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Side-thrust and wash-in

A couple of questions for those with greater knowledge of aerodynamics than I have.erod

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David Davis14/06/2019 20:15:38
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I maidened this model yesterday.

bertie 2.jpg

It is a three channel French trainer called a Baron which I hope to use in La Coupe Des Barons, a light-hearted competition for this type of model which will be held in September. **LINK** It is powered by an OS52 FS Surpass turning a 12x6 propeller.I have incorporated considerable sidethrust into the model. It's the third one I've built, though the second one was built for somebody else,

It is a standard Baron with a 1.55 metre (5ft) wingspan but I have made the following changes:

  1. Nose shortened by 20mm to allow for the weight of the engine. The original was designed for a 3.5cc Micron two stroke.
  2. Fuselage made lighter by omitting all plywood except for the firewall.
  3. Wing made lighter by using a three-spar with D section leading edge instead of the multi-spar arrangement of the original model.
  4. Dihedral reduced to 2" or 50mm under each wing tip, the same as the similarly-sized Super Sixty.
  5. Servos moved to the rear.

Having checked the c.g, I went to the flying field and no-one was there so I set about fine tuning the engine. Then the local non-flying Nosey Parker turned up and stood beside me while I adjusted the engine and throttle travel. I didn't know it at the time but he also filmed me on his Tablet!

I took off. The model flew well enough but required 62% left rudder trim, no ailerons remember, and 22% down trim on the elevator. Having got the model to fly staight and level I deliberately stalled it to see how it would react. It dropped its left wing viciously. I would have to watch it on landing approach. Despite my efforts the engine was not running perfectly and after three or four minutes it cut out. I proceeded to glide it back to the runway but ran out of altitude. The model landed at the edge of a field of wheat and was easily recovered undamaged.

After more fettling of needle valves and end point adjustment I had that engine running perfectly and took off again. I deliberately flew it gently until the fuel ran out in order to ascertain the flight time I would get out of a SLEC 6oz/180cc fuel tank. It flew for eight minutes. I glided it in to a landing but on turning into wind on finals, it stalled dropping its right wing this time, I had no control and it landed in the wheat again. Fortunately it was not damaged.

I looked at the wing and found that I had wash-in on both wing halves, i.e trailing edge lower than leading edge. Black mark for me for not checking earlier. I decided to pack up, go home and sort this out when the oldest member of our club turned up. I explained that I needed lots of left rudder trim for the model to fly straight and level and that I proposed to reduce the amount of right side thrust which I had incorporated. He said that the model needed more right side thrust. Sounds counter intuitive to me. Perhaps it was just the language barrier.

So two questions for the cognoscenti:

1. If the model is turning to the right in flight, do I increase or reduce the amount of right side thrust?

2. I have dealt with the wash-in by weighting down the inner part of the wing, putting a 20mm block under the trailing edge at the tip and using heat from a heat gun to shrink the covering. I intend to fly the model again this weekend to see whether this has made any difference. If the wash-in re-appears is there any alternative to stripping off the covering, chocking up the trailing edge, adding reinforcement and recovering it?

My first Baron flies well at all speeds.

baron ready for the coupe (2).jpg

Percy Verance14/06/2019 20:49:44
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Hi David

Having hopefully sorted the wash-in, I'd try it again before adjusting the engine. If you alter or change more than one thing at once, sometimes you're never quite sure which adjustment has done what........

The warps in the wings may have been the cause of some of the odd behaviour David, as might the slightly shorter nose.

 

Edited By Percy Verance on 14/06/2019 20:53:57

Geoff Sleath14/06/2019 21:00:50
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I'm sure it's better to try it first with the wash-in corrected for the reasons Percy writes but if the behaviour is the same I'd reduce right thrust and increase down thrust. As the engine is out in the open it shouldn't be too difficult to add a few washers to the engine mount at the field.

I've been watching the Criterium du Dauphine bike race near and in the Alps the past few days and the weather has generally been wet and windy (windy enough to blow Chris Froome off his bike and seriously injure him) so perhaps playing with lightweight models like the Baron may be a bit fraught anyway

Geoff

Martin Harris14/06/2019 21:07:13
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From the description, you need to reduce the right thrust - assuming it tracks true in the glide. Side thrust is useful but you can always add a throttle to rudder mix instead.

Perhaps your camera is distorting the picture you posted but it looks to me like you have the tailplane and fin offset to the left - this would work against your sidethrust but might explain the port wing drop. Again, probably camera related but I'd have said the engine has left thrust if any!

Most odd as these observations are at odds with your symptoms but there again, you are in a land where they drive on the wrong side of the road!

Peter Miller14/06/2019 21:10:19
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Personally I never use side thrust on ANY of my models.

The wash in is a real NO-NO.

AS the wing is obviously flexible without LE D box LE just heating the covering andleaving it to set should cure that problem, if not repeat the dose with more twist.

Just make sure that they are the same amount

Edited By Peter Miller on 14/06/2019 21:11:22

Geoff Sleath14/06/2019 21:18:42
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Posted by Martin Harris on 14/06/2019 21:07:13:

From the description, you need to reduce the right thrust - assuming it tracks true in the glide. Side thrust is useful but you can always add a throttle to rudder mix instead.

Perhaps your camera is distorting the picture you posted but it looks to me like you have the tailplane and fin offset to the left - this would work against your sidethrust but might explain the port wing drop. Again, probably camera related but I'd have said the engine has left thrust if any!

Most odd as these observations are at odds with your symptoms but there again, you are in a land where they drive on the wrong side of the road!

It looks to me that the rudder is to the right (ie not centralised) but the fin itself is angled to turn left. It's probably just the way the camera angle is, though, rather than a build error. David is too experienced to get that wrong.

Geoff

Martin Harris14/06/2019 22:02:53
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Was that in the second picture, Geoff? I think that's a different model...

David Davis15/06/2019 05:15:50
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Posted by Martin Harris on 14/06/2019 22:02:53:

Was that in the second picture, Geoff? I think that's a different model...

Yes, the grey model, my first Baron with the Russian cockades, has a warped fin but still flies well! I may have the chance to do some flying today, if not I'll fly tomorrow and let you know how twisting out the wash-in has affected the model's flying.

Jonathan M15/06/2019 08:28:23
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When trimming indoor scale FF models, the two issues are completely separate:

1. Glide test: for correct CG and flying surfaces.

2. Thrust lines: down-thrust to neutralise pitch up, right-thrust to neutralise left-rolling under power.

Normally, the shorter the nose the more down- and side-thrust needed, and changing to a more powerful motor would also need increased angles and vice-versa. But a RC model would need very little (say fine-tuning for the purest of aerobatics?) or usually none at all.

I suspect your biggest issue was wash-in on both wings (radically increasing tendency to tip-stall) together with an imbalance of comparatively more wash-in on the left-hand wing: effectively down-aileron on that side, the rolling effects of which would increase at higher speed but reduce at lower ones, which is not an easy situation to control with just rudder - both trim and normal movements! 

It is difficult to envisage how much effect your original thrust lines had on this: if right-thrust was overly-excessive then under power it could have been that which necessitated left rudder trim, rather than any differential in the wings; the only way to separate these out would would be to climb to height, then throttle completely down and observe the flight path.

So check you've fully and equally cured both wings of their old wash-in, but also check that the two wings are completely true to one another down their whole span. Also check whether one wing might be significantly heavier than the other and add sticky-backed tip-weights to balance if necessary.

Then deal with your thrust-lines.  Start with none (or very little, say 1-2deg each?) and experiment with what happens in the air.  Once the low-power glide is neutral, you can eliminate wing problems, but if there's a pitch-up or left-roll when the power comes on, then add washers to compensate.

Then get nosey-parker to bring his camera! wink

Edited By Jonathan M on 15/06/2019 08:44:19

Geoff Sleath15/06/2019 11:27:04
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Posted by Martin Harris on 14/06/2019 22:02:53:

Was that in the second picture, Geoff? I think that's a different model...

Yes, you're right. I just scrolled up and didn't recall that David had posted 2 pics. Must write down 100 times "I will be more careful before jumping to conclusions"

Geoff

Jonathan W15/06/2019 13:14:05
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How much wash-in have you got? Unless it is very bad, I doubt that will have too much negative effect given that you have a paralel chord flat bottom wing section and a light wing loading.

I am thinking that maybe you reduced the dihedral too much. The Super 60 is a high wing design, but the Baron is a shoulder wing, so I expect it would need more dihedral. I've once flown a 3ch RET model with too little dihedral and it was pretty horrible, much as you describe, very "tippy". Directiional control with the rudder was poor, due to too little dihedral.

kc15/06/2019 13:35:36
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Maybe it's an illusion but in the photo of the roundel model it looks as though the tailplane is not central - i.e. the front of the tailplane is moved over but not the rear. Therfore more elevator one side? A check with a tape measure seems needed to prove if it's an illusion or not.

My question is - if the previous model flew well why reduce the dihedral on this one?

David Davis17/06/2019 06:05:34
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Thank you gentlemen for all of your advice.

I did my best to get rid of the wash-in and I flew the model again yesterday. This time the model flew well at all speeds and I was able to execute a good landing but on the second flight I ran out of fuel and landed in the wheat again! I need to order some wood from SLEC so I'll order a 9oz/270cc tank from them once I've worked out what wood I need. Left rudder trim was reduced from 62% to 40%.

The model still pulls to the right so I'm going to remove one of the washers from behind the engine mount and see what effect this has.

Martin Harris and KC you are quite right, the tailplane is not quite central but it's only a matter of 3 mm and that's assuming that any of the other points of reference, e.g the firewall, are square!

I tried the glide test yesterday, with the power backed off the model dived quite steeply. I think I remember reading that if a model dived on the glide it required more nose weight. That sounds a bit counter intuitive to me and I would welcome the advice on this matter of those who have greater knowledge of aerodynamics than I have.

Three final points:

  1. I reduced the dihedral in order to have a more responsive model.
  2. Even with the washed-in wing the model flew well at moderate and high speeds.
  3. I built a second Baron to give myself a greater chance of having one in undamaged condition when the competition takes place in September! My Russian Baron, Boris, the model with the checkered tailplane, is currently on the repair bench.

Jonathan M17/06/2019 07:16:45
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Glad to hear its better sorted.

I'm no expert aerodynamicist or flier, but its fairly simple mechanically:

If a model is nose-heavy then it will need some up-elevator trim to maintain level flight at say half-throttle. So in a fairly steep dive-test (with the motor throttled back to remove any influence from that - see note *) as the speed increases this up-trim will make the model pull up (i.e. towards to canopy). Moving the CG back to its neutral position will mean no up-trim in level flight and therefore the model will continue straight in a dive-test. Moving it too far back will require down-trim in level flight, so the downward angle will steepen in the dive-test (i.e. pull towards the UC).

Excess right-thrust will reveal itself in a similar way: in level flight bang the throttle open from half to full, and if the model pulls itself to the right then you've got too much. (This effect is normally seen when opening the throttle for the takeoff run in models with no thrust-line adjustments, in which case the pull is to the left initially, therefore right rudder is used to track the model straight until airborne.)

PS * Also, in a dive the effect of gravity on horizontal weight distribution (between the nose, CG and tail) is reduced, until in a vertical dive it disappears altogether.  Therefore the only effect on the model will be airflow over the flying surfaces at increasing speed - which is where the 'CG-correcting' elevator trim will reveal itself fully.

Edited By Jonathan M on 17/06/2019 07:23:44

Nigel R17/06/2019 08:23:59
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DD,

"I tried the glide test yesterday, with the power backed off the model dived quite steeply. I think I remember reading that if a model dived on the glide it required more nose weight. "

Correct, to the best of my knowledge.

However.

It may be that you simply require a bit more downthrust. You have after all fitted quite a large motor for the airframe, and the thrustline as design will be correct for the smaller motor, not necessarily for a bigger one. If your CG checks out ok (the usual 30% chord) then I'd try adjusting the thrustline first - after all, that only takes a couple of washers.

Have you now fixed the warped wing? All bets are off if anything is bent.

The only other thing(s) I can suggest involve measured the usual rigging parameters checking for symmetry -

wing tips to tailpost / tailplane tips to fuselage CL at wing TE / left and right prop tips to tailpost

(prop tips to tailpost will be affected by right thrust of course)

wing incidence, tailplane incidence, downthrust angle

CG, fore/aft and port/starbd

Eliminate these unknowns first!

By the way I think your dihedral angle will be fine.

David Davis17/06/2019 17:03:32
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Thank you for the advice Nigel and Jonathan. I plan to test-fly the model over the next few days and will use the dive test to ascertain whether the centre of gravity is correct or not. I have a feeling that it's about right as it is. After that I will fit the cowling, the machine gun and the dummy wing-warping wires which are essential for all Barons which want to compete in La Coupe.

PatMc17/06/2019 23:41:39
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The dive test is a useful tool for gliders because diving is the only way their speed can be varied over a wide range whilst the model's reaction to this variation is observed. But the diving the model a power is completely unnecessary as the speed variation can be carried out simply by operating the throttle in S&L flight without changing the elevator trim.

brokenenglish18/06/2019 07:49:49
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I agree with Pat.

David, did you know that ARF Barons are available?

If not, check the Weymuller site.

Edited By brokenenglish on 18/06/2019 07:50:37

David Davis18/06/2019 11:58:09
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Yes I know that ARTF Barons are available Brian, one of my clubmates has one, in fact it's his second, but they are expensive at 200€, and appear to be out of stock at most suppliers. Besides, however, limited my building and flying skills may be, I enjoy building and I enjoyed competing in La Coupe Des Barons last year. The reason for building two Barons, is, as I have said above, so that I double my chances of having a model in flying condition on the date of the contest, 7th September.

The Baron in the RAF scheme has a number of modifications to make it lighter. I fitted a bigger fuel tank to it yesterday and I plan to fly it tomorrow. I will report back on the cg situation.

The other Baron is absolutely stock but I crashed it into the "goalpost" during the limbo round of a competition organised by my club and it needs to be repaired. I am taking the opportunity to reduce the length of the nose on this model to accomodate the extra weight of the four-stroke engine and to move the elevator and rudder servos to the rear. It had previously flown with considerable weight in the rear to get the CG correct.

I'm a great admirer of four-stroke model aeroplane engines and with the increased capacity permitted in this year's competition, I hope that they will be more competitive in amongst the ubiquitous OS 35AXs and 800 Watt electric motors. Last year only a handful of the sixty-eight competitors flew a model with a four-stroke engine, this year I hope to see a few more. I shall certainly be competing with a four-stroke and will donate a prize, a box of balsa, to the pilot who finishes in the highest position while using a Baron with a four-stroke engine. In the highly unlikely event that it is me, to the second man!

PS. ARTF Barons are penalised 5 points in La Coupe and everyone I've seen is a four channel model. In the competition four-channel models have to have their ailerons disconnected and taped up. French scholars may be interested in the following: **LINK**

kc19/06/2019 10:32:21
5901 forum posts
168 photos

David, I am amazed that you say the tailplane is only about 3mm out - it certainly appears to be about 9 or 10mm back on the right side ( compared to the wing at the wing bolt TE area) and therefore the fin does not look in line. Either some weird optical illusion in the photo or it's out! Could even be the angle of wing tips or the elevators are different sizes so the rudder cutout is not central, but it looks out to me. Maybe my eyes are deceiving me......

I would say that if you put a pin in the fuselage centre just behind the wing bolts securing a piece of cotton it would not reach equally the tailplane LE tip on both sides and that the differnce is 9mm or more!  Maybe even more if measured to the elevator tip TE.

Edited By kc on 19/06/2019 10:44:29

Edited By kc on 19/06/2019 10:46:23

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