Thread Tools
This thread is privately moderated by navigator2011, who may elect to delete unwanted replies.
Oct 03, 2013, 12:58 PM
Folding Spacetime
navigator2011's Avatar
Thread OP
Mini-HowTo

Building a DDVP tail for a 450 Pro


If you've read my blog, you'll already know that I love to tinker and try unusual things with my helicopters. My latest project has been installing a brushless motor onto a variable-pitch tail of my 450 Pro. This particular tail configuration, known in the forums as a direct drive variable pitch (DDVP), differs from most motorized tail systems that incoroporate fixed pitch tail rotors, or even use an airplane propeller. In the case of the DDVP tail, the tail blade pitch is controled by way of a tail servo, as usual, while the motor turns the rotor, either at a governed speed or in combination with the main motor speed. Based on some of the information available on the forums, the DDVP has the potential to reduce vibrations and noise often associated with torque tube tail (TT) systems, as well as eliminating belt tensioning issues often encountered with belted tails. However, the weight of the motor also has the potential to affect tail performance due adding inertia to very end of the tail. Even so, I wanted to give building the DDVP tail a try.

For my tail, I chose a Tarot 450 Pro TT tail section because they are less expensive than Align parts and they are readily available. The motor I picked for this project is a Turnigy Park 300 1380KV brushless outrunner because it should produce a tail RPM similar to the TT tail, and because this motor has a 3.0mm shaft diameter, just like the stock tail shaft of the 450 Pro.

A primary part of this project is mounting the motor onto the side of the 450 tail gear box; and to do this, I needed an adapter that cannot be purchased, but rather must be made. I started out by taking measurements of the existing tail part that would be replaced by the adapter and then drawing the adapter in AutoCAD with dimensions. This drawing formed the basis for the adapter that I would need to mount the motor onto the side of the tail. The next step was to draw a scale drawing that I could use as a drill guide for making the adapter. Figure 2 shows the scale drawing taped onto a workpiece before drilling.

At first, I tried making the adapter out of carbon fiber, but I found it very difficult to accurately drill such a small part in such a tough material. I don't have any sophisticated equipment like a drill press to make high precision parts. So, I needed a material that is strong enough to use on the tail, but also is easy to work with. I ended up going with 3/16 inch thick ABS sheet, which is very easy to work with--almost too easy. The ABS is also reasonably strong on the small size scale I'll be using for this DDVP tail.

I began by simply taping my scale drawing of the adapter to the ABS sheet, as shown in Fig. 2. The drawing formed a decent drill guide for use with my hand drill. I drilled 3/32 inch holes for the screws that will mount the adapter onto the tail gear box. The two motor mount holes are each 1/8 inch diameter. The large hole in the middle of the adapter has to be large enough to accept a locking collar that will go on the motor shaft. The large hole shown in Fig. 3 is 1/2 inch diameter, which is just large enough.

Once the holes were finished, it came time to cut the adapter out from the ABS sheet. I did the rough cut with a hacksaw, and the finishing touches with a variety of files, while continually checking for a perfect fit. As shown in Fig. 4, the three tail mounting holes are countersunk with a 3/16 inch diameter drill so that the bolt heads will be hidden, ensuring that the tail motor will mount onto a nice flat surface, as shown in Fig. 5.

Referring again to Fig. 4, it is easy to see that one side of the ABS material has a surface texture whereas the other side is smooth. At first thought, I was concerned that the texture coupled with the pliability of the ABS might cause the tail motor to later loosen from the tail gear box. With this in mind, I chose to place the smooth surface of the adapter in contact with the tail gear box, and the motor in contact with the textured side of the adapter. The contact area of the motor is much greater than the area of the adapter that contacts the tail gear box, and also the tail motor is mounted by two 3.0mm bolts. As an additional measure, I decided to use red threadlock on teh 3.0mm bolts--so now they're tough to remove, but are still removable. For these reasons, the presence of the texture appears to have no bearing on how well the motor stays tightened to the tail gear box.

Figure 6 shows the underside of the tail section with the tail motor installed. Installing a longer 3.0mm shaft into the tail motor is another critical area of this project. There are a wide variety of vendors that sell 3.0mm shafts having a variety of tolerances and lengths. My plan for this project was to purchase extra long shafts, like 200mm, and then cut the shafts to the desired length. The length of the shaft will depend on the particular type of motor used for the tail. The shaft shown is Fig. 6 is a 316 Stainless Steel 3.0mm diameter from McMaster Carr, and it is cut to about 76mm in length. I have not yet come to a conclusion on the perfect shaft length, but 76mm is pretty close.

Upon receiving my shafts in the mail, I was a bit surprised to find that they were just a bit too big to allow the tail bearings and the pitch slider to move freely on the shaft. After a lot of sanding with 600 grit sandpaper, however, everything now is smooth as silk. Sanding the shaft was a lot of time consuming work, and the take home point is that there will be no quick fixes at the field unless already prepared shafts are on hand.

As shown in Fig. 6, I used two Esky main shaft lock collars (EK-000231) purchased from Helidirect--a first one against the motor, and second one against the tail bearing. The lock collars merely serve to keep the tail bearing and the motor bell housing in place. Since the lock collars are not subjected to a terrible degree of strain, I didn't go to the extent of grinding notches in the shaft. I simply thread locked the grub screws tightly in place. At present, I think this approach is fine, and if they collars ever do slip, then I will gladly grind corresponding notches into the shaft.

Figures 7 and 8 show the tail motor installed onto the 450 Pro tail boom. As shown in Fig. 7, the tail motor fits neatly behind the tail fin--no modification was needed to the tail fin, due to the thickness of the ABS adapter. As best shown in Fig. 8, I also drilled a small hole in the adapter to receive a zip-tie, which serves to keep the motor wires away from the tail shaft. I also soldered 22 AWG wire extensions onto the motor leads, which pass all the way through the boom to a HobbyWing Flyfun ESC 10A.

Installing this motorized tail is as easy as sliding the boom into the 450 Pro's frame, tightening the bolts, attaching the tail servo to the linkage rod, and plugging the motor leads into the 10A ESC. Although I have bench tested this tail, and it runs increadibly smooth, I have not yet installed it onto my 450 Pro because, well, my TT tail is still working so nicely. I worked hard to get my TT tail to run smoothly, and now I'm not terribly excited about tearing into a perfectly working helicopter. The very next time the TT gives me trouble, or I just drive it into the ground, then I will give the DDVP tail a real proper test.
Last edited by navigator2011; Oct 17, 2013 at 07:47 AM.
Sign up now
to remove ads between posts
Oct 04, 2013, 10:39 AM
Custom User Title
very interested, how's it fly??
Oct 04, 2013, 10:44 AM
Folding Spacetime
navigator2011's Avatar
Thread OP
I have not yet installed this tail on the 450 Pro because, well, my TT is still working just great. I'm waiting for things to go south before tearing into the 450 Pro again. Have a look at this thread--it's full of guys that are already flying these DDVP tails.

Direct drive tail on a trex450.
Oct 14, 2013, 02:14 PM
Registered User
Cougar429's Avatar
I have been looking for something this type to maintain tail rotor speed when I slow the scale 5-blade head.

Curious as that ESC does not seem to have governor mode. How do you maintain RPM.
Oct 14, 2013, 02:21 PM
Folding Spacetime
navigator2011's Avatar
Thread OP
It would tie into the motor throttle port, and work much like any other heli--the tail turns at a ratio of the main rotor (about 4:1). Since I run an essentially flat throttle curve, the tail RPM would be essentially flat, as well. Others that I know who've used a DDVP tail do run governed tail motors and report that it works very well.
Oct 14, 2013, 04:18 PM
Custom User Title
the HobbyKing YEP ESC's are supposed to have REALLY good governors
Oct 14, 2013, 05:37 PM
Registered User
Cougar429's Avatar
That was my plan, just a few other projects ahead of it and I was hoping to find an ESC the correct size with governor mode.
Oct 14, 2013, 05:41 PM
Folding Spacetime
navigator2011's Avatar
Thread OP
Quote:
Originally Posted by Cougar429
That was my plan, just a few other projects ahead of it and I was hoping to find an ESC the correct size with governor mode.
Be sure to check these guys out--they go into some detail about ESCs with governor mode: direct drive tail on a trex 450
Oct 14, 2013, 05:48 PM
Registered User
Cougar429's Avatar
I read though some of that earlier.
Oct 15, 2013, 12:57 PM
Registered User
gsenroc's Avatar
Actually I think this direct drive setup will be very useful on some clone helicopters. Most of the time these clones comes with bad torque tube or gears that just ruin the tail performance or even destroy the helicopter. Of course change the gears into good ones and you're already golden, but sometimes the alignment of the frame is too bad and the gears will never fit well

The extra rotating mass at the tail does change the dynamic characteristic of the helicopter, tuning the PID of the flight controller (autopilot or fbl controller etc.) will solve the problem if your heli is equipped with such controller. Assume that the servo's have enough power to maintain it's speed at higher load, the dynamic characteristic of the heli won't change that much to make you feel uncomfortable during the flight.

After all it's just about tuning, but be careful:

1. You have eliminated some bad vibrations due to the gears. This is the pros.

2. Now the cons. The elasticity of the boom and the mass of the motor/pitch mechanism form together a mass-spring system which has a much lower eigenfrequancy(resonance frequency) than the unmodified tail. Now there is a higher possibility of resonance at the tail so just keep an eye on it when tuning, you will see the tail shaking when the motor hits the exact frequency(RPM) or within a certain range, just like the crazy shaking you may have when the main rotor rotates at a certain speed.

Avoid this frequency range and you'll be fine. It's also possible that the frequency is higher than that the motor can reach and this resonance will not happen, depends on the boom and motor you use
Last edited by gsenroc; Oct 15, 2013 at 01:03 PM.
Oct 15, 2013, 01:06 PM
Folding Spacetime
navigator2011's Avatar
Thread OP
Quote:
Originally Posted by gsenroc
1. You have eliminated some bad vibrations due to the gears. This is the pros.

2. Now the cons. The elasticity of the boom and the mass of the motor/pitch mechanism form together a mass-spring system which has a much lower eigenfrequancy(resonance frequency) than the unmodified tail. Now there is a higher possibility of resonance at the tail so just keep an eye on it when tuning, you will see the tail shaking when the motor hits the exact frequency(RPM) or within a certain range, just like the crazy shaking you may have when the main rotor rotates at a certain speed.

Avoid this frequency range and you'll be fine. It's also possible that the frequency is higher than that the motor can reach and this resonance will not happen, depends on the boom and motor you use
Thank you for the interesting points to keep in mind.
Nov 18, 2013, 12:04 PM
Folding Spacetime
navigator2011's Avatar
Thread OP
Hey guys, due to other projects and life issues, I decided not to this tail myself, and have since sold it on to another member of RCG. He immediately installed the tail and began testing it out. He resported his initial results in an email, which I reproduce here in the hope that it will aid anyone else in the future that considers a motorized tail on a 450-size heli.

Test flights of DDVP tail

Got the tail motor on the heli and flying it around. You really see the benefit of a motorized tail...it never loses control due to the contstant RPM.

I have it governed at around 60%(it's pretty powerful for a 450) with a midrange setting at 25% and 0% for hold. Got a programmable 18a Turnigy esc for this.

If you do another one of these you have to set it up as governed...not slaved to the throttle...you lose all the benefits and just might as well keep the TT on.

Now it goes permanently into an elevated tail project...I'll have to post the build on RCG when I finish it.


Quick Reply
Message:
Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion DDVP 450 tail Ken59 Electric Heli Talk 8 Jul 15, 2013 11:15 AM
Sold Tarot CNC Tail Boom Mount Upgrade For TREX 450 Pro wc_wickedclown Aircraft - Electric - Helis (FS/W) 2 Jun 18, 2013 08:26 AM
Found WTT 450 Sport Belt Tail (for Pro) for 450 Pro Tail SYMAWD Aircraft - Electric - Helis (FS/W) 0 Apr 11, 2012 07:28 PM