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Old Apr 02, 2013, 04:17 PM
Rangers Lead the Way
Joined Mar 2010
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Correct head speeds on scale heads

I am working on a Trex 450 ~> MD500 with 5-blade scale head. I am relatively new to helis. My question is: how to determine the correct head speed for the scale rotor. This would not even be a question, if I were not seeing repeated comments to the effect that the scale head should be turning at something like 2/3 (2200rpm) of the speed of the stock head ~ 3,000. The most common reason given is that the head will tear itself apart.

This makes no sense to me. The centrifugal forces on each of the blades and grips of the multiblade head are exactly the same as those on the 2-blade head at the same head speed. And the requirement for an adequate head speed (stability and responsiveness) still applies. So I am not sure where this "conventional wisdom" comes from. Is there any mechanical reason why the scale head cannot operate at the same speed as the stock head?

Key assumptions on my part are:
1. The scale model will be flown in a scale manner: loops and rolls, perhaps, but no 3D flight.
2. I will be using an FBL controller (ZYX-S)
3. I have ordered plastic scale blades as well as standard 325s
4. I will be running a HDX 4-blade tail
5. I'm planning an analog servo set (HS 65MG) all around. I have a S9257 but may not use it because it cannot run on 6V: I will likely use a LiFe battery to power the RX as I plan nav lights as well.

Thanks
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Old Apr 02, 2013, 04:30 PM
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thenodemaster's Avatar
Hemet, CA
Joined Aug 2010
516 Posts
Quote:
Originally Posted by TTRotary View Post
I am working on a Trex 450 ~> MD500 with 5-blade scale head. I am relatively new to helis. My question is: how to determine the correct head speed for the scale rotor. This would not even be a question, if I were not seeing repeated comments to the effect that the scale head should be turning at something like 2/3 (2200rpm) of the speed of the stock head ~ 3,000. The most common reason given is that the head will tear itself apart.

This makes no sense to me. The centrifugal forces on each of the blades and grips of the multiblade head are exactly the same as those on the 2-blade head at the same head speed. And the requirement for an adequate head speed (stability and responsiveness) still applies. So I am not sure where this "conventional wisdom" comes from. Is there any mechanical reason why the scale head cannot operate at the same speed as the stock head?

Key assumptions on my part are:
1. The scale model will be flown in a scale manner: loops and rolls, perhaps, but no 3D flight.
2. I will be using an FBL controller (ZYX-S)
3. I have ordered plastic scale blades as well as standard 325s
4. I will be running a 4-blade tail
5. I'm planning an analog servo set (HS 65MG) all around. I have a S9257 but may not use it because it cannot run on 6V: I will likely use a LiFe battery to power the RX as I plan nav lights as well.

Thanks
The reason for the lower head speed is indeed because of the added load of additional blades on the rotor head. While it may be balanced, the amount of force on the rotor head goes up with each blade that is being spun. You also have the advantage of more blades = more lift at slower head speed. And with those added blades, motor/speed control power consumption goes up. add to that the addition blades on the scale tail, and, you can see why you need to slow things down.
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Old Apr 02, 2013, 06:05 PM
Love my scale Whirlybirds
Keyrigger's Avatar
Mississauga, Ont., Can.
Joined Sep 2009
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Hundreds before you have tried everything under the sun and the general consensus is that a 450 5 blade head will give you the best performance from 1800 to 2000 rpm with or without scale narrow cord blades. A 450, at the best of times, is a light, twitchy helicopter that responds in a millisecond to collective inputs, so if you have an ultra high head speed per stock motor and gearing, the extra lift and torque will give you instant response and not what you want. It will torque around on you as you input collective movements like you did with the stock 2 blade head. Also, a number of scale blades are not designed to be run at over 2000 rpm (or there about) and spinning them at 3000 rpm could have serious consequences. I run a four blade head with scale blades on an Agusta A109 and it has lots of lift, pleasant to fly, and looks pretty scale with the narrow cord blades on it. It's running around 1900 to 2000 rpm.

If you want to strike out on your own and visit a land of discovery running high head speeds on a five blade head, remember, you have been cautioned not to. Take care.

Don
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Old Apr 02, 2013, 07:17 PM
Rangers Lead the Way
Joined Mar 2010
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OK thanks, those reasons (especially the plastic scale blades not being designed for that RPM) make sense to me. As it so happens, I just purchased a 2700 Kv motor with better sustained amp capacity than the 430BL thinking I'd go to 4S. Looks like that will put me right at 2000 head speed on 3S at the target lift of 42-45oz.

The low head speed just did not make sense to me given the typical emphasis among heli guys on high head speeds, but I guess that's really more the 3D way of thinking.
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Old Apr 02, 2013, 07:25 PM
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Marysville, Ca., US
Joined Jan 2007
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When I started in this hobby a mere six years ago, the normal head-speed of a T-Rex 450 SE was right around 2400 to 2600 RPM. If you were using wooden blades recommendation was for 2000 to not more than 2400 RPM. Those helis did not have any problems with stability. The only reason 450 head-speeds are up around 3000 RPM is to be able to do more extreme 3D moves, and to provide instant, insane response. Exactly the opposite of what you are trying to accomplish with a scale bird. Much like what Keyrigger said, my 3-bladed 450 for an AS-350 is turning between 1600 and 1800 RPM, which gives a nice, smooth flight, even without FBL controllers, and just looks and sounds much more realistic than it would if it was running 2500 RPM and over. To get that head-speed down I didn't use the typical Align 3550 kv motor, instead running a Scorpion 2250 kv, and an eleven tooth pinion.
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Old Apr 02, 2013, 07:33 PM
Rangers Lead the Way
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Thanks. I wonder if that lower level is not about where I am now. I am flying in normal mode, stock everything, with liftoff at 50% thro at about 60% pitc. I'm using the ALign 325 yellow and black carbon blades. The heli flies very mellow and stable.

EDIT - I revised all this over the weekend. Both my 450s were bought used, already set up and both swashes were set up incorrectly. One too low, the other too high. No wonder people have problems. Now the birdie lifts off in Normal mode just past 1/2 stick with Pitc set at 50% at midstick and Thro at 75% on one and 80% on the other. I'm learning...
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Old Apr 07, 2013, 08:12 AM
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Canada, ON, Windsor
Joined Apr 2008
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Sorry to jump in here with this question, but it's related and one that has been hanging me up for quite a while.

I have a 450 size MD500 kit sitting in pieces for a while now trying to figure out the solution to the lower head speed required by the 5-blade head since as you reduce main rotor RPM you also cut the tail, just when you need it the most.

NOTE: The reason you want to cut the main rotor RPM with multi-blade heads are many. First is due to what is called "Disc Loading". You will now share the total aircraft weight over many blade and if spun at the same speed the reaction times are MUCH quicker. This can make it anywhere from uncomfortable to unflyable. From what I gather the smaller chord and flexible scale blades were intended to reduce that twitchiness.

Two other factors come into play. At faster speeds you also encounter increased gyroscopic forces. Those, combined with the loss of the original flybar characteristics means ALL of the forces involved in flying the rotor head must be countered and controlled by the servos alone. In the original head the servos were directly connected to the flybar and used the paddles on the ends as aerodynamic levers to then control the much higher loads of the main blades. The flybar served two purposes: the one just described and also as a damper to counter SHORT DURATION excursions from either your inputs or from outside the system, (gust loads).

You obviously lose that with multi-blade setups so need servos that are strong enough for the job. Difficult to accomplish with a 450 size machine.

The final factor, and the one that is still hanging me up, is that drag, and the torque countering required of the tail, increase as a result of the number of blades and RPM. If you reduce the main rotor RPM you can negate a lot of the drag, but there will still be considerable torque required to drive the higher number of blades. With direct correlation between the main rotor and tail you will also cut the torque countering benefits of the tail system when you actually want it to INCREASE. The stock system may be able to counter a slight increase if it remains relatively stable, but at a correspondingly higher blade alpha which in itself increases drag on the power system, but also runs the risk of blade stall.

For my own setups that drops the comfort level far too much and the solutions I have been able to come up with are either too complex, too expensive or not to scale. I have made up a reverse pitch lever to move the blades over to the left side as per the real machine I flew. That was the easy bit. To maintain or increase tail rotor RPM I thought of replacing the tail rotor belt drive with a small brushless set on governor mode. Have not found one that really fits and works well yet.

The two other options were to direct drive the tail from the main rotor shaft via a drive cog and longer belt. Have some bits but difficult to fit within the originally intended SuperFrame.

Lastly I have thought to replace the auto drive gear with another motor drive gear and move the tail drive system aft. That increased drive diameter and tooth count may work IF I can then find a corresponding belt length.

Did not want to hijack this thread, this was to simply list the problems you can encounter with the change and hopefully some others can reply with thoughts and solutions.
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Old Apr 07, 2013, 09:38 AM
Love my scale Whirlybirds
Keyrigger's Avatar
Mississauga, Ont., Can.
Joined Sep 2009
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There are lots of five blade head 450's out there running two blade tails. They seem to be working fine so the question now goes to you, have you actually put the system together and flown it and if so, what motor/gear combo did you use?

Don
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Old Apr 07, 2013, 12:41 PM
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Canada, ON, Windsor
Joined Apr 2008
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I first built another mechanically identical setup for a friend and, along with a whopping 47A recorded on spool up, I found it nearly impossible to keep it straight on the turntable. Does that count? It never flew.

Rather than experiment again I thought I would solve this during the build, rather than after.

As for my own, I opted to run another Hacker A20-6XL 2500Kv motor. Same as my stock 450. The exception was to flip to a 75A ESC, (had a spare).
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Old Apr 07, 2013, 02:36 PM
I convince things to work
The Netherlands
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Quote:
Originally Posted by Cougar429 View Post
You obviously lose that with multi-blade setups so need servos that are strong enough for the job. Difficult to accomplish with a 450 size machine.

The final factor, and the one that is still hanging me up, is that drag, and the torque countering required of the tail, increase as a result of the number of blades and RPM. If you reduce the main rotor RPM you can negate a lot of the drag, but there will still be considerable torque required to drive the higher number of blades. With direct correlation between the main rotor and tail you will also cut the torque countering benefits of the tail system when you actually want it to INCREASE. The stock system may be able to counter a slight increase if it remains relatively stable, but at a correspondingly higher blade alpha which in itself increases drag on the power system, but also runs the risk of blade stall.
The servo-problem is easily solved: Hitec HS65BB are completely up to the job. If you insist on running FBL electronics, use their digital equivalent.
Needless to say, anything with higher quality, is more than adequate.

The tail is also VERY easy sorted, I have mentioned this over and over here on various 450 size multiblade threads: use main blades for an MCPX, shortened to 62 mm (2.5 "), from tip to centre of bolt-hole.
Virtually all fuselages benefit also in looks from a slightly larger tail diameter.

Using semisymetrical narrow-chord mainblades does really unload the tail, and keeping the machine as light as possible also does a lot.
I can recommend the Spinblade 325 series for 5 blade, and the 350 series for 3 or 4 bladed heads. They fly very good, even without FBL (a bit twitchy, but VERY accurate and controllable).

But even with the heavy and draggy Copter-X plastic mainblades, a tailrotor equipped with the shortened MCPX blades will hold, no problem, even with rate gyro.

In case you're wondering: yes, I have actually maidened my buddies Align MD500 fuselage, with Cheap clone 450 inside, Copter-X fully rigid 5 blade head, Copter-X plastic mainblades, and MCPX tailblades, using Hitec HS 65BB on swash and Futaba FS61 CarbonSpeed on tail, Walkera GY011 gyro and no FBL, and it flew on first attempt, without any issues....
Headspeed not measured, but guesstimated at 1700 RPM

Brgds, Bert
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Old Apr 07, 2013, 06:20 PM
Love my scale Whirlybirds
Keyrigger's Avatar
Mississauga, Ont., Can.
Joined Sep 2009
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Although not a five blade head, I have a four blade running narrow cord scale re-enforced plastic weighted blades with a set of 61mm KBDD tail blades. The tail was never a real problem, only its behaviour in flight and that was settled with a 3 axis gyro. I am running a similar 2580kv motor and 13t pinion on it and it is installed in an Agusta A109 Align body. That is why I asked as it is not that difficult to get one running with the stock 35A speed controller. The body does not have any side windows so there is lots of air getting inside to keep things cool but the ESC has never got hot enough for me to even notice that things are anything but fine. Hope this helps.

Don
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Old Apr 07, 2013, 07:53 PM
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Do the MCPX blades fit the grips without any mods to the root thickness?
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Old Apr 07, 2013, 08:28 PM
Love my scale Whirlybirds
Keyrigger's Avatar
Mississauga, Ont., Can.
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I think they fit from my recollection of trying it. I don't remember having to make up a washer or file the root of the blade down. I found the KBDD blades worked better than the cut down mCPX blades but many others have used it and it does work.

Don
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Old Apr 08, 2013, 11:48 AM
I convince things to work
The Netherlands
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The MCPX blades fit directly (unless Blade altered them for the BL version, but I think not).

I have no access to the KBDD blades, so I would not know about that.
To be honest, I was never intending to say that MCPX blades were the best solution (but you can cut them to size so it's a step in the right direction experimenting with them) but I was not aware there are blades availlable longer than the usual "T-rex standard"....

I find however, that the MCPX blades, due to their very thin profile seem to have a noticeably positive effect on flight time, it seems that the power needed for driving the tail reduces significantly for the same or more sidethrust.

Brgds, Bert
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Old Apr 08, 2013, 12:38 PM
Rangers Lead the Way
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I am going to try the 4-bl HDX tail rotor just for fun (I know, it's not scale). But I have read in multiple cases that this is not needed if using narrow-chord main blades.
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