700 DFC tail issue

[rcflyerheli]

Originally Posted by XSNSX

Is anyone else noticing the roots of their tail blades are being eaten/chaffed by the blade grips whenever the hum starts? Just a 20 second hum is enough to bite into the roots so the blades don't rotate smoothly in the grips. This would be an indication that the tail blades are fluttering, however we're being told that lighter blades will fix the problem. This is contradictory, because lighter blades should flutter before heavier blades.

Flutter can also be caused by the relative position of the center of lift with respect to the center of gravity. The effect will be dependent also upon the airfoil of the blade. There was a thread or post regarding the CG of the old Align blades. If the CG varies along the spanwise length and the C/L is constant, then that can definitely lead to flutter. Theoretically, to have a positive stability on the blade, the CG needs to be forward of the CL. Neutral stability needs to have the CG and CL coincide.

Not trying to bash Align blades, but maybe the other brands have that particular issue figured out.

[michael503]

I just made some washers for my 600N from a .006" thick piece of blister pack plastic. Anyone who's tried to open a modern blister pack knows how tough the stuff is. Took the slop right out!

I used a sharpened K&S brass tube (left over from my plank days) and a hammer to stamp them out,then just made a hole in the middle, but scissors will work too.

[LizardMan]

Originally Posted by rcflyerheli

Flutter can also be caused by the relative position of the center of lift with respect to the center of gravity. The effect will be dependent also upon the airfoil of the blade. There was a thread or post regarding the CG of the old Align blades. If the CG varies along the spanwise length and the C/L is constant, then that can definitely lead to flutter. Theoretically, to have a positive stability on the blade, the CG needs to be forward of the CL. Neutral stability needs to have the CG and CL coincide.

A friend builds high-power rockets. Most of their models reach at least trans-sonic speeds. Well below Mach 1 air flow around sections of tail fins becomes super-sonic. Air velocity increases in areas where air is displaced by body/fins. Localized air flow becomes super-sonic significantly before the rocket does.

Airflow patterns suddenly change with seemly minor increases in a model's airspeed. At trans-sonic speeds CL shifts, but CG doesn't. A fin stable at lower airspeeds suddenly begins to flutter at slightly higher airspeeds.

Flutter is very destructive. Extreme forces are at work, combined with rapid motion. A fluttering airfoil extracts energy from the air flowing over it. Flutter easily becomes a runaway condition. A fluttering surface becomes a free-running power oscillator with almost unlimited energy available to it.

When a surface deflects under this load it has a higher angle of attack relative to airflow, further shifting CL and increasing energy extracted from air flow. This caused it to deflect more. This feedback cycle repeats. It happens in a split second.

Higher airspeeds increase risk of CL movement and energy available to be absorbed by fluttering surfaces. A change in tail drive gearing could place tail surfaces in trans-sonic flow. The critical airspeed varies with airfoil design, temp, pressure and RH. Add grip to blade lash, torque tube flex/resonance and uneven gear mesh to create a complex non-linear problem.

I've seen rock-solidly mounted thick stiff fins (FG/CF/metal) ripped from body tubes in an instant. The airframe is fine, stable and clean, until reaching a critical airspeed where it suddenly self-destructs.

It's amazing to watch, if it isn't your rocket. (or tail blades)

[tutelarRC]

I really like flying my 700E DFC. It is the best heli that I own. But this tail problem is really annoying.

I blame myself for the crash. I have been following this thread for over 6 months. I know there is a problem and I upgraded all the parts. But the issue is still there. And I chose to fly. Parts cost $400.

The "good" news is none of the gears stripped.

There is a lot of wear going on in the tail area. I can't imagine why there is wear at the torque tube.

XSNSX has identified why there is wear on the tail thrust bearings.



If you continue to fly this heli, make sure you check all tail components, including the torque tube, every couple of flights.
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Hurry up HeliFreakers and solve this problem! This issue needs to be fixed and Align is not stepping up to the plate.
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[rando]

Ah that sucks -- Did you determine exactly what part of the tail failed (servo, binding from wear, loosened tail hub grub screw, ...)? I didn't see anything fly off before it let go.

[ShaneJourdan]

I don't see how this is in anyway related to this thread?

[Bonkievan]

Sorry to see your crash, Tutelar.

I didn't seem to hear the 'hum' like some other vids though?
Did you find snapped tail shaft etc on your crash inspection?

[rando]

Originally Posted by DoctorJ

I don't see how this is in anyway related to this thread?

It could be, if the vibration/hum lead to wear which eventually caused the in flight failure.

It appeared the tail got stuck AGAINST the direction of head torque. This can happen if your tail hub grub screw comes loose (improperly installed, excessive vibrations or both). The hub slides away from the tail box assembly a bit and the plastic link arms extend past their 180deg point and effectively lock the tail slider in place. Of course, there are other possibilities. Additional details from the post-crash analysis would be helpful.

[tutelarRC]

The tail actually took very little force of the impact. The heli landed mostly on the main blades and front right skid. Surprisingly the landing gear did not break but the aluminum skid was severely bent. The blades and head took most of the impact.

Upon inspection, all tail components are still moving smoothly. No pieces missing. All screws and bolts still secure with loc-tite. During my complete tail tear down, I could feel the loc-tite breaking free.

I was running plastic tail blades made by a major manufacturer. I was using them because I enjoyed how tough they are when learning how to auto. The blades had recently been replaced and had not had any impact.

The only part I could not find was one of the tail blades. At this time, I am guessing that the severe vibration/flutter/whatever it is that occurs in the tail eventually caused one of the plastic blades to fatigue and break off.

The point that I am making is that there is excessive vibration, wear, and fatigue occurring in the tail. The absence of the hum does not mean the tail is not vibrating. You don't know when a part is going to fail. For me it was a plastic tail blade. It could just as easily be a bearing or one of the screws in the tail slider.

Making the tail shaft bigger simply chases the problem to another part of the heli.

If I fly it again, it will be with CF tail blades and continual tear downs to ensure no other part is on the brink of failure.

Check your bearings. Check your tail blades. I doubt there is any 700E DFC without the tell-tale signs that have been shown in this thread.

[XSNSX]

I believe the dust you're seeing in the tail mount is residue from the splines of the new black umbrella gear. It's well known that the new black gear-to-spline fitment is much looser than the original white gears, resulting in excessive backlash. It appears your TT aluminum splines are reaming out the plastic in the umbrella gear receptacle. This could definitely be a source of vibration. This joint should be tight, even though the system in 'loaded' during flight.

When I built my 700, I wrapped Scotch tape around the splines, then cut slots along the spine edge to help fill the gap. On my recent tear-down to try a new TT with triple bearing set, I did not use tape because I wondered if it could be contributing to the problem. I tested over the weekend. I am still unable to go above 2000 RPM without the hum beginning. This was with my tail grips tightened down so the blades are not free to rotate. If I loosen them up, the hum only gets worse. Go figure!

I'm still waiting for the Rail tails to arrive...

[ghost2212]

From that video I'm pretty sure your tail is humming, While it's hard to tell it definitely sounds like it which would definitely cause a failure. On my 700 and numerous others running the rail blades and new gears the hum is non existant.

KBDD blades are definitely not one of the lighter blades which would lead to the hum and the eventual failure of the tail blade. Had you been running Rail blades I don't think this issue would of occured.

[ddj74]

Never saw a tail blade fly off. Definitely hear a hum but to me looked like a different type of failure. With the tail speeds this thing puts out i wouldn't run plastic blades.

[XSNSX]

The tail is absolutely humming! And to put another theory to rest, it's humming inverted which means the resonance is mechanical, and NOT an interaction from main blade downwash.

So there you have it. Failure from fatigue. Exactly the point I've been trying too convey!!!

God forbid this happens with spectators nearby!!!

[XSNSX]

Originally Posted by ddj74

With the tail speeds this thing puts out i wouldn't run plastic blades.

Would you like to quantify that statement with a basis of reason?

[Steemo]

Originally Posted by ghost2212

...KBDD blades are definitely not one of the lighter blades which would lead to the hum and the eventual failure of the tail blade. Had you been running Rail blades I don't think this issue would of occured.

+1
I stopped using KBDD tails as it was making my tail hum, rails fixed my problem.
pitty since I love the visibility you get from orange KBDD blades

Originally Posted by ddj74

Never saw a tail blade fly off. Definitely hear a hum but to me looked like a different type of failure. With the tail speeds this thing puts out i wouldn't run plastic blades.

It's always a concern isn't it... this is why I'd never buy a Logo 600: plastic main and tail grips on a 600-700 size heli? fiber reinforced plastic or not...

[rando]

I agree, your tail had a vibration though perhaps not as severe as others.

I don't think you lost a tail blade pre-crash. I didn't hear any difference in sound when the tail let go.

[ShaneJourdan]

Looks like a servo related problem?

[beenflying]

It definitely has the hum. It's quite clear at 20s in and remains while inverted. If it threw a single tail blade, it would rip the tail apart, so I doubt it was that. More likely a tail thrust bearing seized under load or the tail servo overheated from the load.

tutelarRC, sorry to see your crash. Is the tail servo still working? No plugs came loose?

[tutelarRC]

I am not positive it was a blade coming off. But I have found no other problem and was unable to find the other blade - we searched the area quite thoroughly. Thrust bearings, although showing good signs of wear, were not seized. Everything moving freely. Tail servo no issues. Nothing unplugged - even after the crash.

As a point of clarification, I did not say they were KBDD blades.

Regarding the sound, I don't think my tail blades sound any louder than those in this video.

Alan Szabo Jr. ALIGN Trex 700E DFC New Align Blades Test 1 3/9/13 Vegas FF (3 min 13 sec)

(Although my mains are getting nowhere near the same workout!)

The tail on this heli deserves considerable caution. I appreciate all the advice and direction found in this topic; from needing thicker washers, to the best tail blades, to torque tube bearings, and setup tips. Thanks HeliFreaks!

I don't understand why we aren't hearing more from the extreme talent that Align has on staff (and my hobby dollars are paying for). I find it hard to believe they can punish a 700E DFC and not have similar issues.

[ShaneJourdan]

Mine sounds no worse than yours and I beat the crap out of the tail reversing piro's.

I'm fearful this was just a failure unrelated to vibs.... Just my 2 cents.