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Low Head Speed Helicopters Low Head Speed Helicopter Setups and Flying info


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Old 12-26-2016, 11:50 PM   #1
prudhomme
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Lightbulb Heavy Heli with 700mm low speed

Hello to all;

I'm making my own helicopter with 3 blades in main rotor from Spin blades 690mm and 78mm chord asymmetrical.

I'm thinking in use the motor KDE 700XF-295-G3 with Kv of 295, with 2 lipo batteries of 6s 20,000 mah in serie to have a 12s with 20Ah. The ESC will be the Castle Creations EDGE 160HV.

I'm not interested in 3D operation only stable and long endurance flights for video operation; so I'm thinking in use the governor at 1200 to 1300 rpms for main rotor and 9000 rpm for tail rotor.

The specifications of KDE indicate that for 12s power at 1200-1300 rpm the gear ratio will be 9 to 9.5, so I think to use the 112th with 12th gears that gives 9.3 of ratio.

The problem is that the minimum weight is 13Kg (26Lb), and the maximum will be 18Kg (36Lb).

Using the spreadsheet "Basic Rotor Spreadheet" and the results that gives are:

For 26Lb the power for main rotor is 2.3HP or 1.85Kw + Tail and looses
And for 36Lb the main rotor power is 3.7HP or 2.85Kw + Tail and looses

But I bought the eCalc program license and with the same variables the program indicates that the motor don't have the enough power to fly.

So, I'm very confused about the viability of that configuration.
Do you have some experience in this kind of helicopters with high
weight???

Thanks
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Old 12-27-2016, 02:15 PM   #2
amphibia
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hi there,

If you are making your own heli, can you make it to run longer 780-800mm main blades and longer 120-130mm tail blades.

Longer main blades give higher efficiency for sure. Longer tail blades also means high efficiency. Another benefit is that you dont need crazy high tail rpm, which could lead to extreme vibration and stress.

I have some experiences of low rpm flights, my forza700 (5.2kg take off weight with 2x720mm symmetric F3C blades) can hover at 1100rpm at about 7degree pitch with a battery of 6s10600mah for 30minutes consuming 80% battery capacity. My calculation shows the battery output a constant 400watt power. Based on linear extrapolation, if it hits 14degree pitch, it has a maximum lift of 10.4kg.

Your bird is only 2.5-3.5x the takeoff weight of my forza700. It is not that challenging for 800size bird.

- 1. motor power most likely is not a concern at up to 400 x 3.5 = less than 1500watt. Most 700-800 size 3D helicopters can handle up to 6000-8000watt (12s x 3.8v x 150A)

- 2. lift could be a concern at 1200-1300rpm. As you know, most airfoils work below 15degree pitch. And for safety you need some reserve power during flights. Therefore during normal hover, you probably should not run above 10-12degree pitch. Based on linear extrapolation from my bird, you would need 1200-1600rpm (800mm blade x 3). High quality 800size asymmetric main blades certainly help. But be careful of the asymmetric blade design. I remember reading about asymmetric blades have different lift center point during different pitch angle. This could introduce vibration and/or danger if not tested properly. Your max lift is not that extreme. You might be able to just run the normal symmetric 3D blades.

Cheers and let us know how your project goes.

PS, there are many rc helicopter weighlifting videos on youtube. Check them out.
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Old 12-27-2016, 09:06 PM   #3
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Mhhhh - the post made me think about what to write.

I have been lifting about 20 - 25 kg on the HD750 for testing earlier this year. I normally do fly a lot at low and very low rpm. I agree with user amphibia and also have some doubt that you could lifts that weight with 700 size machine at your target rpm. I used 1600 rpm on 753 mm blades with 13 degrees pitch. It worked but I think lower rpm would have made it more difficult.
The weight I lifted is above your target but not very much and if you would run a 700 size machine you need more lift = more rpm.
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Old 12-27-2016, 11:40 PM   #4
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Thanks for your very complete answers.

Well after your recommendations I began to search for an 800mm blades and I found the Align 800 with 70mm of chord symmetrical. I liked the 690mm from SplinBlades because the chord of 78mm and them are semisymmetrical, but itís true that 690mm can very small blades.

The tail rotor will have a diameter of 330mm with 2 blades of 115mm.
Asking to the people of KDEDirect they recommend the same motor, the 700XF-295-G3 with Kv of 295, and 7kw constantly.

What do you know about KDE motors?

To check the different options of gearing I bought the license for the eCalc program and test different configurations with the ESC and the motor with 12s batteries (attached the config). I found that the gearing ratio of 7.5 : 1, will give a good range of combination of lift and time, for example with 1350rpm it can lift 13Kg for 21 minutes in hover with 13į; and at 1600 rpm it can lift 18Kg for 12 min with 13.5į.

Iím not very sure about the results of this program, because the blades are very generical, the chord and shape are not considered, also the power in the tail rotor is not considered. I think that the time and lift can be bigger that the program indicates.

With the HD750 you can lift 25Kg but how many time and with what kind of battery?

What do you think about this configuration?

Thanks a lot

Regards
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Old 12-28-2016, 06:20 AM   #5
amphibia
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Let me write you a spec sheet. First thing is that your task is not that demanding as most 800size helicopter can handle 8000watt power.

1. motor: any 800size probably will do. Please refer to my recommendation below at #5.

2. battery: 12s, that is 2x6s10000mah or bigger in series depending on your flight time requirement. Battery output rating of 25c will give you enough juice. 25c means burning it up in less than 3minutes. honestly 15C is probably good enough. Buy decent batteries such as Gens Ace tattu series.

3. blade+rpm: that depends on your rpm requirements, which then depends on blade size and number of blades. Let me assume you use 810mm x 3 blades (i.e. align trex 800 with 3-blade upgrade on main and tail), then I think 1500rpm at 10degree pitch will probably give you enough lift for 18kg.

4. Power and ESC options: remember, your task of just lifting in hover is not that demanding. I think 1500watt - 2000watt at most (many youtube videos can confirm this) Remember power = voltage x current. We know the voltage is 12s x 3.8v = 46v. Even if I use the higher 2000watt, that gives you a current of less than 50A. For safety margin of 2x, Kontronik Jive 120A or hobbywing v4 130A will certainly do the job already. These two have active-freewheeling function. They can handle partial throttle without overheating. But a strapped on fan is still recommended. You surely can go kontronik 160A or hobbywing v4 160A if you want more safety margin.

5. Gearing: this is crucial. It is crucial because you dont want hovering at lower percentage throttle to heat up even the advanced ESCs with active free-wheeling. Going back to #3 above, we want to target 1500rpm at a constant 80-90% throttle. You can then adjust the lift with pitch angle up to 15degree. Don't worry about the current Amperage requirement. Your ESC will increase automatically to hold your designated headspeed.

Here is how I would seek the optimal pinion size. Do not worry about current at this stage. We already solved that in #4.

1500rpm = 12s x 3.8v x motorKV x (pinion teeth) / (main gear teeth) x (80-90% throttle) x (90% electric efficiency)

You really can use any typical 700-800size motor such as scorpion 5035 500kv or align 850mx 490kv. I am not familiar with KDE. But if you just mention a typical align 800size 3D requirement, KDE sales guy will tell you which motor.

Let me plug in 500kv, in my equation above. And let me assume you end up buying align 800 as a starting point, then the main gear has 110teeth, your pinion should be around 9T or 10T. Just buy them both and test flight to fine tune the head speed and throttle percentage based on emergency reserve requirement, flight time and motor temperature after landing.

6. Servos and Gyro: Since I assume your equipment is expensive, you should buy high quality reliable servos and a reputable gyro with self-leveling functions. I'd say futaba brushless bls275 x 3 for cyclic, bls276 for tail, and vbar neo with a dedicated 2s4000mah receiver battery.

7. Transmitter: Buy futaba 14sg or 18sz with futaba 7008 receiver. Please remember to set the receiver to run Sbus connection. Do not use SBus2 connection per vbar instruction. With a futaba system, especially the 18sz, you can plug in many useful accessories, such as GPS module, voltage sensor, and temerature sensors.

Good luck with you.
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Old 12-29-2016, 05:40 AM   #6
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To add a bit of advice on the swash plate - check out the system used by Velos helis. Its a 4 servo swash adding a bit safety in case one servo fails.
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Old 01-02-2017, 04:39 PM   #7
Dennis P
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Quote:
Originally Posted by prudhomme View Post
Thanks for your very complete answers.

What do you know about KDE motors?

Regards
There is no problem with KDE motors, top quality and easily up to the task at hand.
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Old 01-02-2017, 05:07 PM   #8
prudhomme
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Hello again to all and Happy New Year!!!

Thanks amphibia for the specs was very useful.

Great to know your opinion about the KDE motors Dennis.

I bought the KDE motor with 295Kv to run the main rotor from 1400rpm to 1550 at maximum. I'm excited waiting for the motor, and I defining the rest of the equipment.

I forget to tell that the control will be with the Embention Autopiot, I was using in and it works fine, I hope we don't have problems with this heli and the autopilot.

In base to the equations in past posts the best ratio for 1400 rpm will be main 8:1, and to have a good answer of the tail rotor I want to run it at 9500 rpm so the ratio from main is 6.7:1

For main I don't have problem with the commercial gears, because I will use a 112th with 14th and both are commercial, the problem is the tail rotor because the common ratio are between 4:1 to 5.5:1.

One option is to use timing belts but I don't feel very comfortable with them, I feel it a little insecure. So, the other option is to design our specific helical gear and develop them in PA66, with 3 gears in series of 10th-80th-12th.

10th for motor, 80 for main rotor and 12 for tail rotor.

Maybe I kind find some commercial gear of PA66. Do you know some company that sell plastic gear for this size??

Finally, for batteries I will use two lipos of the TATTU PLUS 22000MAH 22.2V 25C. Itís hard to find here this kind of batteries because the restriction of aerial transportation of lipos, and the terrestrial delivery itís sucks, so I need to find the lipos in stores here.

So, once I solve the gears problem I will finish the designs to send to CNC and begin the production and then the tests

Thanks
Regards
Alberto
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Old 01-02-2017, 06:46 PM   #9
amphibia
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Quote:
Originally Posted by prudhomme View Post
Hello again to all and Happy New Year!!!

Thanks amphibia for the specs was very useful.

Great to know your opinion about the KDE motors Dennis.

I bought the KDE motor with 295Kv to run the main rotor from 1400rpm to 1550 at maximum. I'm excited waiting for the motor, and I defining the rest of the equipment.

I forget to tell that the control will be with the Embention Autopiot, I was using in and it works fine, I hope we don't have problems with this heli and the autopilot.

In base to the equations in past posts the best ratio for 1400 rpm will be main 8:1, and to have a good answer of the tail rotor I want to run it at 9500 rpm so the ratio from main is 6.7:1

For main I don't have problem with the commercial gears, because I will use a 112th with 14th and both are commercial, the problem is the tail rotor because the common ratio are between 4:1 to 5.5:1.

One option is to use timing belts but I don't feel very comfortable with them, I feel it a little insecure. So, the other option is to design our specific helical gear and develop them in PA66, with 3 gears in series of 10th-80th-12th.

10th for motor, 80 for main rotor and 12 for tail rotor.

Maybe I kind find some commercial gear of PA66. Do you know some company that sell plastic gear for this size??

Finally, for batteries I will use two lipos of the TATTU PLUS 22000MAH 22.2V 25C. Itís hard to find here this kind of batteries because the restriction of aerial transportation of lipos, and the terrestrial delivery itís sucks, so I need to find the lipos in stores here.

So, once I solve the gears problem I will finish the designs to send to CNC and begin the production and then the tests

Thanks
Regards
Alberto
Good luck to you. Apparently you have resources and experiences in design and machining. I have no doubt you can achieve your goals. Please be prepared for unintended wobble of death during spool up and/or spool down. It might take some debugging (lube/change spindle o-rings, blade grip tightness, tweaking landing gear,... helifreak has many posts about this topic). At least dont strap on your expensive gear during your maiden flight.

Please keep us posted on your design and parts selection.
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Old 01-03-2017, 03:12 PM   #10
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Hi:

This may or may not be too much help by I have the 295kv motor installed in my MD7, weighing in, fully loaded, at 15 pounds. The specs for the motor state that it will produce 7125kw of power which works out to 9.55hp. At 15 pounds and a fairly brisk full pitch climb out, the maximum draw was around 50.5A and produced 2166 watts of power. The average draw was 14.2A at 562 watts, clearly drawing nowhere near peak potential power. I am using 35C batteries at 12s with a gearing of 12:121. I cannot stall the motor at full pitch. I am waiting for a spare battery mount to arrive so that I can add additional weight to this helicopter as I am using it for a large scale trainer. Eventually, it will be converted to an 800 size but for the time being, the 700 size works just fine. Once I convert it to the 800, I will have a full up weight of 25 pounds which should not tax the system at all. Good luck with your aerial helicopter filming platform.

Don
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Old 01-10-2017, 09:55 PM   #11
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eCalc's algorithms are complete garbage for helicopters. I've seen them predict setups that I know work to be not capable of flight.
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