Phoenix 6 Meter K8 - Page 25 - RC Groups
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Sep 05, 2012, 11:47 PM
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A meaningful load test on a wing should be done with a few critical points sorted out.

Firstly doing a test by hanging the glider from the wing tips is not representative and will over stress the structure even at very low loads. Furthermore, by doing so it might cause irreversible structure damage that might lead to catastrophic failure when the structure will be loaded again (even with only normal flight loads).

When doing a load test you have to realize that if you go over the max load, you might not destroy the structure but create permanent damage. This might eventually end in a catastrophic failure even at a much lower stress level.

When doing a load test it is as critical to monitor deformation(s) than monitoring the load(s) levels.

The load distribution on a wing is (more or less) elliptical. The normal load on a wing is equal to the mass of the aircraft for strait and level flight. If not doing aerobatics or extreme flying, for a glider a +6G -4G is a good base to work from. It means that you want to test your structure at 6 times the mass in positive attitude and 4 times the mass in negative attitude.

The span of this specific glider is 6 meters. After drawing the ("ideal" lift distribution) ellipse using the wing span as the big axis (to scale: again 6 meters in our example) you create stations every 30 centimeters. You measure each segment between the big axis and the ellipse. Adding them is "equal" to the mass multiplied by the load factor. It is now easy to deduce the load at each station. NOTE: you will have zero load at the tips and the max load at a given station right in the center of the fuselage.

Now for the load test itself: you need to mount the wings on the fuselage, turn the assembly upside down and support it on the fuselage (top of) as close as possible to the wing mount area. Ideally you need to load each station from the root to the tip (on both sides) progressively and equally in a way that is as close as possible to an elliptical distribution. The best is to use sand bags (or lead bags) that are placed on top of the spar at each stations. 100, 200 and 500 grams bags should do the trick.

IMPORTANT: planning such a test is a prerequisite. It is NOT a one man show and a three person crew is a minimum. Ideally you need 1 person in charge telling one right wing loader and one left wing loader the bag mass and the station to be loaded while one person at each tip measures deflections. And KEEP written records. A good video of the process might help for the post analysis as well.

It is of no use of trying to speed trough the process: take your time and measure very carefully the deflections at each tip. Make sure you have plenty of tip deflection measurements especially when loading from 0 to 2 G's. This will allow you to monitor deformations once the loads are heavier: as soon as the deformations are not proportional to the loads the structure is getting to its over stress load.

Of course you should keep your ears open to monitor any "suspect" sounds beside the normal "moaning" of the structure under load. Once you reach the maximum load you have to unload the structure by exactly reversing the process. Repeat the operation for the negative loads, the glider being now resting on its belly.

Disclaimer: this method is good enough (and safe enough) for model application. This is not a scientific test but it works and has been used successfully even for full scale gliders. Nowadays, the use of waffle tree and/or hydraulic pistons linked to computers allow to recreate very representative flight loads and monitor accurately the behavior of wings under stress all the way up to catastrophic failure.
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Sep 06, 2012, 05:53 AM
Registered User
I don´t understand the problem. Neither the original Ka 8B than this model is designed for aerobatics. Admittedly the guys in the Phoenix video are flying loopings. Regarding guarantee, they're completely stupid. Please make a big stamp on the package " No areobatics - No bodi bill - No beginners" and every thing is fine.

Thermal scale flying the model do very very well. The models cost for the size is only a few bucks and thereof are maybe 5$ for the gluing people. You only get what you pay for.

Everybody who´s buying ARF must know the risk of possibly bad finish. It is everybodys own decision and risk to fly without previous control.

Sorry for my clear words.
Last edited by axl*; Sep 07, 2012 at 07:30 AM.
Sep 06, 2012, 10:16 AM
Registered User

The pull ups you were doing in your video under power with your K8B were much more severe and at higher speed than what I was doing when my wing failed. Yes my intention was to perform a loop but my wing failed the moment I gave the plane up elevator. To say there is not a problem with the plane is just plain wrong. It is obvious that some are better than others as Roscoe and others as well as the guys on the video on Phoenix Models site have successfully looped their models several times. As you mentioned if a model is not rated for Aerobatics it should be labeled as such and a warning sticker put on the box as well. If my plane had come down in a crowded area instead of a wide open field someone could have been seriously injured or killed. Also, why do you feel it is stupid to think that Phoenix Models should not be responsible for warranty or guarantee on their product. Not sure how much these cost in Germany but I am out over $1,000.00 for plane that I got to fly maybe 7 - 10 minutes tops before it failed and crashed destroying the model. I can assure you if I had known that I was spending a $1,000.00 on plane that was not airworthy I would have saved my money a little longer and bought a LET Models ARF and gotten known European Quality Construction. Not much savings involved if you have to strip the plane down and spend hours of modifications to it to make it safe to fly.

See Ya,

Sep 06, 2012, 02:23 PM
Registered User
Pat, please do not get me wrong. I know the feeling loosing a model. The problem is the ARF way itself. The construction mostly is generally ok, but the build quality is not veryfied enough. ARF RC models worldwide are not made with industrial standarts. The final quality is only a question of cost. Regress against the manufactores will fail, cause it is almost in fact impossible to prove the guilt there. It will be always a pilots fault. In Germany the Ka 8 cost ist 699,- €. German bulid scale planes are up to ten times more. Many, many years ago grandpa says "Never use a model that´s not build by yourself". Wise man...
Sep 06, 2012, 08:57 PM
Registered User
maybe I spoke too soon...
Asia is Asia you need to go in with both eyes wide open, I think it is human nature to want to buy cheap then demand premium support when something goes wrong.
Well you can bet your booty Great planes will not bring more of those in country for a while. How they extricate themselves from a complicated public relations/warranty situation will prove interesting.
Last edited by DanJudson; Sep 06, 2012 at 11:43 PM. Reason: fact check
Sep 06, 2012, 09:23 PM
LSF303-AMA Fellow
tkallev's Avatar
They'll probably be available through Hobby King in about two weeks
Sep 06, 2012, 09:29 PM
Chicago Mayor: A Terrorist
pilotfrank's Avatar
If anyone is interested, I'm thinking about selling mine. Not because it's junk, btw. I would still finish it after checking the wing underneath the cover if I had more time.
UPDATE: Got a down payment on it today, so I will not finish mine. Should bring it to the Coachella event in November to finalize the deal.
Last edited by pilotfrank; Sep 08, 2012 at 06:03 PM.
Sep 06, 2012, 09:47 PM
SZD16's Avatar
This plane is made in Vietnam .....not China.
Sep 06, 2012, 09:56 PM
Lbuff1's Avatar
Mr Wood never met Mr Glue. And introduction is in order.
Sep 06, 2012, 10:00 PM
SZD16's Avatar
Or Mrs. Glue for that matter either!
Sep 06, 2012, 11:40 PM
Registered User
Ho Chi... I didnt realize it was made in vietnam.
probably a semantic difference at best.
Sep 07, 2012, 07:17 AM
Registered User
Guys, I'm kinda amazed at some of the more recent discussion. It's like the K8 is one of the very first instances of poor quality out of Asia. Perhaps memories are short but I clearly remember very recent and significant, widespread catastrophic problems with 2.4 radios.

I've personally witnessed 4 loss of signal failures with JR Spektrum radios, with the latest one being at our club picnic last Sunday.

And there have been dozens of reported 2.4 problems with outright loss of signal and resulting total destruction of plane, radio and engine. All Asia made products.I don't recall modelers requesting reimbursement of all destroyed RC you?

So what's happened? Does the industry shut down? Does the AMA ban the offending gear? Do modelers pick up their picket signs and protest? I don't think so.

In fact, the truth of the matter is we may grumble a bit and then buy more of the same stuff hoping the U.S. importers make it clear to the Asian manufacturers to fix the problems in the next shipment.And not everything gets fixed the next time around. Who knows how many iterations of Spektrum and FAASSSSTTTT gear there have been over the last few years in an attempt to deliver a truly reliable radio?

R/C historically has been fraught with products that have failed including radios, batteries, switches, airframes, etc. The list is endless.

The situation with the K8 is no different. If there was widespread popularity in large scale aerotow, Hobbico would probably push the manufacturer to fix the problem. But since we're such a tiny niche market, they'll probably just drop the model from their line.

For the life of me I can't figure out why some apply a different yardstick to this situation vs. the many others in this hobby.

I'm not trying to rationale or excuse the problems we face with this model. My point is that the situation is no different than many we've faced in the past. At the end of the day, this is a high risk hobby. If you expect perfection, this is not the place to be.

My 2 cents,

Last edited by SteveR; Sep 07, 2012 at 07:35 AM.
Sep 07, 2012, 08:12 AM
Registered User

When I spend over a $1000.00 on a product I do have certain expectations of the product. Did I expect it to be perfect, heck no I didn't. Did I expect it to be as airworthy as the plane in the video on the manufacturers site advertising the product? Yes I did. Do I expect Phoenix Models to pay for my radio gear or hours of labor I spent assembling the plane? No I do not. It would be great if they did but it is not expected. Expecting them to warrant their product is not out of line. If they choose to bring a product into our market to sell, then they should have to expect the same expectations their customers would have of any product made in the USA, Europe or any other place. I am sorry but we cannot and should not excuse them of their responsibilities just because the product is made in SE Asia and sold at reduced cost. Thinking like that is just plain dumb on our part. I never expected this plane to be on par with a LET Models or a Bayer CNC Model as far as overall fit and finish, but I sure did not expect it to be what it turned out to be either. My hope is that when all is said done that I either get my money back or a new plane with redesigned wing. Of course my expectations may seem way out of line to you.

See Ya,

Sep 07, 2012, 06:26 PM
Registered User
BobK3597's Avatar
I agree 100%. I have not even completed my K8 and expect Phoenix to provide a redesigned wing at a minimum prior to any attempt to fly.
Sep 07, 2012, 07:08 PM
Registered User
Bob it will fly fine ,
You saw mine and Toms at the SKSS
Just no loops till you reinforce
I did some reinforcing when I added the double spoiler
Had a hard hit trying to win the spot landing prize and just some damage to the bottom of the fuse no damage to the spar

Fly it , I will gladly tow it up
Enjoy the plane as it thermals well

I was going to strip mine this winter
Planned that before I ordered it as I need to scale the plan up

I plan on testing strength on the mountain next weekend

Weather looks awesome for Cumberland next weekend!


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