I have posted this picture and message on behalf of Mr Bob Birkett, Morecambe, England. he owns this jet.
This Hotspot has had less than 10 flights, it's last flight was made with wing tanks and powered by an AMT Netherlands Mercury turbine.
I have flown every flight, on this occasion during the flight I handed the box to a trainee, he proceeded to fly very well, without warning a loud bang was heard and a large piece of the aircraft fluttered to earth.
I immediately landed the aircraft and was shocked and amazed to see the damage.
Our English Graupner agent has told Bob Birkett there is nothing wrong with the kit and he is not prepared to do anything.
Ive heard rumours of this type of delamination in Hotspots before.
The reason for this posting is to warn others to reinforce the fuselage around the gear apertures and to ask if this has happened in the States, and what was the view of your Graupner agents ????
This should not happen, we were very lucky to get this aircraft back, you can imagine the result if the batteries had been in this area

Mark Diggle

Wow. Thanks for posting this, as i have just finished a new Hotspot and am going to test fly this weekend.


It's not suprising that this happened. The back part of the fuse probably sees a lot of pressure. The back egde of the gear opening acts almost like an air scoop. At 200 MPH for example there is close to a 1 psi (0.7 if i'm correct) of dynamic pressure available. If all that dynamic pressure gets stagnated inside the back of the fuse it would truly make it bulge out like a ballon.

Good sealing gear doors would be a nice fix i think... a lot of work though.

It's probably happened quite a few times.


Jon

Hi Guys,

Would it help to make some small openings in the wing skin near the trailing edge to let any accumulated pressure vent out?


Garrett

Should we have to flight develop and modify a kit that is sold for beginners by a large scale manufacturer.
This failure has really shaken my respect for Graupner quality.
I'm sure most experienced jet modelers can stop this happening by either injecting foam or resealing the door areas, as long as they know it can happen !!!!
Iam a reasonable jet pilot with hundreds of flights behind me, I doubt a beginner would have saved this aircraft, and these are the people this kit is being marketed to

I'm curious, Was the initial failure in the bonding joint near the retract opening and if so was it the bond that broke or the 1st layer of fiberglass locally pulling away from the laminated surface?

Like Jon, I will be testing out my HotSpot for the first time very soon, probably the Friday before the Princeton Jet Meet.

Thx,
Kelly

It would appear to be poor bonding to the skin, you can see an imprint of the skin in the glue.

Sounds like there needs to be a Airworthiness Directive (A.D.) out on this aircraft in this area of structure.........this could cause a serious or even fatal crash.


Jackjet

I think the problem started at the retract opening, but equally the whole area could have ballooned, it happened so quickly I don't really know

Now where have we seen that before????


Turbulence

Guys just a thought! as I do not own a Spot , but I do have a Roo....
What I am doing is sealing the wheel wells on my Roo, using the old trick of cutting the base off a plastic soda bottle, and using this to line/seal the wheel wells stopping any air getting beyond the wells... This will definetly avoid the pressure scenerio as a concern. Its an easy fix and I am surprised the manufactures have not added simple well liners to the Spot and Roo designs... a piece of plastic or aluminum tubing cut in half would serve well also to line the strut recess.
Just my 2c worth : )

Cheers-

Hot Spot failure
(post # 1)


Well...here is what I have seen...

Several Hot Spots that I have seen including my own have all suffered from shall we say "Firm" landings either on grass or on
pavement. At some time or another the bond has broken on all of the aircraft due no doubt to the forces of the hard landing and the
insuing "hop" if you will. Check your bond all along where the gear retracts and make sure that there is no separation. The airflow
can and will peel this back like a sardine can if you allow it to go unrepaired. Nothing wrong with the aircraft as far as I have seen
...only when left unattended would this probably happen. JMO

Scott Strimple

Thanks for the reply Scott, I agree with you that the bonding may become detached by poor landings, however this is supposed to be a beginners aircraft, it will be bounced.
The manufacturer should at least warn of this possibility of this happening, my view is that the bond is not strong enough to cope with an experienced flyer, never mind a beginners landings.
My purpose in placing this thread was to inform people of this problem, ask for their experiences and debate in a constructive way this experience, so far so good, alot of people have been made aware of the problem and it may have stopped it happening to other people.
I still think that an international manufacturer should warn of this in the instructions or remedy the problem

Scott is right on. At first, when the delams first appeared, I didn't believe it, but then my first HS started to delam along the back side of the strut opening. This is very serious, since once this lets loose, only a 6 inch or so piece of plywood, running fore-aft holds the lower skin on near the midline of the fuselage. This is very short, up near the gear well, providing no support for the aft end of the lower skin. Out laterally, the glue line does run to the aft of the fuse, but if the front and midline separates, then the lateral aspect will never hold. If there are any glue voids or bad bonds to improperly prepared glass, this will sardine can like in the photo.

Since it has happened so infrequently, I think when it has happened, it was due to an unrecognized glue flaw in this area, but Scott's point about heavy landings is well taken, and I believe that is what lead to my initial separation. Fortunately I caught mine.

This bond must be checked very carefully, and I would now recommend putting another piece of quarter square plywood along the edge of the aft side of the strut opening, running in a transverse direction between the gear mount and the former with the oblong holes. This will prevent the front-edge of the delam from starting to peel back.

This is hard to explain and difficult to photograph, but I will try to post a photo if it can be done.

Gear doors would help, but I see gear doors on the photo, so they are not the answer. Eventually, all gear doors open and let the air hit that leading edge of the aft strut opening.

Tom

If the Graupner agent says there is nothing wrong with the Hotspot kit in question then he is deluding himself and trying to delude his customers.

One cannot make an accurate assesment of any such failure without a very careful inspection of the airframe in question and taking the approppriate action to prevent a recurrence.

(ask the accident investigation branch at Farnborough if you don't belive me, I believe Ken Smart is the chief inspector of accidents) In the last couple of weeks or so we have heard reports of structural failures of both a Kangaroo and Hotspot. The situation cannot be allowed to continue, and both manufacturers should must, take urgent action to investigate these failures. If another structural failure should occurr and injury or death result, then both manufacturers could be in very difficult position indeed when previous structural failures were known to have occurred.

If we are to continue to operate high performance turbine pwered model aircraft without much interefernce from or regulation by the authorities thn we must keep our hous in order and that means putting safety at the very top of our prorities, just as is done in fullsize operations.

Anthinhg less could mean th end of this part of the hobby and I am not prepared to see that happen as a result of short term commercial expeciency.

And if Graupner and Fber classics think I am being difficult they are wrong. Proper investigation of thse twoincidents to prevent a recurrence is in THEIR commercial interest.

BRG,

David Gladwin

There is a simple fix. Don't buy more.

Darryl Usher

[QUOTE]Originally posted by Darryl Usher
[B]There is a simple fix. Don't buy more.

Here's another simple fix:scratch built your own turbine jet-then you will KNOW if it is strong enough or not........


Jackjet

once i heard a high frequency wine/flutter at high speed on my HS. it was the fuse seperating near the retract opening (probably because of firm landings). zap fixed the problem.

the loss of a plane is always a personal tragedy. but there are hundrets/thousands of roos and HS out there flying flawless over and over.

nevertheless: always check these bonds.

mec

Hi All,
Just looked at my own yet to be flown HotSpot and I think the seperation may just be simple flutter induced at high speed due to the lack of support of the very thin skin forward of the ply structure. My usual remedy for flimsy GRP parts is to reinforce with CF and Epoxy and this I did around the Nosewheel-well and spoiler. Did not do it around the main gear as it looked like a chore. My fix will be to add a solid ply/CF support right up to the edge of the rear well skin. My demised ROO suffered wing flutter with complete failure of the main spar at about 240mph and it was rather scary....
Cheers
Andre

Thank you all for your replies.
It would appear from the postings that we were not the only ones to suffer this problem. The hotspot delamination problem has occured all over the world. The cause would appear to be a poor bond in the retract opening.
This thread has served it's purpose of alerting others to this probable failure and hopefully prevented unecessary further loss of aircraft.
Iam disappointed that a representative of the manufacturer has not asked to inspect the damage and establish the probable cause.
It is in their own interests to remedy this problem, they would find it hard to protect themselves from litigation should a serious accident occur, bearing in mind this problem appears to be commonplace in their product and is now publicly acknowledged.
If I were selling this product now, I would think it prudent to insist that the manufacturer either fixes the problem or notes in the instructions a warning of the possibility of delamination.
The response that there is nothing wrong with the kit is no longer a defence in law.
Thank you all once again and enjoy safe flying

Mark Diggle

referring to the picture in post #1:

one can see that there is a servo for the rudder installed in the fuse.
is ist possible that servo movements helped that the fuse bonds came loose?

just one thought

mec

The servo was not attached to lower skin, it is contained in a box that was attached to the upper skin.

I have been pretty quiet regarding the subject as some of you might remember the previous saga from the similar topic I will let the two pictures speak for themselves. The two where taken with ZERO FLIGHTS and ZERO hard Landings and no pressure added to make the pictures.





Pay special attention to the light GREEN color under where the skins separate. No that is not the glue!!!!!!, The glue used is white in color in all other areas


Turbulence

As the importer of this HS kit I find the way in which people talk about something they have absolutly no knowledge increadible.
I have personally been flying a HS for over two years and I have people very close to me flying two more. None of these have had a problem with the fuselage even though my own example has been flown from rough grass as well a runways. It's even had some rough landings, but I always check the model over before re flying it.
The fact that three HotSpots haven't had a problem is not a Guarantee of quality, but I have sold over 50 Hotspots in the UK and this was the first one of kits I've supplied that I've been aware of having a problem.
I did hear of the "exploding" fuselage rumour through another HS dealer. I was told that having the U/C cutouts open would pressurise the fuselage and make it split, I decided this was unlikely and did some test with things in the fuselage to see what happended to them. I concluded that there wasn't enough pressure to worry about. Two years later I still think that.
I have seen plenty of HS flown VERY fast without gear doors and are still flying like that today without problem.
The model in question has been heavily modified from the kit design. the Retract openings have sharp corners where the kit shows round smooth openings, the fuselage has other openings cut in the top surface for rudders. Modellers will modify kits and that's fine, but it is up to the individual to decide whether the mods are strutually safe.
The HS has proved it'self to be more that capable of handling the sort of flying it was designed for. It's a Jet Trainer/Sports model, not a BVM Bandit.
I don't recommend to any of my customers that they fit P120 size Turbines to a HS, The few that have we have de-tuned the motor back to 18-20lb of thrust.
The operator of any Turbine powered model is personally responcible for the airframe they are flying, if this failure happend as descibed to me then the weekness was already there and should have been sorted before flight. This model flys from grass a far as I know, which can put a lot more load through the U/C mounts especially on heavy landings...

Dave Wilshere

I have read all the theory's and solutions submitted by subscribers with interest. But I would like to throw one more possibility into the cooking pot. I purchased a HS kit from Mr. Wiltshire some months ago, on receipt it was found to have sustained damage in transit, resulting in some delamination of the stucture. The kit was duly replaced, but on inspection of the second kit, damage in transit was found again (a small ding to one of the wing skins).
Could this mean that manufacturers and suppliers of this kit need to look at packaging methods. Rough parcel transit could cause unseen delamination problems?.

I am sure that Dave Wilshere is absolutely correct when he says that a correctly manufactured and assembled Hotspot is capable of handling high speeds. However, I have heard a number of stories of delamination with these kits and in -flight structural failure of such high speed, high energy models is unacceptable in the greater interest of safe flight operations and the interest of turbine modelling as a whole.

Any structural failure which results from the difference between wheel well contour shapes is unacceptable and the manufacturers should enure that this weakness is eliminated, perhaps by incorporating wheel well liners.

If reasonable structural modifications to incorporate rudders, an essential flight control on any aircraft which has freedom of movement in the yaw axis, can so weaken the model so as to cause structural failure then Graupner needs to develop a suitable and structurally sound rudder installation design.

Without, or even with, the detailed inspection procedure such as we have here in Australia we can never prevent inexperienced modellers from operating poorly built models but even a whisper of delamintaion or structutral failure needs a careful examination by the kit manufacturers and the issue taken very seriously indeed much as any airframe defect is most carefully and rapidly assessed by a fullsize airframe manufacturer.

If firm landings or grass field operation can cause damage in a predictable fashion then Graupner needs to strengthen, within reason, any weakness, and suggest an inspection prodedure as a guide to inexperienced jet modellers with limited knowledge of model airframe structures, particularly if internal and difficult to see internal structure is affected.

It is a fact that Hotspot/Kangaroo type prefabricated models are frequently chosen by new and inexperienced entrants to turbine operations and , therefore, these models need to have great reserve of structural strength and the highest standard of manufacturing quality assurance.

I have no hidden agenda but I strongly feel that if we dont have the highest standard of safe turbine operations of which airframe structural integrity is an essential element, then it is only a matter of time before there is a serious accident and a fatal accident could well be the end of turbine flying as we know it. I saw two crashes (not Hospots) on my recent trip to the UK which could have been, in slightly differnt circumstances, catastrophic. As we know only too well in fullsize jet operations, saftey demands eternal vigilance.

I, for one, am not going to wait until such an accident occurs before speaking up about safety matters. I earnestly want, so far as possible, to ensure that turbine modelling continues with its current freedom of operation but for that to happen safety has got to have a higher priority and the manufacturers and distributors have a vital role to play.

BRG.

David Gladwin