ORIGINAL: WhoDaMan

Hi Gordon:
Is it not true that a aileron servo will un-load were as a flap serv will not ,,,, thus the need for more torque on the flap servo. I also remember Johnny Hernandez losing his F-4 on the flap servo nylon craking and giving way thus the role on final and lost of his plane.

Dave




Rolling an aircraft requires a lot less twisting moments than doing a loop for example. You have two aileron servos working together to twist a plane in direction of flight. Flaps have almost an opposite function (almost) they are trying to lift a jet against flight path and they are bigger in surface area. The same for elevetor servos. With the price of these jets (6-9k dollars) it is almost silly to save a few bucks on aileron servos. I would use 8611/5955 everywhere on flying surfaces if at all possible. I am sure mathematically that would be an overkill, but better be safe than sorry in this particular case. The hitec 6985 at $79 is also a very good choice for any aileron...

Accually it was not meant to be a joke. Wouldn't be the first plane with catastrophic wing failure.....
I've done the calculations, assuming that the carbon rods are extending halfway into the wing, are solid (error-free) high tension rods with 600MPa bending modulo of 1/2" diameter each, one going all the way through, the other extending 6" into the fuselage.
This setup gives me a burst load of about 140-150lbs, or slightly more than 63kg to be more specific. Estimating an optimistic 22lbs weight, this gives the airplane 6Gs max performance....less than the original had. Everything above 6Gs load will be due to wing mounting....but from an engineering standpoint, this system is bound to fail at a very very optimistic 10G's max, if it were a real airplane it would be limited to 3-4Gs...
These are the textbook numbers i came up with, as an oncoming mechanical engineer in aeronautics, not the opinion of some guy wanting to bash skymaster....it's simply a concern after seeing the setup and going through the numbers.

Best regards
Hank

Hi Eric: Ok, thx. Will you have the #413 color scheme? When do you think the shipment will arrive in the us and what is shipping to California....

Aren't you glad you posted, Adil?
Now you have had your amateur aileron setup corrected, and now you have just had expert calculations showing that the wings are going to fold ANYWAY. Ahhh...the internet!

ET:

My comment on the "joke" was related to the original post about the wing mounting being "flimsey" and the impression that the spars did NOT go all the way through - I could be worng.

What I was letting people know is that the wing does have two spars and one does go completely across. Based on my experiences with SM planes, this seems definitely an improvement and feels a lot tighter than before. I liked the idea of the spars sliding in a slot inside a 1/2" former vs just on the side of it and bolted with some bolts.

I was wrong in my "guess" of the diameter of the Rods - It is NOT 1/2" but rather 14mm (I believe it is slightly bigger). anyway, I could have worded my rebuttal a little better and for that I apologize to Hank.

Regarding the Flap and Aileron servos, I have changed them to be the "Digi's" with double torque ratings than what I was originally planning

Adil

Adil you are right. 14mm is bigger than 1/2", and this boosts rod performance to about 9-10Gs total. Still not much for a jet model....considering that 9Gs are reached when traveling with 120mph in a turn with 1000ft radius (=2000ft diameter).
Especially on the F-4, large diameter rods can easily be used......25-30mm in the skymaster size. Considering that diameter flows into the calculation in the third power this is a huge hop in rigidy.

Eventually you have to decide for yourself wether or not to trust the system....i do wish you best of luck and wings that hold up :-)

Best regards
Hank

This is a big plane and pulling that many G's is not out of a normal flying session. Can SM share with us if simple high G testing was done to test the Stab and the Wing system? I personally want all manufactures out there to be successful and competitive, it provides us with safe models. I am seriously considering buying this jet or the CARF lightning jet but I am holding off at this time to get more data.. So, please modelers and manufactures out there, keep the constructive info flowing back and forth with the intent of providing the best and safest products that allows these companies to be profitable too!!!!

hey Adil, lets put my Eagletree G sensor in the F-4 and let me have a go at it I'll let you know exactly what the G limit is


Voy

We all know that in that case, the "G" limit will be just the takeoff !! LOL.

Seriously though - I doubt SM (or similar offshore companies) do any kind of sophisticated data gathering. I'm not saying that they just slap things together either and in SM case they do some stress flying.

My plan with this plane is to have a graceful big scale flier. I do some "wild" Flying satisfaction on sport stuff all the time.

I believe that the above comments are great and hopefully SM will read and update stuff (If they agree) as they get closer to full production.

Adil

I know for a fact that high G testing can be done very easily. Done properly and scientifically, improvements to air frame and hence increasing customer satisfaction can easily justify the cost...."IF" some companies are mass producing jets without proper engineering and testing they "may" put this segment of our hobby at risk..

PLENTY of companies are mass producing stuff with minimal testing and engineering, if any at all.
Is there a risk to the hobby? Sure.
But it's part of the price of the price.
Want something extensively tested for a year or more, with thirty years of experience behind it? Buy BVM. And pay double or more the price. And realize that sometimes even Ol' Bob misses something, and a plane goes in.
Some people are willing to pay the premium for all that testing. Most are not. Most of the time...most...it works out okay. I know Skymaster flew at least one of these, I saw the video on their website. Is it possible the wing will fail? Sure, it's possible. But if you want to be the first kid on your block with an ARF Phantom, you are taking something of a risk, yes. Adil knows that, I think. If this were a BVM ARF, it would be $10k or so. It would be better tested, sure, but very few guys would be willing to buy it. There is always comprimise...guys want cool jets for cheap. Nobody can afford to spend a fortune in time and money testing and engineering and still keep the dirt-cheap pricing, I guess.

Like I said in my post, I know it can be done for a lot less than one thinks. There is a balance between no testing/minimum testing and a year of testing extremes....Stress testing carried out with inteligent planning can be very cost effective!! and I mean well within the means of asian jet companies...

Just give Wojtec a copy .......He has his own STRESS test maneuvers.......
Semper Fi
Joe

Guess its time to bring out the sand bags, support the model inverted on some foam saddles, and do a static load test. I ladled out 5-lbs sandwich bags from 50-lbs sand packages (they're quite reasonably priced at the local home center) for a total of 24bags to test my 12+ Lbs all-up JHH F-4. I suppose 180-Lbs would have been more appropriate; but me & my test pilot are usually easy on the sticks If it was to fail, I thought having the wings snap over the garage floor was more salvageable than if they folded high in the air.

Cheers,

Phil

I think static loading done properly is ok, but G forces and dynamic forces measured while the plane is flying is more valuable

Dave,

Can you crunch the numbers for the Yellow F-16? It's got two solid carbon rods for spars. The front one is 6mm and only extends into the wing about 4", the aft one is 10mm (I'm pretty sure--this is off the top of my head) and extends about 5.5" into the wing. The wing is relatively thin, but has carbon cloth laminated between the foam and balsa skin. Both spars extend into the fuse about 3.75" into clamps.

Thanks...

Hi All

I agree with what Ehab says I think that it is very prudent to test fly the airplanes and stress them beyond what you would normally see during flying.

We have a team of volunteers in Hawaii that works with Eric of Airpac Models to test fly all of our new stuff. I almost have the Skymaster 1/6 F-16 ready to test here as an example. Many people just don't hear about what we do here because we just don't bother posting every little thing that we do. Most of us have full time jobs to pay for the jet stuff and we just get to darn busy between the building and work . Our main goal it to make sure that the things that Airpac sells are safe within reason, of course we cannot afford to test to destruction so we try to push it as much as we can with the aircraft. If there are some big problems we usually find them. There have been several design changes to Jets that Airpac sells due to issues we found during our testing. Other dealers that sell the same kits as us also were involved in testing, Ali from the UK, Morne from South Africa.

We also do low speed flying testing, stall behavior, hi AOA behavior, CG optimization. You can do a reasonable amount of testing for low cost I agree.

In all its a lot of work for us here but we get to fly some cool jets, and it's sure great to get a new jet every couple of months! Having said all of this, we have not gotten an F-4 to test yet, or the L-39. I’m waiting like the rest of you to get ours!

Nice to see you on RCU again Ehab!

Aloha Duke, how are you?

I truly want all the jet manufactures out there to be successful for selfish reasons...I want more quality ARF jets. I do, however, expect them to do quality building and testing. Most of the jet guys have $5000 to $20000 or even more De vested (not invested) in these things so we want to keep them flying. Spending a little time and money upfront in quality testing will go a long way. You guys need to think Looong term business strategy, not just a few models worth!! So keep up the good work.. That F4 looks really good!!!

Well, accually a lot of the "expensive testing" can be omitted by doing the paperwork properly. Stress calculations on the type of connections used on model airplanes (usually rods, tubes or flat steel stripes) are very very basic. Calculating a wing rod takes about 5 minutes plus a letter size sheet of paper, plus about 20 minutes research to find the specifications on strength and stiffness of the individual material.
Doing that you can be sure your airframe holds up to your specs, and can test it without having to worry about loosing a plane because of underdimensioned components. Yes it is more work, and yes you have to invest some into someone who is willing to do the math, but all in all that will be cheaper than loosing just a single test airplane. Static load testing will give very realistic results also (to verify calculated numbers).

Best regards
Hank

Dear Hank,

I would like you to calculate the load on the Comp-arf mig 29! It uses a single 15mm spar for wing. The model weight 50% more than F4 and uses double the turbine power!

I have seen Thomas Singer fly this beast to the limited in Germany! And yet i have to hear of an in flight explosion.

The F4 uses 2 x 14mm carbon rods. Total 28mm. One in front and one at back.

The elevator uses 4 bearing mounts.

Skymaster have tested this model with P-180 power and is satisfied with results. But if you do i vertical dive and pull full up elevator, there will be no gaurantee that it will last! For normal scale and aerobatic flying it should be fine!

Regards

Morne

Dear Morne,

if you give me detailed dimensions, materials and perhaps a picture of the wing mount i'll be glad to go over the numbers. I don't know my way around the FC/Composite MiG because i've never seen it disassembled....there are several constructions out there where the rods only carry about 20% of the load, the rest is induced over a different type of wing mount similar to that of full scale aircraft (which, by the way, usually do not have rods running through the plane at all, but in which steel bolts are used). These different types of mounts cannot be directly compared to one another....

Your equation "2x14mm=28mm" does not work. The loads induced on a round spar/rod climp to a fourth power to the radius, or the distance from the "neutral phase" to the point furthest away. Adding all other components to the equation (e-module, geometrical components, max bending module...) a third power to the diameter remains. With a 14mm rod, the highest radius you will get is 7mm, no matter how you turn the dice.
Two 14mm rods will be able to support twice the load of a single one. A 28mm rod has twice the diameter, thus 2³ or 8 times the burst-strength of a single 14mm rod!

As ET said, hardly anyone goes over the numbers. Some have a goot "gut feeling" on dimensions....others do not (those companies are usually those found in the forums when wings fold, control surfaces flutter, landing gear is worn out prematurely). Some do the math, thoise are usually the companies that charge a lot.
In some cases it will hold up to the personal "normal flying", with others it may not....we will see sooner or later. I just wanted to show with the help of some numbers that the wing mount appears to a weak point of the airplane, and appropriate care should be taken.
These are the numbers from an engineering standpoint. 10G, 12-13 with an excellent fastener. Whatever you do with these numbers is up to the individual reading them...

Regards
Hank

The problem with doing stress calcs like this are many. Sounds like what Hank is talking about is doing simple Mc/I calcs on wing spars and comparing that to published ultimate strength values. Well, that works OK if you have a simple spar in bending made out of the exact stuff that was tested to come up with the properties - but that is rarely the case. You REALLY need to build extra spars and do some destructive testing to see what you really have. And to be statistically meaningful, you are gonna have to test a lot of them. We pretty much never have a simple idealized spar in bending. We normally have some compression of the wing root that helps take some of the bending loads too. Then there is the issue of a spar section's stability failure (on the compressive side), that could drastically weaken a spar. If all you are doing is looking at a published Ftu, then you have totally ignored that possible failure mode (for thin spars only). Then there's the issue of estimating the loads on the spar: not always straight forward! You are assuming a max G load, assuming an aero center locale, and so on. Heck, we have an entire Loads & Dynamics group just for this and NASA's Level II double checks it all independently!

I am not saying that you can not do an accurate stress analysis of the spars, all I am saying is that it is easy to do it, but not easy to do it accurately. You have to know a lot of inputs to a high pedigree for the results to be accurate. Otherwise your errors are compounding errors. Usually, however, the errors are conservative and the answer will show that a spar that works just fine in real life will fail prematurely in flight.

Woketman you are right, there are a lot of different factors. These make it really complicated for full scale layouts that have to be approved by authorities. But laying out a spar for worst case scenario (no support from fuse, loads induced at FCL and WS/4) will get you in the ballpark plus safety. I've taken average strength of market-usual HT carbon and placed them in the standard ME formulas for 2-dimensional loading. This gets you a good impression of where you are, and a test on a crashed wing (not damaged on critical points) has shown that using this method you will be within 15% of the actual burst load....close enough for 99% of all modeling needs.
Also, using average strengths published, you will probably land somewhere near the stuff used by most companies...

Interesting conversations. I am now wondering how much of the G load is actually carried by the wing. Isn't a portion carried by the fuse? If so, wouldn't that mean there is less stress on the spar?

Mark

Absolutely Mark. As I said, usually you end up on the conservative side, analytically.