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Calculating Washout

How do you determine?

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Nigel R16/05/2018 15:55:33
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A modified Clark Y, but how modified is modified, will that modified section have a stall that warrants washout? As much as a piece of string is long.

In my unqualified opinion, if the section is constant across the taper, and the taper is as mild as listed, then I would not bother, just build the thing all dead flat on the board and have done with it.

"Just to an another variable swept wings or even swept back leading edges can reduce the need for washout."

Hmm. I'm certain I have read the opposite. Sweep increases the chance of tip stall - but does produce a small dihedral type effect.

Hence why a million and one pattern models have gone with a tip 2/3 of the root, and the LE swept with TE straight. A bit of stability, cleaner to snap & spin, but not so much taper that it bites.

Sweep forward promotes root stalling first.

David Mellor16/05/2018 16:57:29
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Posted by Nigel R on 16/05/2018 15:55:33:

In my unqualified opinion, if the section is constant across the taper, and the taper is as mild as listed, then I would not bother, just build the thing all dead flat on the board and have done with it.

I would not bother putting washout in such a wing either. Why the modification? That seems to be a more potentially significant factor than washout.

Unless your design flight envelope is slow flight with high payload/wingloading, then I don't understand why you think washout is required on what sounds like a relatively high lift wing.

If it is important to you, perhaps you should calculate the plane's stall speed first?

PatMc16/05/2018 17:16:40
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If I read it right this sketch is roughly what a half span will look like ?

stfs wing.jpg

It's relatively low aspect ratio with very little taper.
I wouldn't consider any washout.

PatMc16/05/2018 17:30:41
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Posted by Jon - Laser Engines on 16/05/2018 08:47:05:

-2 at the tip relative to the root is perfectly normal i agree but that was not the issue. As the incidence was wrong (between 0 and - 1/2 at the root) the tips were then at about -3 in level flight. If i had a root incidence of 3 and the tips were at 0 i would have been fine.

Just to make sure we have all of our terminology straight incidence is angle of the chord line to the longitudinal axis of the model. angle of attack is the angle of the chord line to the relative airflow.

In this case, in level flight (ie, longitudinal axis horizontal or very slightly +ve) i was left with the outer wings at a negative angle of attack and hovering around their zero lift AOA. This meant that very (and i mean VERY) slight changes in this relative airflow caused massive trim changes as the wing either produced or didnt produce lift. The ailerons had a similar effect. level flight was impossible to maintain as it was continually trying to diverge either up or down.

As an example, if i went in to a loop the elevator would go ineffective towards to top and even with full up applied the model would push forward until near vertical and then stall. once pointing nose down, it would continue to tuck, even with the elevator up, until a speed was reached where it would start flying again. With the recovery in hand it would then accelerate to a point where it would become divergent in pitch and pull up almost uncontrollably. It would also do weird things like push out of a turn and was almost impossible to land. I suspected a wing incidence issue and checked. Then started packing the trailing edge down and flight tested the model. The difference was huge and while still not perfect its much improved. I would have liked to add another few degrees but it would have meant major mods and i couldnt be bothered. The flight performance in its current state is acceptable and i will fly around its remaining strange traits.

Jon, are you saying that the only change you made was to increase the root incidence but you didn't alter the washout which is still at -3 degrees relative to the root ?

supertigrefan16/05/2018 17:36:30
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Posted by PatMc on 16/05/2018 17:16:40:

If I read it right this sketch is roughly what a half span will look like ?

stfs wing.jpg

It's relatively low aspect ratio with very little taper.
I wouldn't consider any washout.

You have it spot on!

The consensus seems to be not to worry so I'll build it flat,

Thanks for the response guys.

Biggles' Elder Brother - Moderator16/05/2018 18:05:36
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Yeap, I'd focus on building that wing as straight and true as I could - no deviations!

BEB

Jon - Laser Engines16/05/2018 21:26:34
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Posted by PatMc on 16/05/2018 17:30:41:

Jon, are you saying that the only change you made was to increase the root incidence but you didn't alter the washout which is still at -3 degrees relative to the root ?

Yup. The washout itself, although slightly excessive, was only giving the problems it was due to the low incidence of the whole wing taking it beyond a certain threshold as -3 seems to have been somewhat of a critical angle between lift and no lift from the outer panels. I have not measured it yet but based upon my initial measurements I think I'm probably at +2 - 2.5 at the root and -.5 - -1 at the tips. Not ideal but a considerable improvement.

PatMc16/05/2018 23:05:41
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Sorry but if you haven't altered the washout it can't have been the problem. The incidence is only a rigging angle, all you've effectively done is changed the relative angle between the wing & the thrustline. For any given power setting the model will be flying nose down (compared with it's previous incidence) by the angle of the increase in incidence. The angle of attack will remain as it was & the lift generated at the tips won't have changed.

Jon - Laser Engines16/05/2018 23:38:40
3686 forum posts
146 photos

Err...no. The whole wing now generates lift more evenly as it is all now at an AOA greater than zero lift. Previously AOA at the tips was so low in level flight that small pitch changes would give lift on either the top or the bottom surface of the wing producing large trim changes. The attitude of the model in flight is completely unchanged and it has a wider speed range at constant trim than before as lift is produced more evenly.

While the washout itself was not the root cause of the problem in terms of how to fix it, it was the cause of the strange behaviour in the air as the model in effect had extreme washout. I would have either knocked all the washout out of the wing (not easy being foam) or adjust the whole thing back to where it was supposed to be in the first place.

PatMc16/05/2018 23:56:47
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The AoA doesn't change when you change the incidence. Only the angle that the fuselage adopts in flight has changed & with it the thrustline.

Jon - Laser Engines17/05/2018 08:47:09
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Posted by PatMc on 16/05/2018 23:56:47:

The AoA doesn't change when you change the incidence. Only the angle that the fuselage adopts in flight has changed & with it the thrustline.

Angle of attack is the angle between the chord line and relative airflow. With the fuselage at the same attitude it was before, the change of incidence will have changed the AOA as well. If it was the same, the model would still fly as badly as before.

Its not a case like the old Whitley bomber where it flew with nose down all the time. The attitude of the model remained the same but now no longer needs massive amounts of up elevator at slow speed like it did before.

In any case as you don't seem to understand the issue or the fix, and we are miles off the op now i wont be discussing it here any further.

Nigel R17/05/2018 11:34:25
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Jon, have to say, I'm with Pat here.

You've adjusted only the angle between fuselage and wing.

As a knock on, correspondingly changing the downthrust.

You would re-trim elevator for level flight in either circumstance.

In both circumstance, the wing will be at the same AoA in flight.

The Wright Stuff17/05/2018 12:24:05
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Hmmm. It seems as though the logic that Nigel and Pat endorse holds true. After all, the AoA is the only thing that affects the lift coefficient for any given wing.

However, unless we are calling Jon a liar, I think we have to respect his observations.

Can I propose that the discrepancy is a result of second order effects, usually disregarded by for some reason more sensitive on this particular model? I'm talking about the change in longitudinal dihedral by varying both the relative incidence of the tailplane and wing chord, and also the vertical offset between the centre of lift and centre of mass. additionally, the fuselage itself produces lift, which while sometimes negligible, perhaps can't be ignored in marginal cases such as this...

David Mellor17/05/2018 12:54:39
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Things that fly enjoy complex relationships with the fluid through which they move. And that fluid itself (air) varies a fair bit from one day to the next and one place to another - especially at the scales we fly.

So.... it isn't really possible a lot of the time to change one thing without interacting with a host of other things. Sometimes the interactions are insignificant, other times they aren't.

Observations are fine. It is conclusions that are drawn from observations that we need to be careful about.

Nigel R17/05/2018 12:55:29
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Not doubting Jon's observations at all, WS. But I agree, another cause there is.

Jon, what did the model do (previous to fixing the incidence) when cruising at 70/80% throttle, straight and level, and you cut the power? Did it climb or dive significantly?

I'm thinking the downthrust change was more to blame, with wing as was, there would be less downthrust (or even upthrust) so flight at cruising speed and power (or more) would be then be trimmed level by down elevator. With power off (dive, top of loop) there would be tendency to do what was reported.

At a loss to explain the ineffectiveness of elevator though. Fuselage effects?

Jon - Laser Engines17/05/2018 13:53:30
3686 forum posts
146 photos

I was going to abandon discussion of my Hurricane as i have no desire to 'argue' with Pat or anyone else. I stand by my comments so far and am confident in my analysis of the model as i have now fixed most of its strangeness. perhaps my example was slightly tenuous when it came to relating to this thread and the fact that i left out a great amount of detail (due to time/not wanting to take over the thread) meant that perhaps my assertions do not make sense.

if there is sufficient interest to discuss the symptoms i experienced with the model then i will start another thread dedicated to that discussion as i have to confess it was an interesting experience.

Edited By Jon - Laser Engines on 17/05/2018 13:54:06

PatMc17/05/2018 21:12:47
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Posted by Jon - Laser Engines on 17/05/2018 08:47:09:
Posted by PatMc on 16/05/2018 23:56:47:

The AoA doesn't change when you change the incidence. Only the angle that the fuselage adopts in flight has changed & with it the thrustline.

Angle of attack is the angle between the chord line and relative airflow. With the fuselage at the same attitude it was before, the change of incidence will have changed the AOA as well. If it was the same, the model would still fly as badly as before.

Its not a case like the old Whitley bomber where it flew with nose down all the time. The attitude of the model remained the same but now no longer needs massive amounts of up elevator at slow speed like it did before.

In any case as you don't seem to understand the issue or the fix, and we are miles off the op now i wont be discussing it here any further.

I do understand the problem you are/were having with your Hurricane & the fix. But this thread was opened by supertigrefan requesting advice on "washout". One thing that is abundantly clear is that washout can have played no part in the problem or the fix related in your anecdote.
IMO the advice you offered regarding washout was wrong, misleading and confusing thus defeating the object of the thread.

I also understand the probable causes of the problem with your Hurricane but as it has nothing to do with this thread I'll leave it there.

PatMc17/05/2018 21:16:32
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Posted by The Wright Stuff on 17/05/2018 12:24:05:

However, unless we are calling Jon a liar, I think we have to respect his observations.

I don't think anyone has questioned Jon's honesty, I certainly haven't.
It's the conclusions that he has drawn from his observations that I doubt.

Denis Watkins17/05/2018 21:46:06
2671 forum posts
136 photos

Italians can appear impolite when speaking to very close friends and family as they are very direct.

But in most cultures, rude, is just plain rude, and warrants an apology.

I hope you see sense

john stones 117/05/2018 21:54:35
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Some words creeping in here that'll do no good, and leave the Italians be, they've said nowt. wink

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