Down Blades for Blade CP

Old 10-13-2005, 10:29 PM
  #1  
Keven
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Default Cut Down Blades for Blade CP

I cut a set of blades down an inch and a half and I think it is more stable. It was my best hovering yet. Now I don't have to wait till Monday for my new blades to arrive so I can fly!
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Old 10-14-2005, 09:23 PM
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watt_the?!
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interesting. less inertial moment and remember though, far less thrust.

id suspect that the better stability comes from these factors.

i.e. far less sensitve to throttle inputs due to a reduced coarsness in thrust increments, plus far less cyclic response with the shorter blades.

the tradeoff is agility though.

Nice discovery.

Tim.
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Old 10-14-2005, 11:28 PM
  #3  
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Your new found stability comes from the higher HS that the shorter blades allow. If you go to a better motor put the long blades back on and feel the benefit.
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Old 10-15-2005, 01:31 AM
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Hi Oz,- good to have you here mate!

I wouldve thought that the headspeed wouldnt be much different as it is governed by kv and the gains come from the physical difference.

i understand that the load will be effectively less, thus drawing less amps and therefore resulting in higher V, but that would be more a function of battery capacity.

i suspect that overall the combination of this and the physical differences are in effect.

Tim.
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Old 10-15-2005, 06:04 AM
  #5  
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with the smaller disk area the hs certainy is higher and thats where most of the difference is. The smaller disk also makes the heli "heavier" requiring more control input to get the desired effect. We used to cut the blades in the early days of micros to get the hs up before the availability of lipo and B/L motors. Now if you are prepared to spend on the right motor there is no need to cut the blades.

HS is critical on micros, now many run close to 3000rpm on th ehead to get good results, that wasnt possibe just a little while ago and is certainly not possible with a brushed motor.
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Old 10-15-2005, 06:43 AM
  #6  
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this is an interesting topic that i want to explore.

lets assume that you reduced the rotor diameter by, say 10%.

this means that the reactive force that used to occur at x from the centre now occurs at x-10%, or 0.9x.

so the greatest moment is now 10% less, so effectively less agile and would require greater throws to achieve the same movement as before.

However, the load is reduced by that same airfoil length, rsulting in less thrust to the square and less power needed to the cube. BUT, as you say, teh headspeed then goes up...but how far?

This has to be a function of the battery capacity primarily..i.e. to delive current under a prescribed load and motor kv.

if we reduce blade by 10%, does motorspeed (headspeed) go up by 10%?...keeping in mind that 10% of V x kv = (for 3 lipo) 0.1 x 10x kv

i.e. 1 volt?...i doubt it. perhaps 0.5V.

lets say it goes up by 0.5V. So you have a new headspeed of 5% greater but have 10% less rotor diameter.

elts say you were at 2200rpm. 5% more is now 2310 rpm..

but 2310 at 10% less thrust is actually LESS than the before capability.

As a result, I'd sugeest that headspeed is not the major factor in the new stability.

it is more to do with the force vectors and the new moments.

I think that the thing here is to recognise that an increased headspeed will give you a better ''flying'' heli, as it will react faster, and with the same diameter be more manouvrable. But for a beginner the opposite may be the requirement...although it would also mean it's harder to correct a mistake.

ok thats my math for today...need a rest now.

tim.
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Old 10-15-2005, 06:45 AM
  #7  
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let me just clarify one more thing..the average force now occurs closer to the heli and is, in magnitude, less than before.

i re read my post and the x-10% thingy isnt technically correct, it is more for point laods/force, and in this example it is the distributed force that is important.

tim.
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Old 10-15-2005, 07:37 AM
  #8  
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Tim you think too much
IF all else remains the same, a smaller disk will require less effort to turn. Considering the brushed motors are working harder than they are designed to anyway, its easier for it to spin the disk. Therefore the smaller disk ends up spinning faster providing a more stable heli. As the disk is smaller it doesnt provide the same lift as the bigger disk unless it is spinning faster.
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Old 10-15-2005, 08:01 AM
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think?....my problem is i speak/write what i think....lol...

your explanation seems to me to make every bit of sense, except for the ''more stable'' bit.

im still not convinced that the loss of the extra moments arent dampening the response, thus making it less reactive and therefore more stable.

the problem is here is that to quantify it might not be easy , or would require someone who really gives a ****....(like me maybe.lol...nah, just curious)...

without it being quantified, i like the combination explanation of moments v headspeed giving more ''puffs per second'', which gives it more frequent, but less aggressive lift as the blade passes through the air.
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Old 10-15-2005, 10:02 AM
  #10  
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The stability comes from the gyroscopic effect of the higher head speed.

I cant be any more technical than that, i just know it works, as does Keven.
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Old 10-15-2005, 10:48 AM
  #11  
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now that makes sense....also, i was thinking that, as in real helis, the greater the diameter of the blades, the greater stability...kind of like a tripod with well spread out legs.

the greater the headspeed, the more effective ''spread''.

im happy now....nice.

Tim.
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Old 10-15-2005, 03:28 PM
  #12  
Keven
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I have noticed that the heli does not seem as responsive with the short blades. But that is good for me, being a beginner, I need all the help i can get!
Keven
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Old 10-15-2005, 08:53 PM
  #13  
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Default another 2 cents shortened 1/2"

Great topic. Lift is proportionate to rotor diameter squared x pitch x rpm. So with shorter blades or less pitch you can run higher headspeed and gain gyroscopic stability. Shorter blades become shorter "levers" so you lose some cyclic response, but a one degree change in pitch has more effect at 2100 rpm (for instance) than it does at 1900 so you gain back some response. One step forward, one step back, turn right.

Full length blades with higher headspeed give you the best of both worlds but you're crippled by the stock throttle curve.

Any ideas on "rule of thumb" tip speed range? A Blade CP @ 2100 rpm is ~190 ft/sec (mach .15 ?). Ever seen data on this? Tom
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Old 10-15-2005, 09:03 PM
  #14  
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i think mach 0.3 is the factor from memory...dunno..maybe it was 0.3 less than mach 1..

and im not sure where it is, id assume at any point that meets that mach number.

so it would creep up the blade me thinks.

tim.
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Old 10-15-2005, 09:15 PM
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Mach 0.7 kind of rings a bell for max tip speed for propeller on fixed wing aircraft? Tom
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Old 10-17-2005, 10:33 PM
  #16  
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The "stability" issue due to gyroscopic forces has been debated endlessley on many forums. Basically everyone is arguing over the fact that the rotor disk is either spinning at a slower speed, with more mass (longer blades) OR spinning at a higher speed, but with less mass. Do they equal eachother out? I don't know. I'd like to hear from an expert, but my experiences is that the headspeed is increased, the heli is more agile and somewhat more stable.

Just my 2 cents.
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Old 10-17-2005, 11:16 PM
  #17  
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yeah, its a toughy....as you can see i tried to quantify it....but so many assumptions and variables.

i bet, though, that there is an ''optimum'' headspeed v blade length that is achieved. I think that this is where the mach number comes in.

the problem is, that max lift or best mach number may not be most stable or most agile, particularly when the pitch is changing so much and is dependent on weight, environment etc etc.

I love this debate/topic though.
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Old 10-17-2005, 11:25 PM
  #18  
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it is an interesting topic but not often done anymore. We used to do it when we didnt have much choice in motors or batteries. Now we can just put a suitable motor in and the batteries are no longer an issue. Back then we were limited to speed 300 and 8 cell 750mah sanyo aaa packs.
(hornet and pic days)
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Old 10-18-2005, 05:17 AM
  #19  
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Originally Posted by watt_the?! View Post
i bet, though, that there is an ''optimum'' headspeed v blade length that is achieved.
I agree, but the problem may be that the "ideal" headspeed may be so high that the gyroscopic forces are greater than what the head can sustain. Perhaps gentle hovering and FF may work, but try and throw in some crazy 3D stuff and you might suddenly have a very pretty lawn dart...less the main blades!
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Old 11-02-2005, 07:14 AM
  #20  
rotorman
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Default Been cuttin' blades for 2 months

I'm new to rotary flight and got my Blade in June. Mastered a pretty good hover now, and can even do it in a decent wind. Every time I do a tip-over tho, the ends of the rotor blades get beat to death. Rather than buy the expensive replacement blades, I've been cutting the damaged blades back about 1/4" at a time. After each cut, I weigh the blades on a gram scale (resolution = 0.1 gram) and push small brads into the end of the blade to adjust the weight. I match the blades within 0.1 gram of each other. Apparently the factory inserts weights into the ends of the blades, as they are exposed after I do the cuts. I add a small amount of superglue to the factory weights and my brads after balancing to be sure they don't fly out when the blades are up to speed. If one of the brads comes out at full rotor speed, it could be real dangerous.

Regarding the improved stability some have noticed with shorter blades, the reason is most likely higher rotor speed. A gyroscope's stability is directly related to it's angular momentum, and the angular momentum is the simple product (multiplication) of it's mass times the angular velocity. Since the shortened blade obviously has reduced mass, it's angular velocity must be higher by a greater degree than the mass has been reduced. This makes sense because a shorter blade will have less drag force. For a given propulsion force, the velocity will vary as the inverse square of drag. For example, if propulsion force is constant, and drag force is reduced by half, the velocity increases by a factor of 4.

'Nuff science, time to go trim the blades again.
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Old 11-02-2005, 07:23 AM
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rotorman...welcome aboard...nice post mate...
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Old 11-02-2005, 05:39 PM
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Default Experimental Rotor Blades

Jsut for fun, I'm going to try an experiment with redesigning the rotor blades on my e-flite Blade.

Here's the concept....has anyone ever seen the small "winglets" that are attached to the wing tips on many commercial aircraft (on most Airbus planes and on many 737's)? These winglets are small vertical surfaces attached to wing tip and their puprose is to reduce/eliminate the wing tip vortex that occurs as a wing flies thru the air. These winglets dramatically reduce drag and improve wing efficiency. My neighbor flies for Southwest Airlines and has told me it costs $500k to install the winglets on a 737, and they pay for themselves in 6 months with the fuel savings.

Here's the plan....I'm going to add very small winglets to the rotor blade tips. They will be short (3/4" tall) surfaces attached to the blade tips and oriented vertically. Probably start with a simple plastic disk or washer (3/4" dia) and screw it to the end of the blade. If it works, I'll try shaping it to be more aerodynamic. In theory, these should improve blade efficiency, reduce drag, and reduce motor load. One effect at least might be longer flight time per battery charge. Another effect might be more responsiveness because the rotor blade tip will now be actually flying and lifting, and not wasting energy generating a vortex. That's the theory anyway.
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Old 11-02-2005, 05:43 PM
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what about just twisting the entire blade to produce constant lift across the surface?
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Old 11-03-2005, 07:34 PM
  #24  
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Originally Posted by rotorman View Post
Jsut for fun, I'm going to try an experiment with redesigning the rotor blades on my e-flite Blade.

Here's the concept....has anyone ever seen the small "winglets" that are attached to the wing tips on many commercial aircraft (on most Airbus planes and on many 737's)? These winglets are small vertical surfaces attached to wing tip and their puprose is to reduce/eliminate the wing tip vortex that occurs as a wing flies thru the air. These winglets dramatically reduce drag and improve wing efficiency. My neighbor flies for Southwest Airlines and has told me it costs $500k to install the winglets on a 737, and they pay for themselves in 6 months with the fuel savings.

Here's the plan....I'm going to add very small winglets to the rotor blade tips. They will be short (3/4" tall) surfaces attached to the blade tips and oriented vertically. Probably start with a simple plastic disk or washer (3/4" dia) and screw it to the end of the blade. If it works, I'll try shaping it to be more aerodynamic. In theory, these should improve blade efficiency, reduce drag, and reduce motor load. One effect at least might be longer flight time per battery charge. Another effect might be more responsiveness because the rotor blade tip will now be actually flying and lifting, and not wasting energy generating a vortex. That's the theory anyway.
I'd be curious to see the results however, I'll be the devils advocate here. The winglets (sp) are designed for straight flight. A heli's blades turn in a radius. I would imagine if winglets were applicable for FULL size helis they'd already be in production = they're not
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Old 11-04-2005, 05:10 AM
  #25  
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Default Winglet Experiment

1LO64 - I tend to agree with your assessment. If these winglets aren't on full size rotor blades, there must be a reason. Either they don't provide any benefit, or there are structural/mechanical reasons that make them impractical.

I'm certain the blade tip creates a vortex. As long as there's a pressure differential from the bottom to the top of a blade (creating lift), a vortex will appear at the blade tip. This will happen in either straight or rotary flight. In fixed wing A/C, the tip vortex bleeds a lot of energy from the wing and gets worse as gross weight increases. It's possible the tip vortices on a rotor blade are small and don't reduce efficiency by any significant amount. If true, they simply aren't any benefit.

However, it may be possible a winglet does help efficiency, but isn't practical to add to a full size rotor blade tip for structural reasons. Adding any mass to a blade tip creates huge problems. Full size blades are long and rotate at high angular velocities, so a small mass at the end of the blade creates very large stresses along it's length. A quick calculation shows the problem...add one pound at the tip of a 20 ft long rotor blade, then rotate the blade at 300 rpm. The centrepidal force on the blade increases by 616 lbs. It may be impossible to strengthen a blade enough to support a winglet of a couple lbs at the tip.

It's almost certain engineers have looked at this and problaby know a lot more about it than I do. They've decided not to do it for some reason. OR, it's so obvious that no one has ever considered it, and maybe I should file for a patent. Who knows.
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