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64 Thousand Dollar Question in Pattern Flying

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64 Thousand Dollar Question in Pattern Flying

Old 06-27-2014, 08:47 AM
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Malcolm H
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Default 64 Thousand Dollar Question in Pattern Flying

So what do you do when your new model bobbles about in choppy air? Some models just appear so locked, like they are flying through treacle and others jump around dropping wings and never look settled.

In my thirty year pattern career I have had settled models but I've had a few dogs too. My Asyuler is festooned with zig zag turbulator tape and it made an improvement but it's not perfect. I once spoke to a European pattern ace who admitted his solution to an unsettled model was to throw it away but then he wasn't paying for them!

What had prompted this rant is I have just returned from a trimming session with my Contra equipped Oxai Hybird and the air was bumpy today. The Hybird draws nice lines but bumped about all over the sky. Having spent a large chunk of money on this setup I'm pi**ed that it doesn't seem to be the groovy aeroplane I hoped for.

So here's the question, what do I bolt onto it to smooth it out? Tip fences? More turbulator tape? Lumps of lead in wing tips lol? All suggestions gratefully received.

Malcolm
Old 06-27-2014, 09:34 AM
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big_G
 
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I'll assume the c/g is set correctly....I found that my plane performs much better with an aft c/g in calmer winds, but loses directional stability if the winds kick up. So I moved the c/g a bit forward and mixed out the side effects. Having the AUW at the weight limit might help also.
Old 06-27-2014, 10:29 AM
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From experience using turbulators and other methods of aerodynamic tweaks may result in an already good setup being better but it won't make a bad setup good.

I would be investing my time on getting the CG and wing incidence correct. You may need a more forward CG and possibly more positive incidence. This is we're most of your gains will be made.
Old 06-27-2014, 10:58 AM
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serious power
 
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Hi Malcolm,
If your CG is at the 90mm point it is already forward.
Niall has moved his Acuracy cg back and found it helped.

With my bipes I found the strut fits critical. The contact points on those struts have a very short chord (assuming they are the same as the Midrex) so a little slop can have the tips twisting as much as a degree or two. To move a 1/4 to a 1/2 degree needs very little slop indeed. I shimmed mine to true and very tight - it takes effort to push the pins into place.
I had previously done this to the Ventura and before I did it was as you describe - especially in pushes and pulls.
You saw the Midrex fly at last years TC !! - only you can compare but it was quite windy for that event.

Then there is the issue of the two wings having different incidences to begin with !??
With a very forward cg there is a large moment between CG and AC. I'm not sure if I'd like a set up like that working on wings with different incidences ??!
If then there is any slop at the struts adding into this mix then it's likely to make for a lively/busy cocktail.

I don't know if any or all of this is relevant to your current set up, but if it is then when you add an inconsistent outside force like turbulence the outcome is likely to be along the lines you are describing.

Brian
Old 06-27-2014, 11:36 AM
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Hello Malcolm,
In level flight and everywhere?
Are your 'issues" in pitch or in roll/yaw?
Are they only in roll/yaw while pitching or looping?
If so, is there a difference with positive versus negative "G"?

sorry for the interrogation,
Dean Pappas
Old 06-27-2014, 11:39 AM
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Malcolm H
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Brian,

CG is indeed at 90mm point as recommended by Suzuki. I did try moving it rearward but found it was beginning to feel unstable in yaw so moved it forward again. Your strut fixing comments are interesting. I did add some hard foam packers under the tops of the struts but did notice some play today at the bottom. I will definitely work on this to tighten them up but to be honest I will be surprised if this makes a big difference to the way it sits in turbulent air, but I freely admit that my biplane knowledge is zero!

To be clear I'm not talking about low frequency oscillations in roll, I'm talking wings which wobble about with the odd roll through 15-20 degrees one side to another.

Will report back once I tighten the struts and recheck incidences.

Malcolm
Old 06-27-2014, 11:48 AM
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Malcolm H
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Dean,

Not at all, I'm pleased to try and describe the problem.

The model is actually very good in pitch, it draws very straight lines and it's easy to lock into verticals and back to horizontals. It's pretty good in yaw too showing none of the direction changes going round inside or outside corners that some models do. I have some experience of this as I run a Contra in my Asyuler which isn't quite as good in this respect.

The issue is purely in roll where the wings oscillate around and occasionally drop unpredictably. It's fine in calm weather.

Malcolm
Old 06-27-2014, 12:12 PM
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Don't know if it's relevant, Malcolm, but these bits of depron make a big improvement on my monoplane (Episode) wings.
More stable in roll,in particular at spin entries---without them, almost completely unpredictable wing dropping----with them, just a nice, straight, dropped nose.
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Old 06-27-2014, 02:43 PM
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How are servo pushrods, clevis, and rudder cable (if used) tightness? Are control horns flexing (if using thin AL fancy ones)? If you move the CG back, reduce rudder and elevator throw accordingly to offset the stability loss.
thanks,
Jim
Old 06-27-2014, 02:46 PM
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Granpooba
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My question to you is, how are you going to pay the 64 thousand, after you get your answer ? I hear the European economy is in pretty bad shape.
Old 06-27-2014, 05:34 PM
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Hi Malcolm,

Hope you are enjoying the Oxai Hybird, other than the bobbling of the wings in windy/turbulent air. I too have experienced the same with my Oxai Midrex and recently the Oxai Acuracy bipe. (Both electric)

I agree with everyone and making sure everything is tight fitting with no slop.

Both airplanes are some of the best I have flown in calm non turbulent wind, but in windy, hot, thermally air they both bobbled the wings. It is worse when I fly slower and better as I fly faster and further out.

I feel that it is merely the amount of wing area biplanes have compared to monoplanes and the light wing loading bipes have, especially as electrics compared to glow. The Midrex bobbled much less than the Acuracy, which makes sense to me, as the Midrex has aprox. 200 squares less than the Acuracy. Both weigh aprox. 4950 grams RTF.

My friend (Matt Stringer) just started flying an Oxai Acuracy glow version that weighs the same as my Acuracy without fuel, however on takeoff with 28 oz. of glow fuel it weighs almost 13 lbs. When it lands it still weighs more than my plane, unless he flys 15 min or more. His bobbles much less than my 11 lbs Acuracy.

I feel it comes down to wing area/wing loading and the distribution of wing area on the tip of the wings vs the root of the wings.

Matt Stringer and I tested my electric Acuracy in very bumpy 20 mph winds back to back after we added over a 100 grams of lead on the CG. We both thought it flew better and bobbled less with the extra 100 grams of lead on the CG.

I would imagine the Hybird being a very good design as it has the same wings and stab as the Midrex, and the Midrex I thought flew very well and handled the wind better than my Acuracy Bipe. Of all the popular bipes, the Midrex/Hybird has the smallest wing, which I think should be better for the wind. There are some new bipes, like the Finals Touch with a slightly bigger wing than the Hybird, however they are not staggered which handles the wind nicely.

Also, I feel that we see bobbles on our own planes when we are flying them as compared to others that are watching us fly them.

Try flying a little faster, higher, and further out when it is bumpy/turbulent air and see what you think. I would really like to hear your feedback. These are just my personal thoughts.

Mike Caglia
Old 06-27-2014, 05:55 PM
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Wing loading is the biggest factor. Heavy planes with small wings will look rock solid. I had a very heavy Super Star years ago and it would fly rock solid in any wind due to its high wing loading. Lighter planes I was flying like my Firebird pattern ship were much more affected by the same gusts. Add some weight to your plane and see how it flys when the winds are blowing.
Old 06-27-2014, 11:41 PM
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Hi Malcolm,
To shim the wing struts I used some pre-lam carbon/epoxy plate.
It takes a little time with test fitting, sanding and gluing.
I glue them to the strut bases at the heels and or toes. I glue them to either the strut or to the base or to both.
At each strut/wing interface I keep test fitting and adjusting so as to set and or maintain incidence before gluing.

I shim so that the pins are tight going in - they really need a push.
I then check the fit by engaging each strut one end at a time (with the other end free moving) and then trying to wobble the free end to and fro - there should be NO wobble.
If there is any wobble at all then the wing can move/twist at this point. If they can they will in turbulence.

A foam shim will give !!

Brian
Old 06-28-2014, 12:23 AM
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Hi Malcolm,
I should have said.
You need a really good incidence meter to set up bipe wings. Probably a good digital inclinometer is needed.
We use an Aeroperfect - unfortunately they are not currently available.
You are measuring in at least six places ,roots and strut points, so the cumulative error from crude readings can be great.
The meter should be as light and well balanced for tip readings as these wings flex a little very easily.

Brian
Old 06-28-2014, 01:53 AM
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Malcolm H
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Brian,

As always, your advice is excellent and while I can understand how incidence issues can affect overall trimming I don't see how they are impacting on the wobbling issue other than that slop in the struts might cause twisting under turbulence loads and that I will fix along the lines you have suggested.

To those that have asked about linkage/servo slop I can assure you that all is pretty much perfect in that department with ball raced clevises both ends and stiff alloy tapered rods. Servos are all Futaba Sbus BLS HV on 7.2 volts regulated.

Regarding wing loading, I have to say that my general perception has always been that heavier models are not buffeted as much as light ones, that is until the Sebart Mythos in both 125 and 2m sizes started to show up in our club. These things are very very light and yet they all look like they are flying in treacle! There were a couple of 2m ones on the field yesterday and I got to fly one back to back with the Hybird and it was light years better in the roll stability department.

One thing that characterises most Sebart models is they carry wing fences. Once I bottom out the strut issues I think I'm going to investigate tip fences a la F3a05's suggestion and the Citrin.

Unless anyone has anything else to add?

Malcolm
Old 06-28-2014, 02:13 AM
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Jason Arnold
 
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Hi Malcolm,

Have you tried different props to get more speed in the wind?

Cheers,
Jason.
Old 06-28-2014, 02:36 AM
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Malcolm H
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Jason,

Overall the Hybird is a slippery model and using the same Contra setup as in my Asyuler 10.15:1 ratio 22x20 rear 22x18 front, I have had to dial the throttle curve back substantially to fly at the same speed.

One of the reason for trying a bipe is to fly slower.

Malcolm
Old 06-28-2014, 03:05 AM
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Hi Malcolm,
On the Midrex struts there is only a single pin support in the centre of each strut end. Most have two !
The top one in only 20mm in chord and the bottom one 25mm. These are very short spans to support wing tips.
I have a spare set of Oxai Midrex struts and bases here for a spare wing we got due to transit damage to the original. There is at least 3 to 4 degrees of movement in these - unused !!
That would have to miss-behave badly in the air especially when turbulent.
I tightened mine up before I ever flew it so I can't say how bad it would have been in it's original state.

I think the CG and incidences may also be in play.
I think a large CG to AC moment with the possibility of different incidences and or twisting tips feeding into and through that 'large moment' may be a factor here.
Seba's recommended CG's are normally rearward - as far as I know !

Brian

Edit;
After posting I thought 3 to 4 degrees sounded extreme.
I've measured them.
The best fitting moves over 1 degree.
The worst moves 3.5 degrees.
This is with some hand pressure applied - 1/2 lb or so.

Last edited by serious power; 06-28-2014 at 03:32 AM.
Old 06-28-2014, 05:52 PM
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flycatch
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Originally Posted by thailazer View Post
Wing loading is the biggest factor. Heavy planes with small wings will look rock solid. I had a very heavy Super Star years ago and it would fly rock solid in any wind due to its high wing loading. Lighter planes I was flying like my Firebird pattern ship were much more affected by the same gusts. Add some weight to your plane and see how it flys when the winds are blowing.
You win the prize and if the poster did a goggle search he would discover what a ballast box is used for.
Old 06-28-2014, 10:27 PM
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Malcolm H
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Flycatch,

You know its snide smartass posters like you that are gradually stopping people who genuinely want help and to promote informed debate to come on here and ask for it.

I posted this thread with a humorous subject line because I'm at a loss what to do with my expensive dud and all you can do is take a cheap shot.

Thanks for that,

Malcolm
Old 06-28-2014, 10:52 PM
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Jason Arnold
 
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Originally Posted by Malcolm H View Post
Jason,

Overall the Hybird is a slippery model and using the same Contra setup as in my Asyuler 10.15:1 ratio 22x20 rear 22x18 front, I have had to dial the throttle curve back substantially to fly at the same speed.

One of the reason for trying a bipe is to fly slower.

Malcolm
Hi Malcolm,

Does it behave the same in an up line and a downline with rolls?

Cheers,
Jason.
Old 06-28-2014, 11:59 PM
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Wing fences make sense as bumpy air is vortices, and any kind of wing fence or vortice disruptor (as used on older model F-18's) are there to take advantage of or to control that vortex energy.
Old 06-29-2014, 12:20 AM
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Ryan Smith
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Originally Posted by Malcolm H View Post
Flycatch,

You know its snide smartass posters like you that are gradually stopping people who genuinely want help and to promote informed debate to come on here and ask for it.

I posted this thread with a humorous subject line because I'm at a loss what to do with my expensive dud and all you can do is take a cheap shot.

Thanks for that,

Malcolm
Have you ventured outside of the pattern forum here, Malcolm? It's quite dead, and this is quite possibly the most exciting thing that's popped up on the front page in a while. People don't get how intricate F3A is...I'm not sure I would have dignified that moronic post (that professed your question as a simple one, yet the guy couldn't even spell "Google" properly) with any sort of response.

Ignore and move on. Ordinarily I would not argue that airplanes are too light, but given the sheer volume that models of today have reached, I may reconsider that theory. At 4650 grams empty (plus an additional 650g fuel load at takeoff), my PL Partners flew great at 1007 sq in wing area. The Splendor that I flew last year was 4875 RTF and had about 5% more wing area. Remember that not all of a biplane's wing area produces lift, so a little over 1400 squares on a biplane is equivalent to 1000 squares on a monoplane roughly.

Do airplanes really need to be as large as they are today? I'm not flying F3A (I'm flying Masters class infrequently...life has a pesky way of having plans contradictory to your own), so I won't profess to know the answer to that question.
Old 06-29-2014, 01:35 AM
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Originally Posted by flycatch View Post
You win the prize and if the poster did a goggle search he would discover what a ballast box is used for.

Hi,
While accepting that to respond is to, an extent anyway, dignify - here goes;

Malcolm's model is right at 5000g.
The weight limit is 5000g.
What would you suggest he use as ballast ?? - Feathers perhaps !!
Old 06-29-2014, 04:20 AM
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Malcolm H
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Wow a lot of stuff here to try and comment on!

Jason, I'm not too sure what you mean about how it behaves in verticals and rolls? Are you referring to the bobbling about or that it seems to need less power than the Asyuler? Can't say I've noticed much bobbling in the verticals or through rolls but it does definitely need less power to roll nicely and to go vertical. You see the Contra equipped Asyuler is very much a single speed aeroplane, when the power setting gives a nice cruise speed it's also fine for rolls and verticals. Obviously in certain conditions you might need a click or two more especially if you are trying to hold position in the wind. This Hybird definitely needs more throttle control to fly at constant speed.

Ryan, yes I'm into turbines too and over on the jet forum it's much worse, I think very hard before posting there as do many others but thought it was better here. As far as your comments regarding model size, I personally think the modern large fuselage models present much better but it would be great to have a little more weight leeway to be able to ballast but as Brian has said I don't have that luxury. However you don't here Christophe or Wolfgang Matt or any if the other aces over here (and I bet your top guys) complaining so they have found the trick to stability in the wind. It might be by using the model elimination method though!

Brian, spent the morning installing carbon plates in each strut socket. Absolutely no slop now and the pins a tight push fit. You are right it's a fiddly job and I had to taper as well as thickness one plate to get the top of the strut to align with the socket. Even with the play that was in there I think the strut was twisting the wing slightly to get both ends aligned. A 1.7mm drill in a pin vice altered the hole angle enough to eliminate that along with the wedge plate.

Malcolm
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