KJKimball

Virg and Motor,

Curious of how you define "design flaw" and in as much, what testing you base you assumptions of pending failure in this case. As an engineer, I am sure you are aware, there is always give and take in structural design. A "flaw" as indicated by improper material selection, lack of continuity or misalignment is a far different thing than chosing to allow a given amount of torsional flex or displacement in an effort to control total part mass, etc. It may be possible that in this case, the structure, as designed, functioned well with an acceptable level of torsional deflection which remained in the plastic region of the materials involved resulting in no failures. Also, the responses from you two guys seem to bounce between the elevators and stabs when referring to the flex. For example the motor's post #124 in which he refers to "spreading the torsional load further into the stab" which seems unlikely to be true when installing a longer rod in the elevators. Now, increasing the torsional stiffness of the elevator pairing may be possible with this modification but I doubt the stab gained stiffness by doing this.

Virg, you made your change because it made you feel better about the parts. Cool. Did you flight test it before you did the change? Maybe not. Maybe just a gut move. That's fine for you and fine to suggest it to others. But, to disagree with the design of a structure is not the same as a design flaw.

I don't have one of these airplanes. Not a NP or QQ version. I am just an observer in this who is also a mechanical engineer as well as designer of full size aircraft structures. I have designed a few model structures as well. I didn't feel a need to jump in until you two guys basically started to call Octane a dummy (getting close to crossing "Please resist the urge to curse, flame, degrade, insult or embarrass someone in your post" instructions for this forum) and in the same post screwed up on which part of the airplane is the center of this discussion. You guys might want to be more careful and make accurate statements when distributing advice on intelligence displays else you too can seem to have missed the smart boat. I agree with the intent of the last line in post 124 by motor just not the person to which it was being directed (stab/elev)

Build your airplanes, enjoy them, buy more.

Kevin Kimball
www.pittsmodel12.com

mrbigg

My Feedback: (21)

I had the same problem with the elevator half when I built my Balsa USA Fokker DR1. They had you using a dowel to connect both halves. They didn't mention anything about pinning the dowel, but I knew better. I also went ahead and fiberglassed that area. Better safe then sorry.

thevirginian

Kevin;
I have to agree with you in one point, that I haven't test flown this airplane yet and thus not encountered any mishaps, structural wise. The reason I decided to alter the designed stab connection was based on the facts that the provided CF-rod was only extending into the stab ends by approx. 2". The manuals call for the use of CA and not even epoxy. To me it was very clear that this connection would have caused a dangerously weak link between the two halfs. We should be aware of the significant forces effecting the control surfaces of such an aerobatic airplane. One one hand the manuals call for high-torque, fast servos with a minimum torque rating of, let's say, 80-100 oz. or better and then expect a short rod that is just super glued in soft balsa wood to withstand them. My inner voice told me to make a change here. And that's exactly what I did. For any other type of airplane such a connection probably would be O.K. but not on a high performance 3D plane. After the epoxy cured thoroughly I tested the connection by twisting both halfs and was still able to notice a small movement or flex, as you may call it also. But I think I can live with that. I know I did the right improvent on my Yak. One thing else, Kevin: I have seen stock ARFs fall out of the sky, simply because the owner went straight by the books, so to speak. An experienced modeler will always find areas and room for "improvements" on any ARF kit out there. There is no such thing as a perfect ARF kit, or name me just one. Regarding octanhuffer: Haven't you noticed in all his previous posts, how sarcastic and negative he treaded me. I haven't said one mean word against him to justify his actions he exercised toward me. I always try to obey the rules of this forum and, so far, haven't been reprimanded or warned, yet.

BTerry

My Feedback: (8)

Are your shear webs oriented longitudinally as in the other pictures? Just curious.

noahb

My Feedback: (67)

and just because QQ designed the QQ Yak and this one might POSSIBLY be the same one, does not mean that it can or can't have failures. Prime example would be BVM Jets. Wonderful design, top notch materials, etc... However, there has been a few of his kits that have had addendums written to either prevent possible failures or correct issues (possible design flaws). You of all people should know that in engineering, you can't cover everything and that you hope you design a product with a built in margin of safety. I am not a Mech. Eng. but an electrical engineer.

Keep in mind that when this Yak was designed, it was designed to be very light, yet able to handle flight loads. However, in order to keep weight down you design a structure to withstand the loads typical of aerobatic flight. However, you end up having to "skimp" in areas to meet the weight design. Is the elevator in stock form a design flaw? Who knows. If it does not ever fail, then prob. not. However, if you add 1/4 ounce of weight to strengthing the elevator halfs, what does it hurt.

I for one think that either way is good. Why not dist. the twisting to both the "root" rib and the first rib out, instead of just the root rib.

noahb

My Feedback: (67)

Has anybody looked inside the QQ Yak wings to see which way the grain runs on the shear webbing?

I know china products have a very hard time with this. Several H9 kits have had this issue and I am sure these kits are not immune to this problem. And please don't mention quality control with the QQ yak, with the landing gear problem, control horns. Every kit mfg. has there issues and QQ is no diffrent. It's the customer service that supports this stuff in the long run that matters.

octanehuffer

noahb

My Feedback: (67)

It was mistated to the stabs. The stabs are not what they are concerned about. It's the elevator halfs themselves in relation to each other, not the stab. And yes, it does provide better torsional strength then if it was just glued to the root rib.

If you remember the old single aileron servos used that had the "torque tube setup" with the wires that were bent from the aileron out, then up and over so that the aileron servo could hook up to them. Well, if those rods were made of say carbon fiber tubes and they did not twist but very little (as we all know the metal torque tubes where not very good) where would you want them to anchor into the ailerons? Would you want them say an inch from the root? or would you want them to go out further to say 1/3 the way? May not be exactly the same concept, but it's close and serves the purpose of what he is trying to accomplish.

KJKimball

Virg,

Thank you for your response to my post. To reinforce my point, I was not addressing any specific point on any design including the 69" yak in this thread. My post was specifically aimed at the use of certain phrases such as "design flaw" in the context in which they were being used. That being a written tone of fact not speculation. Further, I wanted to point out that the discussion you and motor were having with respect to the elevator connection rod somehow shifted and began to reference the stabs instead of elevators. Your latest post in response to my still refers to "stabs" not elevators. I think this added to the confusion on this by octane and others while you guys may not have realized you were not longer referencing the elevators. It was sort of ironic that motor made a statement to octane about showing intelligence while making this mistake and then you echoed his words.

Virg,

Educate me on the structure of the elevator (or stab as you call it). You state the original carbon rod is inserted 2" into the elevator. What portion of that 2 inch insertion is in balsa? In other words, how big is the balsa section that holds the rod and horns? If the balsa is wide enough in the spanwise direction to incase most of that 2" of rod you refer to, that is a pretty large glue area compared to the 1/4"-3/8" added block you placed at the end of the longer rod. The added end ribs won't see a torsion load until the bond at the original inboard rib fails. Now, since the rod is displaced aft from the hinge line of the elevators (stab), the rod is actually primarily loaded in bending when transferring load from one elevator to the other rather than pure torsion load had it been located in the leading edge of the elevator (stab) concentric with the hinge line. Your longer rod may stiffen the elevators in bending since it is like adding a spar to the structure but really doesn't do much for additional torsional reinfocement of the original glue joint.

Noahb,

Again, read my post #126 and note that I do not address any design features, real or speculative, for the QQ or NP design or airplane. My post was intended to point out that while some posters were presenting themselves as a elite, they too were making some techincal reference errors. You are right. I do know that in engineering you can't cover everything. I will go on to say that those things that can't be covered by engineering are usually covered by experience in that field. Manufacturing errors and flaws are far more common than design flaws. Point being the shear web grain you refer to as being an issue in China, not the fault of the designer.

KJKimball

Noahb,

I know the reference to the stabs was a misstatement. Motor and Virg didn't.

You brought up the exact point about the rod placement to which I referred in post 134. We were drafting our posts at about the same time. Your aileron torque wire example is perfect to show the difference between torsion of that wire and that of the bending in this rod location aft of the hinge line. As long as the elevator hinges are glued in place, the two elevators will have little chance to significantly rotate relative to one another about the carbon rod axis without a fore/aft displacement.

noahb

My Feedback: (67)

I agree with you that most design problems are not relating to the engineer, but the materials and mfg. of said products. In this case, QA in china or lack of. I can see that the torsional load dist. to the ribs in very little as yes the rod is not on the center hinge line. The material that the rod is glued in is ok. It does cover enough of the rod. The problem is that that the elevator MAY not be strong enough to handle the loads without the addition of the longer rod. Yes, the rod acts more like a spar in this case then a "torque tube". However, it serves it purpose. Not sure if the orginal design will serve it's purpose or not. As of today, I have not heard anyone that has had a problem. However, this is a new kit and time will tell.

octanehuffer

Kevin. That second paragraph in post 134 and the post of 135 are the same that I reffered to in posts 132 and 118. Then came all the "I'm an Engineer" stuff. I dont get it.

thevirginian

Kevin;
I am truely sorry if I confused you and others with misinterpreting the elevator halfs and the stabs. My bad[] Sometimes you get carried away with a mindful of thoughts you want to write down and then use the wrong words. I always meant the elevator halfs. O.K. since we got that cleared out of the way, now back to the real topic: The CF-rod goes into the elevator end (closest to the fuse), which is actually not a full block of balsa. It is rather built up. When I stuck the short rod in there I could feel no good fit inside the hole. In other words it was a rather loose fit. Thats' when I desided to go with a longer rod and glue it to the first rib. That way I distribute the twisiting torque that the elevator half is subjected to over a larger area. Ideally the rod should have spanned from the inner end to the wing tip of the elevator half, but I didn't want to open up the whole thing.
Regarding the main wings: Yes, the webbing goes parallel to the spars and not vertically as it should. What I am going to do is simple: I will run some thin CA over the webbing to strengthen it. That should take care of that. A buddy of mine lost a $4000.00 airplane a few weeks ago because of something similar. He flew a brandnew 35% H9 Extra that folded its wings midair by itself. HH confirmed a design flaw in the wing structure. Can happen. At least HH stands behind their products and his loss is not as bad.
This is what a building thread is all about: You tell your fellow modelers about the experience you encounter while building a certain kit. Then it is up to the discretion of each indivitual to either heed your advice of gaff it off. For me this issue with the connecting rod was clearly a design flaw although I have no proof of any mechanical failure. For me it doesn't have to break to be considered a flaw. If it doesn't work the way it suppose to it is already a flaw. But that's me and the way I look at certain things. I hope you don't shoot for that.

Iflyglow

My Feedback: (79)

I made the mistake of saying stab, when I intended to say elevators. I agree with Virg, that there does not have to be a failure to say that there is a design flaw. This Elevatordesign has a higher probability of failing than not failing, and it does not matter if it says QQ or NP on the box, since they are of the same design.
The only reason I ever mentioned anything was because of the constant flaming that Octane was giving Virg.[:@]

For example post #117 from Octane

LOL. A mechanical engineer skimping cost on an airplane to only have to reinvent the wheels?

noahb

My Feedback: (67)

Octanehuffer, I know you have the QQ Yak. Would you mind looking in the root of the wing and see if the shear webbing is running the right direction? Perpendicular to the spar or is it running with the spar (grain that is)?

thanks

octanehuffer

The webs are horizontal. I am wondering why it matters. The wings are fully sheeted.

noahb

My Feedback: (67)

From what I understand, the shear webbing bonds the upper spar to the lower spar. When the grain of the shear webbing runs horizontal, it does not add much bonding strength as the load carrying ability is now in the horizontal plane. When the grain is run verticle, the up and down movement of the spars is alot less because it is running against the grain. I know I did not explain that well, but I hope you understand. Its the diffrence of trying to split the piece of balsa along the grain (easy) and spliting it against the grain (not easy).


There have been a lot of planes (I had a H9 Edge 540 that had the problem and people where loosing there airplanes, H9 came out with a replacement wing set) that the wing has folded because of the grain. Not sure it will in this case as QQ sure did fly the heck out of it.

Iflyglow

My Feedback: (79)

Noahb,
You are totaly correct. The shear webs are supposed to be vertical. Shear stress is a stress state where the stress is parallel or tangential to a face of the material, as opposed to normal stress when the stress is perpendicular to the face.
I have seen this on other ARF'S to, and it is scary, and yes H9 had serious trouble in the past with failures do to incorrectly installed shear webs, and just recently had failures do to the fact they forgot to glue the wing tube sleeve in the wing.

octanehuffer

Oh yeah, I know all about the web and the grain orientation, but this wing is fully sheeted from rib to rib. I wonder what a profesionals opinion is(Kevin). I know my 86" Yak is the same. I wont worry about it one bit. I have seen these planes fly beyond their projected capabilities as well as fly them beyond.

noahb

My Feedback: (67)

well, it's just a lazy way to build stuff. However, if it works I guess it's ok. however, wish they would do it correctly.

thanks guys.

Iflyglow

My Feedback: (79)

Octane,
Any so called expert will tell you the same thing. This is a China thing.[X(]

FYI,
The Aeroworks planes do have them in the correct orientation.

thevirginian

Well, even though you wonder, it does matter, octanhuffer. Motorman and noabh explained the reasons for a correct grain orientation more than adequately to understand the principle behind it. Of course it helps if the wings are sheated completely, but even that is no guarantee for preventing a disaster. Like I said, I am going to drip thin CA on the webbing and strengthen the balsa wood that way. I may even put some small pieces oc balsa strips in the root section as far as I can reach in. I wonder what Kevin's response is going to be. Here you have a classic case of a design flaw and there was, obviously, not even one crash related to it, yet. At least not reported . Even QQ must have missed this problem at QA [sm=spinnyeyes.gif]. BTW last night I tested the strength of my elevator halfs for torsion after I hinged them the evening before. I feel much better now, because there is almost no twisting to notice anymore. I know I did the right mod by lenghten the rod inside the elevators.
MOTORMAN and noabh, thanks guys for your responses. [sm=thumbup.gif]

mvigod

We are locking this thread due to evidence having been provided which clearly illustrated the NitroModels 60" Yak is an exact copy of the Quique Somenzini 69" Yak design. Upon inspection of a Somenzini Yak and Nitromodels Yak it is clear that the construction down to every detail is identical. Additionally the manual was copied from Somenzini including their photos and scheme appearing in the manual. Only minor differences existed such as a CF wing tube and covering/scheme.

The manual as well as the design and CAD drawings are all copyright to Somenzini and unless we receive information which contradicts our findings above we will keep this thread locked and ask everyone kindly to please not start a new thread on this Nitromodels Yak 69" here on RC Universe. We take copyright's seriously as they are a significant part of the protection afforded to manufacturers for their personal investment to bring a design to the marketplace.

Thank you again for your cooperation