Blade-breaker

My Feedback: (4)

Hi, I have been flying a Diamond dust for a while now. I started out with a OS 46 engine and now am running a Jett FIRE 40. The rest of the gear is a Castle creations BEC set at 6.4 volts powered by a Lipo running 2 Hitec 7985 digital servos rated at around 175 oz torque. I have had no problems at all until tonight. First flight started ok. about 30 seconds in the flight I noticed the plane rolling to the left when I pulled hard up elevator. I brought it down right away and did a complete ground check. Both servos pulled hard and battery was fully charged. All linkages were fine.
Second flight did the same thing but worse. I brought the plane down again and the ground check shows no problems.

My first thoughts was that I was having blow back on the control surface. After the first flight I adjusted the linkages on the servo arms for more torque and less control surface travel. Didnt help.


Any thoughts on what could be happening?

combatpigg

My Feedback: (3)

Stick one wingtip between your knees and crank on the wing with medium effort. There shouldn't be any twist to it.
Are the elevons made from hard balsa? They should be.
Are you controlling them from the extreme inboard end? I think the elevons should be controlled from about mid span for maximum control at high speed.
Are the vertical fins tight? They shouldn't be floppy.
Is the programming in your radio uneven?
If the airframe checks out, you must have a tired servo.

Blade-breaker

My Feedback: (4)

Twisted on the air frame some. There is some flex when I twist on it hard but it all seems normal to me. The elevons are from the material that was sent with the kit, seems fine also. The control horns are out side the fins about a inch, just over 1/4 of the way up the elevon. Fins are solid. I did not check the programing yet hasnt been changed since I first set it up. I will check it tomorrow. Serovs are new for this plane, I guess one could still go bad.

Do the servos I have installed have enough torque for this plane? Hitec 7985 MG digital? If not what would should I use?

Thanks
Ryan

combatpigg

My Feedback: (3)

I wonder if those servos are overloading the battery pack during the hard manuever? Those servos are plenty strong, IIRC they're what I'm running in my fastest delta. I've got no experience with lipos and BECs, too high tech for me.

I do know one thing.......problems like this rarely straighten themselves out on their own. Consider yourself fortunate. I would get a big honkin' nicad or nimh pack and go with that. I would also exercise the controls while exerting pressure on the flaps. If you plug a voltmeter in while you check the controls under force, make sure the voltage isn't dipping below 4.8

evan-RCU

Two approaches for me one balance the other radio...

Where is it balanced exactly? If nose happy it could be a snap situation... I've never heard of it with a DD but it could happen.

What size LiPo? Your servos can draw a heck of a bunch of current at high deflection high speed. Plug a voltwatch in an extra channel and see what it shows when you move your surfaces and have resistance on elevon movement, I would bet they pull the BEC and or LiPo down in voltage especially if you have a smallish LiPo. The CC BEC is rated for 5A I think and may be part of it. Going to a 1400-1500 NiMh would be a good step. Also I suggest lowering the BEC voltage to under 6 volts, you are probably heating up the servos.

MJD

My Feedback: (1)

One thing I noticed on my Demon #1 model, is the the two elevon servos don't move at precisely the same rate. That probably means I should remove them immediately and relegate them to throttles, but letting that go for the moment, I notice a good wing dip on fast application of elevator sometimes. I doubt this is the same issue as BB presents, but just thought I'd mention it. It points at the servo in any case.

Is the roll constant, i.e. as long as you hold elevator it rolls? There are deltas flying around on HS-225's and honking .50's and aren't having problems. With the servos you have you shouldn't even need hinges.

Sure sounds like flex of the control surface or a FUBAR servo as CP suggested. Hope you iron it out before it fixes itself..

I remember making tubular bells with my nephew who was a university music student at the time - if you suspend a tube or rod or bar at the 40% length point, that is an anti-node and will damp any fundamental resonance immediately if you ring it. I can't help thinking that might be a prime spot for a control horn. At 23%, it rings forever at the fundamental frequency and I don't think I'd put the horn there - that's where you hang wind chimes for example. Airflow of course doesn't enter anywere into that math.

MJD

MJD

BiggerDanno

Sounds like more travel in one servo than the other. Make sure that deflection angles are the same on both elevons.

Mike Connor

My Feedback: (5)

My first Dust, as it got older, would reach a certain speed and would roll right requiring a significant trim change. I noticed it after changing from a 46 FX to a Jett 50. A couple of years ago there were a few of us having the same problem. Not long after that Dust made a mod to the kit to make a more rigid airframe. Your servos are plenty strong. To make sure it isn't a servo just switch left to right or put in different servos.

I have a feeling you may have an airframe issue. Hope you get it resolve before it goes in.

combatpigg

My Feedback: (3)

This is just a theory. If after covering the plane you introduce a twist in the wing, even slight. Now to compensate, you use a little bit of counter-elevon trim to make the model fly straight. Now you have a constant amount of pressure exerted on the flaps. In the pull mode, one of the flaps should be pretty secure, but the other flap that is in push mode will be less secure and more prone to wimping out.
I would check the straightness of the wing on the workbench.
I think all hot DDs should use 4-40 hardware and C/F tubing slid over 4-40 pushrods.
Planes that are getting the crap flown out of them will tend to loosen up, even if nothing breaks. I tore into an old airframe that needed the spar / rib intersections re-glued.

MJD

My Feedback: (1)

Highly agreed. So easy to do. I like it when you can carry the aircraft around by the elevon pushrods without worrying about the control drivetrain.

MJD

evan-RCU

Should not be an issue when giving full up as both servos are pulling the elevons.... I would have hoped that he would check the travels before coming on the forum asking for help...

combatpigg

My Feedback: (3)

If you are flying a twisted airframe and have to pull one of the elevons past neutral, yes it does matter. You are introducing differential throw.

evan-RCU

I meant flexing pushrods will not matter.......

But if the airframe is twisted there could be a number of issues that may need correction.....

MJD

My Feedback: (1)

One would hope so.

Mine twitches left when I pull the stick back, because of the small servo rate difference. Once the surfaces get there is no roll, but it will roll a bit until then. It is annoying on higher rates and low-med speeds, cooking along with gentle control inputs it is not really noticable. Since the right elevon moves slightly more slowly than the left, until they both stop there is a small left roll input that remains as a net roll to the left once the surfaces stop at the new deflection - i.e. it does not cancel out afterwards.

MJD

evan-RCU

Mike are you down in North Carolina yet?

Blade-breaker

My Feedback: (4)

Hi guys, sorry for my delayed response got called out on a work call for a day or so.

The lipo size I am using is a 850 3s through the BEC and yes it is rated at 5 amps. Only reason I use the 3s is because I have them laying around from my electric planes. Should be plenty of power.

I have checked the control travels and they move exactly the same through out the travel range.. I put pressure on both elevons and the servos perform perfect. Note: this has never been a issue until my last flights, no controls or radio changes.

Linkage is 4-40 hardware with no deflection problems.

The balance of the aircraft is slightly nose heavy, about 1/2"-5/8". When the aircraft does its dip to the left when pulling straight back on the stick it is not a stall condition. I would say I was only pulling back on the stick 50% or so. I have stalled the plane by pulling tight loops in the past and this was not the case.

The airframe is straight. I looked at it last night when i got home. Set it on my building table. With the trailing edge on the table measured the leading edge in a few spots. Difference from side to side was about 1/32", I would say not an issue.

After reading your posts I am concerned with the airframe flexing in flight. I did some flexing and twisting tests last night. Now I cannot remember what it felt like when new. Of course there is some flex and twist if I use enough force. But I cannot feel or see any one spot that flexes or twists more than the other. It is a uniform flex/twist through out the frame.

One thing about the design that has always had me worried was the all balsa trainling edge. Seems to me that there should have been some sort of carbon laminate or something to make that more rigid. No offense to the designer meant, I just tend to like things over built.

In conclusion I guess I am going to change servos, I already ordered a A123 pack to replace the lipo and the BEC. I never have trusted the BEC's. I will do some load testing on the BEC with a volt meter hooked up and possible that is not holding voltage like it should.

The airframe integrity has me worried now. I cant see anything wrong but now I have been thinking about it to much. I have been pulling quite a few high speed turns and level flight to straight up manuevers. Where do these things come apart at if they do fail from this type of flying?? I think I will remove the bottom covering and check out those areas to ease my mind.

Ok one thought I have just before I head out. All the other times I have flown this plane it has been 75deg or so. When I had the problems it was around 93deg and very humid. Could this have something to do with airframe flexing? I have the plane covered in Ultracote, Covering is tight when it is in my houseat 75deg but I did not check it that day at the field. I never noticed any loose areas...

Thanks for all your input!! Crap work just called again...... No rest for the Wicked A.K.A. RC plane addict!!!!
Ryan

BiggerDanno

Usually, a BECs amperage rating is reduced for higher voltage. The rated voltage is usually at 7.4v input. Likely you are only getting 3-4 amps peak with 3 or less continuous. This is probably not enough for those high power servos.

I run the Castle Creations 10 amp BEC with a 1000 mAh lipo on all my planes without issue. 5-8 flights is well within it's capability without re-charge.

MJD

My Feedback: (1)

If the airplane dips to the left with back stick, could it be P factor..? Light airplane, big engine/prop mass in relation.

Not to repeat, but are you sure the servos move at precisely the same rate? Not deflection/travel, but exactly the same speed to get to full up from center? That's my problem I believe, why mine dips. You can see it on the ground, a hair difference that results in roll input on top of the increasing elevator input until they stop at the stick setting.

MJD

Blade-breaker

My Feedback: (4)

Yes I understand. They do move at the same rate and to the same end points. I have checked them visually and also I had someone else move the stick while I watched and held my fingers on both elevons. I could tell no difference from one side to the other. I also checked them while applying a load with my hands. Of course I cannot be sure that the loads I applied were the same but I am sure it was close. No difference.

I suppose P factor could cause this. I would think that would be more of a low speed issue. This is happening at high speeds. FYI the prop I am running is a 8.75x9 apc. I asked Jett to send props he suggested for my plane and this is one he sent.

This is why I started this post. I am baffled!!!!

Ryan

Blade-breaker

My Feedback: (4)

Just looked at my previous post. I said it was a 5 amp BEC, I was wrong, it is the 10 amp BEC. Thanks

Mike Connor

My Feedback: (5)

When I had issues with my Dust there were three of us that had, not asked for, turning issues that started at high speeds. I stripped all the covering off twice and re glued every joint and used extra epoxy and wood at key points. The problem was never solved. I even changed out all the radio gear, including transmitter, with no results. I hope you resolve your problem as I am interested to know what is causing this issues.

That memory is one of the reasons I designed some sheeting into the Screamin Demon. It is a big help to eliminate unwanted flexing but may not have anything to do with your problem.

MJD

My Feedback: (1)

Well, that's good as it rules out servo rate. And as to P factor, I too seriously doubt that high speed flight and a pylon prop are a combo where that is noticeable, or a big deal. Mind you, these things pull some pretty serious G's too!

My thoughts lean towards uneven wing flex under elevon loads.

Oh btw, not directly relevant, but I can say from embarrassed personal experience with my SD at least, that a tiny bit of left thrust inadvertently put in by accident and not my fault at all, , will cause you great grief if the engine/prop combo does not bite hard after launch and the launch is slow and nose high.. esp. as a natural reaction is to grab backstick as it hauls to the left. You can end up in a high alpha tight left turn wondering if you should ditch or slam the nose down and recover with the altitude left. You usually can, but you kinda have to "go for it". I ditched mine from 5 feet up since it had done a 180 in the 15-20mph winds and was hinting at heading towards a Pietenpol on the flightline behind me. So I chopped it and dropped the stick and let it drop in. Got a wee dent and a broken prop, no more.

Mike D

combatpigg

My Feedback: (3)

MJD beat me to it.

Even a tiny bit of left thrust is bad mojo. Some guys put a tiny bit of right thrust in just to be sure that no LT is accidentally introduced. Whenever I tighten any engine into its' mount, I always reef the engine to the right, even if the idea is to have a zero-zero thrust line.

I've never experienced this problem with any of my deltas, except for when I forget to program equal amounts of servo travel into all 4 quadrants of stick movement.

hotrodblazer

I have an older [10 years +] DD that I have recently pulled out of the rafters and started flying.Also a older one in the box that I plan to build and stuff my OPS 45 into.
What changes were introduced to stiffen up the later Diamond Dusts?Is there a link to the differences?
The one I am flying now has a ST45 on it,but will get a K&B 6.5 rear exhaust Quickee as soon as I get adjusted to flying it.

evan-RCU

850mA 3S LiPo running a 10A BEC. Should be good. If yo uare comfortable with balance and the other issues I am too. The only thing I have left is that yo uare powering the servos at 6.4V and this may be heating up the servos causing one to wimp out more than the other. The heat of the day being higher than previous flights may make this more noticeable.

I would still recommend that a normal NiMh battery be swapped out before tearing out the servos.