DG 1000 trouble

[Ian Jones]

This is my FlyFly DG1000. Sadly the image is a cheat - I have removed the person that was holding it up before it's maiden flight.

 

One very short flight only! It was hand launched and immediately it dropped the left wing. After something of a fight against gravity I got it to a safe height under full power, at that time my thoughts were that the launch could have been better. With the benefit of hindsight it probably wasn’t the launch that caused the problem.

I didn’t have any problems with the elevator response and under full power it was going at quite some speed so I turned into the gentle breeze started to reduce power. A soon as I did that the left wing dropped and it went into an earthward spiral. By the time I had stopped the roll I could already picture the bits the spread all over the field – all the same I pulled back on the elevator and at first not a lot happened then eventually it started to level out.

Too late though. Not wreckage spread over a wide area as I had at first imagined and the underneath of the fuselage just forward of the cockpit took the full impact.

There’s a fair amount of damage and there will be a lot of work involved in fixing it, however it’s clearly possible.

So why did this happen? Well my belief at the time was that it must have a heavy left wing and that I must check the balance properly after I’ve fixed it. It would be nice if that fixed it but I’m not convinced. That element of doubt is really putting me of fixing it.

Other thoughts I have:

• The wing chord is very narrow – does that make the CG tolerance narrow too?
• The dropping of the wing was very sudden – was it a stall, if so why?

There’s lots of glider & slope pilots out there and I’d welcome any suggestion that might help me get a successful flight next time.

 

Thanks, Ian

[ken anderson.]

hello ian....sounds like the wing 'stalled'....flying it too slow/in a turn......pulling up on the elevator......

 

ken anderson ne..1.

[Ian Jones]

Yes Ken it seems like a stall but when I turned into the wind it was straight and level, no elevator input - all I did was start to throttle back, it was still fair moving when it tipped.

 

I'm not saying it wasn't a stall I just wouldn't expect this model at a relatively high speed, head on into the wind to fall out of the sky like that. I've seen these just not want to come down rather than rush into it like this one did.

 

So if we presume that it was a stall - what could have induced it I wonder?

 

What if, for example, it was nose heavy and due to the narrow chord the wing just won't tolerate anything other than just right on the CG... and even though I've said there was no elevator input... I wonder if I may have automatically been holding the elevator up without realising it. Could these circumstances induce a high speed stall with this type of aircraft?

[Stefan Hafner]

HI Ian

 

From what you say about there being plety of elevator respense in your OP I don't think the CG was in the wrong place.

 

Going along the lines of a stall, and the very short chord of gliders, if there were slight problems with the build and the wing wasn't quite rigid enough at high speed it may have been the wing twisting under high load, either the right twisting to increase the angle of attack, or the left twisting to reduce the AoA. May be worth checking how much the wing twist.

 

The other possibility that I can see is that there may have been a problem with a servo in the wing glitching?

 

 

[Tom Wright 2]

Seen a couple of these go the same way ,they are C/G critical and require speed at all times, a bit of a "brick" and cant be treated as a thermal soarer ,better chucked off a hill into a good blow.

 

TW2

Edited By tom wright 2 on 16/04/2011 18:39:51

[Tom Wright 2]

Ian.

To be slightly more constructive than my post above, when you reduced the power the speed reduction could have easily resulted in a stall ,and this type of model often drops a wing and enters the incipient spin which usually continues on to a fully developed spin with a high rate of rotation .The degree off susceptibility depends on the build accuracy of the wings ,C/G position,and wing loading, the later of course effects the stall speed.

Do you know the wing loading? if it does fall into the high category i think my previous comments are possibly appropriate.

Did your DG come as an EP or did you convert it ? the examples i have seen come to grief were conversions .Did you intend the model to perform as an EP flat field thermal soar-er ? if so you may have taken on a significant challenge.Sorry about all the questions but the answers may have a bearing on what you do next with the model.And as an afterthought did the test glide incident possibly damage a aileron servo that went unnoticed?

 

TW2.

 

Edited By tom wright 2 on 17/04/2011 01:40:41

[Ian Jones]

Stefan,

 

Thanks you have pointed a couple out of possibilies I hadn't considered and will look into

 

Tom (TW2),

 

C/G critcal eh? Hmm you're tending to confirm my worst fear; that I just hadn't been able to balance it properly, would mind telling me what method you would use?

 

Thanks both, Ian

 

Edited By Ian Jones on 17/04/2011 01:39:53

[Tom Wright 2]

HI Ian.

Or should i say fellow night owl,would need some more info as requested to make further comment.

TOM.

 

PS for test flights i would go for c/g a bit forward of the indicated position,

Edited By tom wright 2 on 17/04/2011 01:50:37

[Ian Jones]

Posted by tom wright 2 on 17/04/2011 01:35:16:

Ian.

To be slightly more constructive than my post above, when you reduced the power the speed reduction could have easily resulted in a stall ,and this type of model often drops a wing and enters the incipient spin which usually continues on to a fully developed spin with a high rate of rotation .The degree off susceptibility depends on the build accuracy of the wings ,C/G position,and wing loading, the later of course effects the stall speed.

Do you know the wing loading? if it does fall into the high category i think my previous comments are possibly appropriate.

Did your DG come as an EP or did you convert it ? the examples i have seen come to grief were conversions .Did you intend the model to perform as an EP flat field thermal soar-er ? if so you may have taken on a significant challenge.Sorry about all the questions but the answers may have a bearing on what you do next with the model.And as an afterthought did the test glide incident possibly damage a aileron servo that went unnoticed?

 

TW2.

 

Edited By tom wright 2 on 17/04/2011 01:40:41

Posts crossed a little Tom,

 

Now you are really getting to to the nuts and bolts of it. The model was available as either a soarer or EP - I bought the EP versiojn version but it appears to me that this just means the nose was cut off and motor mounts provided.

 

My intentions had been to use the motor to gain altitude them thermal soar from there.

 

I'm happy to accept whatever questions you throw at me, even if the answers indicate that I've bitten off a big chunk of something unchewable.

 

I'll look up the wing loading and post again and I'm very interested in your comments re C/G as I suspect that it may be a contributory factor.

 

As far as I can tell all servos were in good in working order before & after the flight to disaster.

 

Very much appreciate your help,

 

Ian

 P.S. Yep a bit of night owl and it does me no good at all it just seem to happen this way.

 

Edited By Ian Jones on 17/04/2011 01:55:12

Edited By Ian Jones on 17/04/2011 02:07:06

[Ian Jones]

The wing loading is stated as 33-35g/dm2 (sq) the aerofoil (translated into English) is HQ 3012

 

Ian

[Tom Wright 2]

OK thanks for that Ian,

Would i be right in thinking that when you bought the model you were seduced by the elegant scale appearance? if so your not alone ,i had similar experience with a Discus ,and eventually did get a decent performance but the modifications i carried out were a bit drastic ,the main objectives were to reduce weight ,find a c/g that suited me ,and adding some dihedral, i then reduced the aileron throws to minimum and coupled the rudder to the ailerons ,the latter illuminated adverse yaw that on its own can initiate a tip stall and develop into a spin ,so in effect i made the model more inherently stable and simpler to fly,but these high aspect ratio models operating at low Reynolds numbers can more often than not represent a bit of a challenge ,part of the price we pay for scale appearance.As i am sure you know if the model was not a hand full in pitch it is unlikely that the C/G was to far out ,but of course with such a narrow chord a bit is a lot !

.

 

Tom.

[Tom Wright 2]

Ian.

I am old school ,very old school,do you know what that is in old English? like oz per sq ft,

 

TOM.

[Doug Ireland]

I wonder if the wing incidence pin on the left wing was a bit out?

[Tom Wright 2]

Now something like this ,really does the job, without any hassle, 8oz per sq ft.

 

[Tom Wright 2]

Ian sounds like a an experienced builder Doug i assumed he would have checked vital stuff like that, but of course your right such an error would have a significant effect........like disaster!

 

Tom.

Edited By tom wright 2 on 17/04/2011 03:10:02

[Tom Wright 2]

Ian do you have the flying weight and wing span, i was assuming it would be 2.5 M and about 3.5 lbs.but it looks bigger in the photo?

TOM

Edited By tom wright 2 on 17/04/2011 03:17:28

[Ian Jones]

Hi Tom,

I'm more at home with imperial measures too but here's what's in the instructions which is one sheet of paper with an exploded view for the non powered version ,

wing span 103"

weight 1000g - 1100 g (don't know what that is in lbs)

Wing area 28.5dm2 (ugh)

CG 30mm from leading edge at 200mm from root

 

 

Oh just done a check on 1100g aparantly it's 2.42 lbs, given that I have a motor & battery too then your estimation would be about right. (I've lost the weight I checked it at but my recollection is that it was nearer 3 lbs)

 

Thanks Doug but yes both wings had the same incidence. As I say all suggestions welcome. As Tom suggested it's a good looking scale glider (even if the motor is in the wrong place) and I really would like to able to enjoy flying it. I'm an experienced sport/scale power flyer but gliders are new to me.

 

Ian

Edited By Ian Jones on 17/04/2011 03:42:58

Edited By Ian Jones on 17/04/2011 03:45:50

[Ian Jones]

Posted by tom wright 2 on 17/04/2011 02:54:25:

Now something like this ,really does the job, without any hassle, 8oz per sq ft.

 

That does look nice Tom, lots of wing to keep it aloft and someone to carry it for you too

Edited By Ian Jones on 17/04/2011 12:35:25

[Tom Wright 2]

Hi Ian.

 

Have been out flying all day and out socially this evening hope to get back to the thread later or tomorrow.

 

Tom.

[Tom Wright 2]

Hi Ian.

It was been established that aspect ratios of above 12 at reynold numbers calculated for 3m span models ,do not produce any significant improvement in performance, but does result in a higher wing mass for a given area,which is detrimental to the characteristics of a true thermal soaring glider.The DG 1000 has an aspect ratio of about 18 so the structure has to be strong to resist the bending and twisting moments,this results in a heavy wing,which returns a poor weight to area ratio.The wing section is possibly designed for laminar flow which makes it efficient but rather temperamental by comparison to other popular sections ,the air flow breakaway can cause dramatic stall characteristics ,the use of turbulators can reduce the tip stall severity but as this type of wing plan form and section are designed to give good L/D ratios at speed the suitability as a thermal soar-er at model size is in my view limited.What can be done to make the best of your DG ? I will post my ideas below .

 

TOM.

 

[Tom Wright 2]

Firstly i am assuming your TX is mode two ? so if you did not have expo applied to the three aerodynamic control surfaces, the possibility of accidental rudder input when using elevator is real enough to produce enough yaw to stall a wing tip. also reducing power can also produce a change in yaw and roll sufficient enough to cause a tip stall as the speed decays, it is quite possible that either or both of these situations caused your problem.Another nasty is the yaw that results from lack of aileron differential, this can also be sufficient to stall a tip .Sorry if all this sounds a bit dry,and i don't want to sound like the pupil instructing the instructor ,but all this has happened to me so i am just relating my experiences on the field ,and from my A/C design projects.

 

MY SUGESTIONS WOULD BE........

 

1) For test flights set C/G at 25% of root chord.

2) Put expo on all surfaces ,try 20%

3) If you can program aileron differential .

4) Climb to a safe height cut the motor and explore the speed range.

5) Keep a good speed on approach and don't reduce until 12 in from touch down.

6) Experiment with turbulators.

 

Don't know if any of this will help in the case of your DG ,but if you do really want a scale thermal glider take a look at the Fred Slingsby classic designs that have been done as model plans or kits,or go for a dedicated thermal model like a bird of time ,bubble dancer, Ava, or the many first class traditional designs from plans or better still design your own .Wing loadings between 6 and 12 ozs per sq ft should result in very long glide times and easy handling ,EP is ideal for getting the initial height.My rough calculations suggest your DG wing loading is 15 to 18 oz per sq ft ,OK for slope soaring not so good for flat field work.I think when all said and done the DG just stalls at a higher speed than you anticipated no more complicated than that .

 

TOM.

Edited By tom wright 2 on 18/04/2011 02:13:04

[Tom Wright 2]

Oh my next post has gone to page 2 ?

[Stuart Eggerton]

Hi all, I fly up at our local flying filed with Tom Wright and I had a similar experience with the DG1000. When I first set it up the locating pins for the wings were misaligned so I tweaked then so that both wings were true. I set up the cg as recommened in the instructions. The first flight was rather interesting as I was using a bungee to launch, the flight was not really flying so much as trying to prevent a crash!, it would drop a wing without warning and glided like a brick. I looked originally at the tailplane decalage to as the DG1000 has a bolt on "T" tail plane. I made some other modes like adding airbrakes which made the structure heavier still which increased the stalling speed. As Tom says it really is a model which likes to be flown at speed all the time, and if you are like me and used to flying "floaters" then this model will be a shock to the system. You can see Tom at the start of the video (hope it's attached). So the model did fly (once I put a hefty elcectric motor in it)

Edited By Stuart Eggerton on 20/04/2011 14:10:53

[Tom Wright 2]

Hi Stuart.

Thanks for your input on the DG ,Ian will be back to the thread at the end of the week,flew the 4m today the air was very buoyant, later we cut the grass on both runways and the dispersal area ,so your Bubble Dancer ,can now arrive even more smoothly, and you wont loose your bits and pieces in the long grass .

PS I don't think you put the video on correctly you need to check the instruction thread or have a word with the MODS.

 

TW2..

Edited By tom wright 2 on 20/04/2011 21:45:07

[Ian Jones]

Thanks for all your responses chaps.

 

I've got loads to catch up on this thread and others, the webbit build in particular but how did I let this situation arise?

 

Well I feel I can now explain. After some years of threatening to do it I finally fixed a date for my BMFA B Test, which was today. I've spent the last week or so puttijng in lots of hours swotting and practising which has left me short of time for other pleasures.

 

I'm pleased (and proud) to have succeeded so now I can re-read the technical comments that have been made on this thread and discuss things further.

 

Thanks, Ian