Dave Presta

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Im sure most of you by now have seen the video of my Yellow F-18 coming apart in flight:
http://rc.ridethc.com/video/dave_f-18_failure.wmv (from page 6 of Best in the West Pilots thread)

Here are the pictures I promised of the failed parts. I know that alot of you will have your own oppinions and have alot to say, but let me just start by saying that I purchased this plane already built, it is the new kit with the epoxy fuse and carbon reinforced wings, very light, and it was powered by an Artes Rhino. As the video clearly shows, the front former let go first then the rear spar bent up and snapped at the weakest point wich just happend to be where the builder drilled a hole in the center for the flap hinge. I agree that drilling a hole in the spar is not the greatest idea, but this did not cause the failure on the plane. The failure was from where the main bulkhead thins down to wrap around the intakes. From talking to respected guy's who are flying this plane succesfully, they say it is mandatory that this part gets reinforced. It is easy to see how true that is.

Shaun Evans

The supplied bulkhead is now of a thicker, much higher quality plywood, and it IS reinforced in that area with carbon and another ply sub-bulkhead. For good measure, we recommend that the builder consider whether or not even that is enough for his particular installation. If you have any questions about it, PLEASE give us a call!

MLaCoste

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Dave Presta

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Sean, with all due respect I have looked at 2 other twins that were built after mine and have the same bulkhead installed. I agree that this needs to be reinforced, but are you telling me that all these builders threw away the stock ones and made there own weaker bulkheads? This thread is not to discredit the Yellow kit, this has been one of my all time favorite planes since I first got into jets. I started this thread so that people who may be building this kit know where they need to deviate from the plans. If there was information like this available prior to me flying my plane, then maybe I wouldnt be out $13K!


Dave

MNModels

Hi Dave,

Thanks. You just saved me a couple of Gee's! I will reinvorce my former immediately. I have an original twin DF converted to single Jetcat P-160. Sorry for your loss.

Regards

JET FX

Hi Dave, I have not seen the video unfortunately! as for some reason when I try to download it I only end up with audio and no video??? therefor could you please possibly briefly describe for me what the circumstances were ie maneuvers etc leading up to the failure? reason I ask is I will be getting a twin soon and curious to learn as much as possible...

Cheers-

mr_matt

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Just a dumb question, but is this cutout for the DF version, or there for the twin turbine?

Cause no matter what the bulkhead is made out of this cutout makes it a lot weaker.

Dave Presta

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Marty, the video shows the plane in a right hand bank turning from downwind to final. The right wing breaks free and folds up and hits the fuse at such a force that the plane immediatley breaks apart. You really have to see the video, go try to open it on another computer.....its worth the effort. I will answer as many questions as I can, but there are many guy's out there with more experience with this plane that can probably answer some of the building questions better then I.

Dave

Dave Presta

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Mr. Matt,
I have no idea, maybe someone else can answer that question. But there was not much clearance between the split intakes (2-1), and that bulkhead so I assume that it was correct.

mr_matt

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OK I never though that this cutout was needed to clear the inlet for a single, never mind!

Shaun Evans

Dave,

No disrespect taken at all. I know what your purpose is in starting this thread, and I'm glad you're doing it. As I already said, I feel terrible for your loss, and am willing to help in any way I can.

As for the bulkhead issue, I'm not sure whose two planes you're looking at, but if they don't have a 3/16" 8-ply aircraft plywood bulkhead that's backed up by a 1/8" 8-ply doubler with carbon sandwiched in between, then they're not what is currently supplied with the kit. I can't say what the builder did or didn't do, but I can say that if he was building this airplane for turbine power, and not for the powerplant installation that it was designed for, then he should have paid some attention to that area. Which, incidentally, is the point of your thread, right? To point out to prospective builders/pilots of this aircraft why that area should be looked at if they decide to modify the kit for turbine use?

I'm no engineer, nor even a particularly talented builder, but I knew enough to reinforce that area on both the kits that I've owned. As Matt stated, that area is too narrow to be left alone, and I added carbon plate to the opposite sides of both bulkheads where the spar receivers attatch. I advised Kenny (and everyone who has ever bought one of these kits from me) to do the same when he began building his. Aside from the notice on the front page of the manual stating that Yellow Aircraft has no control over the assembly, modification or use of the kit--there's also a sensible, well-established, industry-wide expectation that if people are going to be building/flying at this level, then they should know enough to make sensible and prudent choices when it comes to building an aircraft kit for use outside of its intended or specified design. It looks to me like your builder didn't do that. I wish he had.

For the record: Yellow's planes can be easily flown on turbine power. Most of these require mods, though. Yellow Aircraft has TWO kits that are purpose-designed for turbine use. One is the Stingray, the other is the F-15. In those two cases, the builder can expect that the airframe is good-to-go AS-IS, with no strengthening/modifying necessary (assuming he stays within the engine thrust range). Outside of those two kits, builders need to understand that they are modifying the kits for turbine use. Now, this can be done easily and safely, and with engines that are WAY more powerful than what the designer ever intended, but the builder must make some choices and changes that are sensible and prudent. There are several of these models flying with single and twin turbine installations successfully. In fact, all of Yellow's jets have been or are being flown under turbine power.

Gordon Mc

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Given how marginal the two twin DF powered F18s were that I saw fly, I'd honestly be rather surprised if many of the kits of that model that Yellow has sold in the last 3 or 4 years are destined to be flown under DF power. With that in mind, does Yellow provide a 'turbine upgrade" kit, or publish a list of "minimum upgrades reaquired for turbine use" (or even officially "approve" someone else's list) ? If not, it might be something worth doing.

Gordon

Shaun Evans

Gordon,

You're probably right about the DF use. Unfortunately, in this day and age of everyone looking for someone to sue, it's hard for manufacturers to run the risk of making such statements or endorsements, especially when they have no control over how well someone builds or follows directions. Fortunately, there are avenues like RCU for experienced modellers to share their wisdom and ideas.

EddieWeeks

Its been my experience, that the front spar on a wing/stab/rudder need to be Much stiffer
and stronger than the back spar...

And the center of twist of a wing, needs to be in front of the center of lift..

Hope this helps..

Eddie Weeks

mr_matt

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Hi Eddie,

When you say center of twist, do you mean like the center of the wing pitching moment?

EddieWeeks

Yes.. its kind of like pitching moment...

The center of twist, as I call it, is really the line or axis where when you load the wing.. (pull Gs)
Points ahead of the this line move down... and points behind this line move up... This has the same
effect as washout.. This is good.... It is possiable to make a wing that does not do this, but because
most wings have the thinkest point fairly far forward, most wing are fine....

If you mount the wing where the front spar is not as stiff as the back... well you can figure what could happen...

Eddie

KFalcon

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When I built my F-18 I used carbon plates on the front and back main wing formers. I used two pieces of 3/16" carbon plate that had aluminum honeycomb sandwiched between them. My stock former also had two pieces of ply on the front former. It proved to be really strong this way and I was able to avoid a bad situation at Prado at high speed by rolling the plane on its back and pulling the stick to the stop to avoid other traffic. The plane jerked towards the ground violently but sustained no damage. When I build my next twin, I will again reinforce that area with carbon plate but I will use a solid carbon plate closer to 1/4" instead of the aluminum honeycomb.

KFalcon

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last few...

With some modifications this plane can handle a ton of thrust and can be flown hard, ask Joe McBride.

hornet driver

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dave

sorry to hear about your loss.....what can i say


owning 3 yellow twins and having friends that have built 10 of them i have to say i have had the advantage of their tremendous knowledge

thanks to

gary baker
doug arnold
dan massey
roger kirshner
vic montalbano

one of our main concerns were the 2 formers thus we doubled ours...in fact on 2 of them we installed a " stop sign " aluminum sandwitch in the enhancement...we couldnt care less if we added a pound or so as our belief is these are jets not gliders....additonally when we did the enhancement gary baker captured the aluminum spar slots where as they are flush with doubler

also

our engine install is where a the 2 mounts are vertical instead of horizontal which required a top brace from front to rear former....thus the plane can be picked up from the top engine mount and carried around like a suitcase ( if you can pick up 42 pounds ) ...yep we are heavy but STRONG....

i dont want to hear it from the gallery about weight....yea she can be built at around 36 to 38 lbs....but i never worry about structure

ttly gregg bingham

ps...and thanks to the guys mentioned above for all your help!!!!....they are all very smart and have been very helpfull

Dave Presta

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Hey Gregg, how you doin bud? This picture is taken from one of the most well respected builders on the West Coast, his bulkhead is the same as the one in the video. The point i am trying to make is that without notice, someone might just use the stock parts and have a problem on their hands, so this forum is a great place for people such as Kenny, Gregg, and anyone else to share their experience and make sure that others can enjoy this plane without any problems.

Thanks for all your input guy's, keep it comin

Dave

KFalcon

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Dave sent you a PM

hornet driver

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btw

imho

1) the roo was not intended to be powered by more than a 20 lb turbine....but that has been well exceeded over the years

2) bvm bobcat was not originally made to be flown ( with the not made yet ) p-160 ...but that has been well exceeded over the years

also the yellow twin was never designed for whats being put in her now....and all 3 examples have put excess stresses on areas that they werent designed for...thus i concluded that if you go outside the design you need to engineer some mods to account for the excess ...which is part of the hobby

another mod we do is the jumbo mulitplex elevator servos....yep ...over engineered...until you need it....once we needed it...after take off....we had to one lap it and land....with 12lbs of fuel north of the cg we needed all the torgue the jumbos had to pull the nose up to flare....while i`ll never know for sure....i dont think 8411s would of got us what we needed when we needed it!!....add 2 more ounces :-)

hornetdriver

KFalcon

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Good points.

hornet driver

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dave

if you look at your last pic of the biulkhead ..our doubler ( of that former ) captures the female aceptor for the spar...thus we dont have the "lip" you see in your pic

thus not only is the former doubled the capture piece is "locked " in place and cant move

gregg

Shaun Evans

My point, exactly.

Dave,

My understanding is that the 'mystery-builder' from your photo didn't build that airplane from the ground up--but, rather, took it on and completed it after it was started. Do you know anything about that? Also, can you take a look and see if that bulkhead has the additional doubler with the carbon sandwiched between for me? By the way, if you look closely, you'll see that the bulkhead in your latest pic is better than twice as wide/tall (in the area in question) as the one in your plane. I'm not 100%, but I think the additional strength is better than twice, also. Huge difference.



As the photos show, they're not exactly the same at all. The one on the left is what comes in the kit. I still reinforced mine with the carbon plate, but the fact that I had to cut some away influenced my decision to do so.