|Tosh McCaber||22/07/2020 15:05:10|
|234 forum posts|
I went out yesterday to fly my Phoenix motor glider, on its second outing. All was going very nicely, until, coming out of a loop with reasonable power on in about a 30 deg dive at fairly low height, I pulled up to level out, and the damned thing just kept tucking its nose further and further down, until it collided with a cow pat almost vertically!! A messy business!
Can't think of the reason- A few thoughts and suggestions
1) Perhaps c of g too far back? Although it balanced at exactly where it was specified in the instructions- 70mm from the leading edge.
In my previous, first outing, it did the same thing, but at a good bit higher level - I think that then I pulled back on the throttle and regained level flight. This time, unfortunately, close to the deck, there was no time to think of remedies.
2) Or- I bought the model s/h. The specs for the glider suggest 9g servos, except for the elevator, which should be a larger 30mm servo (12g?) The model came fitted out with all 9g servos. Maybe that size was not big enough to counter the additional load in the dive?
3) From mazpr2025 on Andrew Newton's You Tube Channel link
"Its a solid airplane, be watchful when bringing it out of a steep dive kamikaze style. The small elevator and long wiring to servo shows significant lag. Be ready to apply rudder and if need be full flaps to slow and get it out of it." ( I must say that the other reviews of the Phoenix don't mention the elevator being too small.)
Anyway- I really like the flying characteristics- apart from this fatal flaw! So I've ordered a replacement!!! Very reasonable cost from Hobbyking Europe to the UK- 10 day delivery.
This time round, I'll be checking things out before I get into any low level manoeuvres!
I don't like crashing! Especially when I don't know the reason!!
After that long winded preamble, has anyone, especially Phoenix flyers, experienced any similar tendencies to dive under power?
Any suggestions appreciated. Thanks in advance
Edited By Tosh McCaber on 22/07/2020 15:06:12
Edited By Tosh McCaber on 22/07/2020 15:07:10
2418 forum posts
Sorry to hear this, must be annoying to lose your model on the second session by which point it's it's normally well set up and a lot more relaxing to fly!
I don't wish to appear mean, but why did you have the throttle on exiting a loop with a motor glider? A model like this will obviously need power on the way up if you are looping from level flight, but fly it like a glider (i.e. diving for speed before each manoeuvre) and you should be able to get round with nothing; certainly you don't need any power in the second half. The Phoenix is a good model but it's not a hotliner (or even a warmliner) so you do need to remember to fly it within it's limits.
Pretty unlikely to be CG I'd say - if it was keen to tuck under coming out of a loop it would have exhibited that same characteristic at slower speeds which would have made it rather unpleasant to fly. My money is on...
...either the elevator servo wasn't man enough, the linkage wasn't or perhaps both. The v2 Phoenix has a plenty big enough elevator so you can discount that. The only other option is a failure of the elevator horn, but the fact it did this once before and recovered indicates it is more likely to be a twangy linkage. I would put a meatier servo (min HS65, perhaps even an HS81 or 85) on your replacement and examine every aspect of the linkage to make sure it is fit for purpose.
Edited By MattyB on 22/07/2020 15:58:21
|J D 8||22/07/2020 16:16:20|
1749 forum posts
Most modern servo's today have plenty of torque for the job they do but no harm in going up a size if that is what the maker suggests.
However it will not help if as I suspect the model suffers from a condition called "Tuck under "
This was a bit of a problem in the early day's of electric flight when free flight glider's and vintage types were adapted for rc electric. power set ups were heavy and not all that powerful so the light weight FF types with their light structure and large wing area looked a good way to go.
The problem came in an rc induced dive and as up elevator was fed in the light rear end fuselage would bend increasing the angle of attack of the tail plane so you end up with a sort of control reversal situation and the faster the aircraft went the greater the bending, also then compounded by more up being fed in and so on. Tail's of FF types while fine for their intended job would also tend to twist when an elevator was just hung on the back.
Just looking at the Phoenix with its slim foam fuzz makes me think that tuck under may be what is happening given that the first time it happened you pulled back on the power and it sorted itself out.
Matty, came back before me and I agree about twangy linkage as well. Cheers , John.
Edited By J D 8 on 22/07/2020 16:19:30
|Graham Davies 3||22/07/2020 16:22:40|
|189 forum posts|
Bad luck old chap.
I had a similar problem many moons ago where the elevator was set to 'push' for up and there was flex in the pushrod, which was also anchored with insufficient number of points. It was possible to hold flu up elevator and then push the elevator down by hand. Doing so showed the snake outer moving inside the fus, and the final section before the horn bending. Similar can occur with elaborate bends in pushrods to exit the fuselage.
Worth checking; it's always nice to find the smoking (or steaming in this case!) gun in such situations.
|David Hall 9||22/07/2020 16:35:25|
|313 forum posts|
I recently beefed up the elevator joiner, (the part of the elevator that runs through the rudder). Whilst flying, it seemed to have some lag or hysteresis in the elevator. I found that the foam beneath the elevator horn had gone "soft" and was allowing some noticeable flex between each side of the elevator and in the horn itself. I recalled that several flights earlier, I had some flutter in a dive, but couldn't see any damage from it.... perhaps I looked in the wrong places.
I plated the centre of the elevator, top and bottom in 1m ply.
2418 forum posts
The wings are foam but the fus is blow moulded plastic (see this video). I have test flown one of these for someone else and am confident that (unless it was a faulty one) the fuse itself won't be the culprit. Of course it could be the fuse to tail surface juncture of course or the linkage as previously discussed. David's post above on the tail surface going soft beneath the horn is also a good shout too.
Edited By MattyB on 22/07/2020 17:07:16
632 forum posts
Sorry to hear that Tosh
I have a Volantex Phoenix V2 2000 in a box.
Was this yours?
The CG is 77mm +- 5mm in my instructions whereas yours were 70mm so I have my fingers crossed that it is not the same.
|Richard Clark 2||22/07/2020 18:12:42|
|424 forum posts|
GC too far back? Maybe. If so you will be flying with some amount of down elevator (electronic trim or mechanical adjustment) to fly level at 'normal' speed and this may over-compensate the rearward CG as the speed increases resulting in a steepened dive.
Alternatively a too flexible linkage. Where possible I set my planes up so the control linkage 'pulls' for up elevator rather than pushes for it.
632 forum posts
I have checked mine and the cg is marked on the wing at 70mm. The instructions in my box are for the 2400.
Sorry for the confusion.
It's not a CG issue then.
|344 forum posts|
Never had any bad tendencies with mine. I would guess your elevator servo gave up.
|ron evans||22/07/2020 19:45:12|
468 forum posts
I had a similar issue with my P2K. In a dive it would roll and tuck out of control, only saved by height, or rudder, or once by letting it roll inverted and pushing out of the dive
I went through the same process as you, servos? ...linkage?....gremlins?
Comparing the wings on mine to a mates example I noticed that the sun showed through much more on mine than his, and seemed to be of lower density.
I bought some laminating film from EBAY and covered the wings with it, and that completely cured the problem.
It will now dive from height with a rock steady gradual recovery by itself. I have the CG well back with the battery under the wing to reduce the power on zoom. Every thing else is standard.
I've since picked up a secondhand 1600 and given it the same treatment
Edited By ron evans on 22/07/2020 20:07:48
|Tosh McCaber||22/07/2020 20:42:43|
|234 forum posts|
Thanks for all the comments and suggestions.
My further thoughts are:
1) Although it's not a hotliner (or even a warmliner) It's pretty strong in most respects (see some of the Phoenix videos on You Tube). However, there are one or two areas that I'll beef up on the Mk2 version. I wasn't pulling a particularly tight loop, and not at full power when all hell let loose..
2) I'll be putting in a larger elevator servo in Mk2- just in case!
3) Unfortunately, much as I'd like to, the configuration of the top mounted elevator doesn't allow a "pull for up" servo linkage- see photo. I think that the existing linkage- wire through a tube- is reasonable. (Again, I would put something stiffer if I could, but I can't!)
4) Although it's not the Volantex Phoenix V2 2000 version- just the original Phoenix- the (strong moulded abs?) plastic fuselage remained almost completely unscathed, other than being split at joint just behind the motor, which could repaired- no dents. The wings are a different story...
So I'm pretty sure there was no distortion there.
5) I'm enclosing plan of the plane. I'm certain that my CG was at the 70mm mark as noted. It took off perfectly at medium throttle, with no need for any trim compensation during the flight, until the last manoeuvre. I did wonder about increasing the moving elevator area- I'll try increased movement for a start.
6) Just picked up your post Ron. I'm intrigued with your application of laminating film from EBay. Can you give some more details as to what kind of film you used?
Thanks again for all replies and suggestions so far.
|Mike Blandford||22/07/2020 20:51:37|
670 forum posts
Just wondering how stiff is the tailplane. If it is not stiff enough you could be suffering from "control reversal".
Spitfires had an aileron reversal speed of 580mph. What this means is that above that speed applying aileron caused the wings to twist in the opposite direction (ailerons act like trim tabs) with the result the aircraft rolled in the opposite direction to that intended. Later Spitfires had stiffer wings.
If the tailplane is not stiff enough then applying up elevator pushes the trailing edge of the fixed part of the tailplane down resulting in the tailplane as a whole applying pitch down. The harder you pull up elevator, the more the pitch down effect!
|Simon Chaddock||22/07/2020 20:58:17|
5844 forum posts
My money is on fuselage flexing.
The downward force from the elevator causes the fuselage to bend down which in turn alters the tail plane incidence to the air flow. It only needs a degree or two increase on the tail plane and the net effect is no down force despite the up elevator.
I had exactly this effect on a pod and boom plane. the only remedy was to close the throttle, but not completely so the prop did not fold. The prop drag slowed it down enough for a net down force to be slowly restored.
|Vecchio Austriaco||22/07/2020 21:06:31|
1516 forum posts
That's bad luck. I also think of a defect of either the servo or the linkage. I was flying my Mephisto ( warmliner, veneer covered foam wings and FRP fuselage) today, and also on this faster model the linkage to the elevator is using a bowden as in your photo.
By the way the thermals were strong today, I could stay up for quite some time. Location Newport Pagnell, Bury field
|ron evans||22/07/2020 21:25:25|
468 forum posts
Tosh. my rolls of laminating film were 75microns x 300mm x 150mtr, about 5yrs ago from EBAY £25, given most of it away now, club mates love it.
Lots of tutorials online how to use on foam and built up models.
On my P2K it really added lots of torsional strength to the wings, and I can only conclude that they were twisting in the high speed dive, the ailerons being ineffective.
Another advantage is the film stiffens the ailerons, and forms the hinge, the roll rate was much improved.
It would seem most people haven't had this problem, and the three other examples in our club are fine.
Just bad luck to buy a duff one.
|Tosh McCaber||22/07/2020 21:58:18|
|234 forum posts|
Don't think it's elevator flex- its reinforced by small gauge cf spars on both fixed and moveable surfaces.
Just checked out the fuse- it is very strong- absolutely no sign of flex!
Great to hear you've thermals down there, Vecchio! We have miserable weather here in Aberdeen today!
Could only find 50 m long rolls on eBay Ron. How heavy was it compared with say, Solarfilm (which wouldn't be suitable due to the foam wing)?
Also, Ron, and any other Phoenix flyers, I'm changing the motor on my next Phoenix,. Can you or your club mates give me an efficient alternative motor spec? A Turnigy Aerodrive SK3/2836/1040kv was mentioned as a superior alternative to the large Volantex 4023/850kv motor in my original, which is probably knackered now anyway!
632 forum posts
Thanks for the update Tosh.
I have been watching reviews of the V2 in which the original was mentioned. Essentially V2 is much improved and one short coming specifically mentioned was the wings flexing in the original. This has now been cured, so they say. The tail servos are now rear mounted with obviously short control rods, presumable no chance of flex then.
I have started putting mine together and so I will soon find out if there are any issues. As I am still learning, aren't we all, I am not into aerobatics just gentle thermal flying is my goal. No test at all then!
Thanks again for the information which has made me more confident about the V2.
|214 forum posts||
Use of a pull-spring system would effectively make the elevator linkage pull for up, while having the horn on the underside of the control surface.
Gauge and length of the torsion spring would have to be selected to give the required force, a task similar to eliminating slop with a pushrod inside outer tube setup.
To retro-fit to a foam model you would probably have to cut the elavator off the tailplane, make a new hinge, and reinforce the spring entry points..
|Richard Clark 2||23/07/2020 09:42:00|
|424 forum posts|
That elevator rod looks very thin to me. On one of my planes I stiffened it by soldering a length of small diameter brass tube round it, leaving the forward half inch exposed thus allowing for the 'down' movement..
For heat reasons you should do this outside the plane, inserting the rod from the rear afterwards.
And make sure the end of the plastic tube is firmly epoxied in place (roughen it first), and the same at the front end
Edited By Richard Clark 2 on 23/07/2020 09:47:32
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