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Low speed wobbles and flying bricks.


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#1 Plank

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Posted 12 September 2016 - 05:43

Dear Chaps.

 

Does low speed aerobatics feel a bit wrong? ( Virtually wrong ? )

 

Does your plane stall in a way which seems a bit fishy?

 

 

 

Today I was bashing around in the N17 GBR and the N28.

 

Both seem to wobble alarmingly when flown way too slowly. ( I am very very used to flying like this.)

 

Is it the large paddle like wings which for some reason are able move through the air ( flat ways ) like the air is not really there. (?)

 

or am I dreaming.

 

The amount of force you would need to push a wing against the air would be significant.

and yet the plane seems to flop about so, like the air is not really there.

 

(yes I can imagine air recirculating/slipping around the wing as it move "down" etc... but...hmm.)

 

 

and I remember someone suggesting the N28 felt oddly over weight.

 

 

 

Well for those pilots that have actually tried to out turn other planes in it ( the n28) ,

you will have noted it's tendency  to fall out of the sky at certain speeds,

like it's much much heavier than it looks...

 

Which is the thing that struck me just not more than five minutes ago. 

 

The reason why I am thinking about this this is because I prefer slow knife fights and flying on the edge or even outside the flight envelope.

( Not flying like a "bat out of hell on rails".)

 

The slow and protracted fight is my kind of thing but it feels as though the FM has been a bit neglected in this area and is sort of

a bit over simplified. ( I might be quite wrong here and would be happy to be shown why I am so.)

 

To witt.

 

The actual wings pushing sideways against air is not a really big deal when you are slicing fwd through the air really fast.  ( maybe?)

 

However it IS when you are starting to move very slowly through the air in a stall.

 

Air compression.?

 

If you take a sheet of plywood and try to drop it flat, it will resist the urge to fall like a brick.

This is due to air compression. ( easiest way to describe it)

The sheet will try to slip sideways, to cut through the air mas and fall edge ways.

 

The wing of my n28 seems to push flat ways through the air like it's not there, when it begins to stall.

( imagine a fuselage with no wings or a wing with no air. )

 

Is this really what one would expect from a light weight plane with such large wings?

 

It looks like "floating" has not been included in the FM. ( am I wrong ?)

 

Hmm.

 

I realise that programming a FM is not easy and it's going to be hard to please every one.

it would not surprise me if the FM was tailored to "fast flying" and some of the more complex

low speed factors have been simplified out.

 

Such if life. 

 

 

and this is not a whinge it's an observation.  : )

 

Feel free to tell me I am full of twaddle by PM.

 

and Lessons on how N17 or N28's actually work at low speed would be gratefully accepted.

 

Salute!

 

Plank.

 

PS. When they finally put a better engine on the N28 I will be a very very happy hedge hog. More torque, less top end thanks!


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#2 Ice_Age

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Posted 12 September 2016 - 08:02

It's those little men at the tail pounding on it with their hammers, because the turbulent air from the stalled wings is making it impossible for them to blow bubbles, and they are angry about it.


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#3 unreasonable

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Posted 12 September 2016 - 08:46

To put it another way, does the FM model the effects of air-resistance on the various parts of the plane after or close to departing flight?

 

I do not see why not, at least simplistically.

 

Remember even after you have stalled the plane still has momentum, some degree of lateral stability due to the rudder and fuselage sides, turning motion due to the prop ... there is a lot going on, and there is no reason why a plane would automatically turn "sideways". Some were much more stable than others.

 

From the point of view of the game what is important is that a certain AoA the wing stalls and the plane just becomes an object hurtling through the air. As to the amount of force to "move the air": you are talking about hundreds of kilos moving at 100kph. Get it unstalled and you fly again.

 

The N17 in particular was a highly unstable design and would do all sorts of odd things, so "wobbling" if you are getting close to stall AoA is probably entirely reasonable. 

 

I am sure there are aspects of the FMs that are out, but nearly all of the light plane pilots who have posted here over the years have commented that RoF's treatment of how these crates fly is remarkable life-like for a PC game, but I do suspect that many of them a harder to spin by accident than their originals (except the DH2 grrr).


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#4 Plank

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Posted 12 September 2016 - 09:10

Hello yes. ( bodged my retort to Ice... dratted mouse with too many buttons...)

 

Ah, unreasonable ( is that small enough?)    and how many actual pilots put their planes in places the definitely should not?

 

I guess I would have to see planes stall crashing to really be convinced we are getting the jam we paid for. 

not that I am too concerned but it's interesting none the less.

 

any video links?

 

I am assuming that the acceptable aoa decreases with decreasing air speed.

 

Until you are air braking if you lift your nose up at all.

 

this makes sense but the wild flopping over is harder to understand.

 

If there is very little IAS then I suppose it's torque reaction that is twisting the plane.

But still that's a lot of air to move for the wings to flop. ?

 

Back to flight school for me I fear.

 

Salute!

 

Plank. ( Must be time for coco...)


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#5 unreasonable

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Posted 12 September 2016 - 09:43

 

 

I am assuming that the acceptable aoa decreases with decreasing air speed.

 

 

 

No - the stall angle of a wing is not dependent on speed. If you go over the critical angle you will stall, simple as that.

 

When people talk about "the stall speed" they are talking about the minimum speed the plane can fly straight and level without stalling - at that stall point the wing will be at the critical AoA. If you go slower you generate less lift, so to stay level you increase the AoA - which stalls the wing.

 

If you are flying fast and then jerk the stick right back it is possible to exceed the critical AoA and then stall, but this will depend on whether your elevator - and arm - has enough power and that your wings do not break off as well. ;)

 

If you really are interested in learning about how flight actually works, there are plenty of good online guides or even - gasp - printed books. If a non-mechanically minded person like me can understand them (opinions vary) then you can too.

It really is interesting, and it helps your flying in RoF too!


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#6 Plank

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Posted 12 September 2016 - 11:02

Hmm.

 

Reading aoa in Wikipedia. Still seems odd.

 

"A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed.

The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor, the center of gravity of the aircraft and other factors."

 

 

Running out of lift, is it a stall?

 

If I go very slowly and then try to nose up a bit then I stall.

 

What is involved?

 

Angle of the wing to airflow.

Lift from wing shape.

Thrust.

Airspeed.

Drag.

 

all of these combine to keep you in the air.

Change one and all the others change.

 

Probably overthinking it but I guess I would like to see how this all plays out in R.o.F....

 

and those prop hanging Central planes....  can they do that in real life ??? those inline engines must

weight a lot....

 

surely the power to weight of the N17 would be better.... ? it's such a wee thing....

 

Salute !

 

Plank. (scone time.)


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#7 unreasonable

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Posted 12 September 2016 - 13:42

Again, the sentence " The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor, the center of gravity of the aircraft and other factors." is referring to the level stall speed.

 

For instance, if you add weight, you must generate more lift to fly level - by definition, if you are flying level lift=weight. To increase lift you either increase AoA or increase speed. But if you are near critical AoA already you cannot increase it that, so you must increase speed: so your "stall speed" must be higher. If you do not go faster you will not stall, you will just start to descend.

 

Stalling at higher speeds is sometimes called an "accelerated stall". RoF planes do not seem to do it so much, probably because if you are going fast and pull hard on the stick, you will rip the wings off before they stall anyway, but it is useful to get this fundamental point clear.

 

As for lift, weight, drag and thrust - these are the fundamentals of any flight model and you can be sure they are all modeled in RoF. Whether the exact values are correct, and the various complications, well they are approximations of course.

 

You could do worse than have a look at a site like this: www.caa.govt.nz/FIG/basic-concepts/climbing-and-descending.html


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#8 Ice_Age

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Posted 12 September 2016 - 16:15

To be honest, I really have no idea what Plank's question is.  Given the title of this thread, I assumed he was asking if the stall buffeting that he experiences is realistic.  To that, the only answer is yes.  To the rest of what he wrote, or asked, or didn't ask, I cannot translate.


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#9 Plank

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Posted 12 September 2016 - 17:55

Hmm.

I am not sure now either.

 

I think it was about the general waving around of the wings in a low speed "stall" in the N17.

 

pulling back on the stick seems to make it much worse ( even though it's not increasing it's Aoa by much at all)

 

I think it's the dramatic nature of this that I find alarming.

and am wondering how true it is to real life....

 

At super low speed, if you in crease your aoa a tiny bit then it's all over.

 

However that aoa at a much higher speed does nothing.

 

So the airspeed dictates what angle of attack is acceptable. ?

 

any way...

 

The low speed stall spin of the N17 at very low speed is quite a hand full

and I enjoy wrestling the controls to keep the plucky little plane aloft.

but I can't help from wondering if the gods of flight have made it a bit too dramatic....

 

Would it's very serious drama at very very low speed be this intense in real life actual flying?

 

and it's not like the engine has stopped dead at the worst possible moment, it's going fit to burst....

 

I think I will rest here and possibly not go any further. If I get an serious info I might leave a note.

 

Salute!

 

Plank. ( Low speed flat scissors in the SPAD 13 are loads of fun...)


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#10 unreasonable

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Posted 13 September 2016 - 03:10

 

At super low speed, if you in crease your aoa a tiny bit then it's all over.

 

However that aoa at a much higher speed does nothing.

 

So the airspeed dictates what angle of attack is acceptable. ?

 

 

NO!

 

At low speed you are already at a high AoA near your critical angle, if you are flying anywhere near level. You have to be to generate enough lift to stay level. If your AoA was not very high, you would be descending. Hence increasing AoA will push it over the critical angle and you will stall.

 

At high speed, flying level, your AoA is low: it has to be or you would be generating more lift than your weight and would start to climb. Hence you can increase it without stalling - you will start to climb, (or turn if you bank).

 

The stall angle of attack is completely independent of airspeed.

 

I recommend you find a copy of "Understanding Flight" by Anderson and Eberhardt. Amazon or local library? Very clear with good diagrams without too much technical language.


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#11 Plank

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Posted 13 September 2016 - 04:55

Hmm.

 

Fwd movement through the air.

Lift coef adjusted by wing AoA

 

loads of other things.

 

If your fwd speed and lift coef are good you will be flying.

 

If speed drops and you have no lift coef left. ( it goes up then goes down)

you are not flying any more but "stalling".

 

The lift of your wing is determined by how much air is going over it and what "angle" it is on. ( and it's shape )

This "angle" is the A.O.A.  a zero aoa will produce the minimum lift determined by air speed, increase aoa and you increase what lift

you have at that airspeed. ( you are multiplying the lift by a coefficient) however at a certain aoa the lift will reduce dramatically.

( this is not just the coefficient changing, it's the whole equation as the wing is moving out of it's normal operation thingy.

 

 

 

ah, so pulling out of a dive too quickly will increase the aoa relative to the air moving and push you into a stall.

( in our case the wings rip off)

and the ground coming up to meet you means you can't do a shallow dive/pull out more slowly.

 

OK I think I have this.

 

 

If I am going really slowly I am flying at max aoa in level flight.

If I pull back too much I push into the "reduced lift coefficient part of the curve" and stall. Wildly.

 

If I am sinking while at max aoa I am in effect flying too slowly. and need to increase thrust, or decrease drag or increase lift or shallow dive to pick up speed or toss out ballast. etc.

 

That I think sounds right.

 

AND

 

when I am in this position the plane will drop a wing.

( one wing has a tad more lift and it goes up while the other goes down... and engine torque, rudder mashing etc)

 

I think I might be done on this one. Fingers crossed.

 

Will read the book,

 

A funny thing:

 

When I caught the R.o.F bug a few decades ago ( it feels like it) I went to the local Library to find books on how to fly planes.

The how to fly planes for idiots guide that sort of thing.

 

The librarian said:

 

"We don't have them for security reasons" and looked at me like I was a naughty person.

 

I was unimpressed. 

 

Of course I could have taken out a book on "How to raise an army on the cheap" or "Janes guide to tactical nuclear weapons in eastern Europe"....

 

but not how to fly a Vee tail Beechcraft bonanza... 

 

Hmmph.

 

Salute!

 

(Plank) oh I am mucked that bit up...

 


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#12 unreasonable

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Posted 13 September 2016 - 05:12

By George I think you've got it!                

 


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