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HelicopterMain Discussion › Main blades and tail blades doubt on 700 class
04-08-2015 08:54 PM  35 months agoPost 1
Calil

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Brazil

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Dear mates

I'd like some help if possible please. I'm fitting a fuselage on my t-rex 700 wren 44 and I will change the main blades to 720 blade lenght.

My doubt is which tail blade use with it? 105 or 115 mm?

Any input is welcome.

Kind regards, Calil

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04-08-2015 09:10 PM  35 months agoPost 2
ssmith512

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Indianapolis, IN USA

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If you are using the same tail ratio as the stock Trex 700 either would work just fine. I would choose the blade size between those two that best represents a scale look.

Steve

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04-08-2015 09:10 PM  35 months agoPost 3
AirWolfRC

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115 of course . . . But Why ?

Longer blades will be less efficient.

The only advantage of longer or more blades is a bit more float at the end of an auto.

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04-08-2015 09:14 PM  35 months agoPost 4
Calil

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Thanks for the reply mates!

I want a different blade from the stock align blades and I want to slow down the headspeed so that's the reason for longer ones.

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04-08-2015 09:19 PM  35 months agoPost 5
AirWolfRC

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You can slow down without going to longer blades and be more efficient.

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04-08-2015 09:26 PM  35 months agoPost 6
ssmith512

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Longer blades will be less efficient.
Less efficient lift wise or less efficient power wise?

Steve

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04-08-2015 09:37 PM  35 months agoPost 7
AirWolfRC

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Both . . . efficiency is a matter of performance
. . . like power in vs results out.

With more blade, you have to run at a lower Angle of Attack for the same weight of the heli and that means less efficient conversion of power to lift.

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04-08-2015 10:55 PM  34 months agoPost 8
MattJen

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UK

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The reason you would run longer blades is to decrease the disk loading..

On our camera ships we were at maximum power but still needed lift so the only option was to increase the disc size which mean we could lower the power.

I am running a Trex 700 on a Vario 1/6 fuse, I have lengthened the blades to 770 and my tail blades increased to 115, I have also sped up the tail by changing the gear ratio.. as by changing the size of the blades beyond normal,and have lowered the head speed via gear change the tail will not be as efficient so I have increased the tail speed and tail size.

I am not gonna get into calculations etc, IT WORKS and means I can slow down the head speed to a more scale speed and still be able to lift the scale body with ancillaries which is what us scalers want and maintain a good tail authority.

All The Best

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04-08-2015 11:21 PM  34 months agoPost 9
AirWolfRC

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On our camera ships we were at maximum power but still needed lift so the only option was to increase the disc size which mean we could lower the power.
That doesn't make sense.

You may have only needed to increase max blade pitch.

A blade Angle of Attack of about 4º would be the optimum while still allowing a margin of 100% load increase before blade stall.

Notice I said Angle of Attack and not blade pitch angle.

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04-08-2015 11:24 PM  34 months agoPost 10
MattJen

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We were at maximum power, we needed more pitch as the blades we were using were too small for the weight and thin air we were operating in (10,000feet above sea level) hovering at 10o which was our Max range on 850 blades.At ground level they were fine and we had no problem..

So the only option was to decrease the disc loading by running longer blades 900mm which meant we could get more lift without maxing out the turbine.

In Response to Calil

you would not want to run standard blades on a large scale fuse with turbine,it would look silly, and as he is using a turbine power plant you are adding extra weight, in fact a lot of extra weight.. so what most of us do is stretch the boom to a 800 size which enables us to run longer blades and thus run slower head speed. But by doing this you may decrease the tail authority so the option is to run longer blades and or speed up the gear ratio.

All The Best

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04-08-2015 11:53 PM  34 months agoPost 11
AirWolfRC

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hovering at 10o which was our Max range
I suspected something like that. You should allow up to about 16º blade pitch. You don't have to use it all but it's there if you need it. At 10,000ft you have a 32% reduction in air density.

Why 16º ? Because that's about the stall angle in level air. In hover you have some down-flow that effectively decreases that angle.

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04-08-2015 11:56 PM  34 months agoPost 12
dgoss999

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Does that 16deg allow for Cyclic pitch or do you have to factor that in. ie 12deg collective + 4deg cyclic = max pitch.

I guess it will.. Blade stall is blade stall no matter where the pitch comes from.

DG

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04-09-2015 12:01 AM  34 months agoPost 13
AirWolfRC

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On a 600 size at 1800 RPM, I estimate that with about a 0.5 lb/ft² disk loading, you have about a 4º blade angle due to down wash (0º AoA) Now add the AoA needed at some RPM to create the lift needed.

So . . . yes, that covers cyclic. Max pitch + cyclic = 20º

You can always dial it back on the TX.

Try it. Setup 16º and see how low you can get the RPM and still hover.

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04-09-2015 12:06 AM  34 months agoPost 14
MattJen

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UK

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Why 16º ? Because that's about the stall angle in level air.

If above is true, why is max pitch now moved 20o which is 4o above the original stall angle..

I have always been taught blades stall at 15o

All The Best

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04-09-2015 12:08 AM  34 months agoPost 15
AirWolfRC

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16º collective + 4º cyclic.

The down wash in hover effectively gives you a lower angle for the Angle of Attack. You use that reduction to accomodate cyclic.
I have always been taught blades stall at 15o
Close enough. The NACA 0012 airfoil is 16º. the 0014 shape (typical for symetric blades for helis) is slightly more.

PS
To get the º character, hold the control key while entering 0186 on the number pad.

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04-09-2015 12:12 AM  34 months agoPost 16
MattJen

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UK

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Above ok,
let assume helicopter flying forward at max weight and max pitch 16 I then give full right aileron are you saying the advancing blade doesn't go into a blade stall ?

ps
Your keyboard is US mine is UK that doesn't work on ours LOL the character mapping is different

All The Best

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04-09-2015 12:15 AM  34 months agoPost 17
AirWolfRC

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Depends on forward speed and actual disk loading.

The typical operating point should be at least 4º below the max.

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04-09-2015 12:20 AM  34 months agoPost 18
MattJen

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UK

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When I was learning to fly I was told by instructor the angle of attack constantly changes as the blade moves around the circumference of the rotor disk. Other factors affecting angle of attack, over which the pilot has little control, are blade flapping, blade flexing, and gusty wind or turbulent air conditions.

All The Best

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04-09-2015 12:29 AM  34 months agoPost 19
AirWolfRC

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All true.
. . . but it's a bit like flying in extreme turbulence
. . . you fly attitude . . . pick a mean position and maintain it.

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04-09-2015 12:30 AM  34 months agoPost 20
MattJen

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UK

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interesting discussion.

but we are talking about models not full size..

Our small scale models the power to weight ratio is so big its really is irrelevant. Until you get into big scale where I operate..

I suspected something like that. You should allow up to about 16º blade pitch.

When my pitch is maxed out at 10o I cannot add anything further as the washout pins are hitting the swash.

How many modellers think about AOA ? most consider pitch.. and as you said 16o is ideal but in scale you got no chance of that range on a 20 kilo+ model.. the blades would stall and the turbine would over heat and shut down..

All The Best

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HelicopterMain Discussion › Main blades and tail blades doubt on 700 class
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