Lafayette

my plane weighs 2 kilograms. It is powered by a .50 glow O.S. engine. If one of the wings will produce 20-40 grams less lift than the other (this is not a solid statement as the maiden flight has not yet occured), will the plane tend to roll over on that side VIOLENTLY or UNCONTROLABLY? OR will some trimming fix this problem of tending to roll over on one particular side?

Please don't tell me to redo anything .

Yishht.

What I think is that since 2-40 grams is nothing compared to over 2 kg weight, so the roll would not be fatal and the trim will solve it. True or flase?

Jburry

why do you think one wing will make a bit more lift than the other? If a wing is warped, this should be corrected prior to maiden.

How are you estimating the lift each wing will create? Why are you?

Our aircrafts wings operate in a very far from ideal environment. 2 identical wings, one left, one right, will never produce the same amount of lift on a plane. Things like the spiral prop wash (the twisting air stream blown back from the prop) ensure that the right wing will produce slightly less lift than the left, all else being equal.

We use the trim lever on our transmitters to correct for things like this. No model aircraft ever was maidened without needing at least some trim to achieve straight'n'level flight.

Now, assuming your estimate is about right, I think it'll roll gradually (several seconds for 45 degrees) and be easily correctable with trim.

J

j.duncker

My Feedback: (2)

If you have weighed each wing and found a difference in weight of 40 gr. do not worry it will fly ok with maybe a small amount of aileron trim OR you can add weight to the light wing panel.

If one wing is warped [twisted] that is a different story. I WOULD NOT FLY IT! I would fix the warp first.

garys

If you're referring to one wing panel being heavier than the other, and there aren't any warps, it won't be a big deal except tracking through loops, ect, where it will tend to roll towards the heavier side, or in a stall, as it will stall into the heavy wing. Straight and level flight it would be just a minor trim correction assuming no warps, ect.

Lafayette

No, my wing is not warped . I think one wing will produce less lift because on my right wing I accidentally got a 2-4 mm ridge coming out of my wing 3-4 cm back from the leading edge. Thsi was due to my stupidness of unoticing an overlapping spar. I think that wing will produce less lift since of the increase in travel length for the wind on the bottom of the wing- due to the small hill on the bottom. Now, I have a same hill on the other wing, only maybe 1-2 mm smaller. Guys, go to this link and see the three pictures of my plane and you tell me how affected the flying will be. http://www.rcuniverse.com/forum/m_73..._3/key_/tm.htm . There are three attached pictures near the bottom of the webpage of my yellow and blue covered airplane. I really dont think there will be much of a change in flying characterstics, but you guys are the bosses. Guide me.

Yishht87. Happy to yall.

carrellh

If you are referring to the "bump" on the bottom of the wing (center photo) I doubt it will have a huge impact.
I think it looks worse than it really is.
The shiny covering magnifies every flaw and makes it look gigantic.

Jburry

The shape of the top of the airfoil is much more critical than the shape of the bottom. That small amount of ridge will not have a significant effect. It may cause the wing to stall slightly earlier when the aircraft is inverted (you won't likely notice). I would not be concerned, beyond cosmetics.

If you are concerned, uncover the bottom of the wing, sand it as needed, recover and don't worry anymore. But only if it eats at your heart.

J

jaka

Hi!
I get the impression that this is your first airplane...and that you have made it your self...?!? Let me give you some advise.
First! It isn't that important how the surface on a model airplane wing looks like because it will still fly. Understand me right...of course it's important if you are competing in pylon racing , sailplanes and other competitive events. But not on a newbie airplane. What's important is that is build with wash-out in the tips and that the wing is strong enough to cope with the air loads. That's important!
Oracover is the best plastic film to use so I always recommend it. How big is your plane?? An OS .50 engine is rather big for most .40 trainers

I hope you don't try to fly this airplane yourself without help from a knowledgeable flier in your club.
A high winged trainer with a .40 engine is a much better airplane to start with.
Good luck!

Lafayette

Ok, thanks. I am very assured of that my wing is strong enough to cope with a predicted 81 g/dm^2. i accidentally banged it against a pole in a bus once, nothing happened. I also have the tips secured with strings one uses for fishing rods to the vertical tail fin which is complete balsa. The tail ofcourse also gets much stiffer like that, hence if it will try to bend one way the right or the left string would prevent it from doing so. Yes it is my first airplane build, but I have flown a gas .40 trainer once or twice before and yes of course I will give a good flier in my club to maiden it- my trembling knees would be a distraction for flying.

Pardon me, I read a lot about washout and downwash when the air departs halfway through the airfoil to be deflected sligtly downwards, which is a half of the full explanation of how a wing creates lift. Now, please enlighten me what the heck is washout? It is a familiar word I see in a lot of aerodynamic books but what is it? I am thinking of the fins on the tips of the wing, but those are used to somewhat prevent induces drag on the wingtips aren't they? Hmmm....

THank you,

Yishht.

Do you really think the surface of a model airplane doesnt matter too much if it is a trainer type aircraft? What is your reasoning behind that? LIke why do you say that?

Jburry

Washout describes a wing that is twisted such that the wingtips point slightly downward, 1 or 2 degrees, relative to the wing root. This increases aileron authority in a stall, and helps prevent tip stalls. It was famously introduced in the Spitfire in WWII, and allowed its pilots to harness the wonderful efficiency of the otherwise tip-stall prone elliptical wing planform. It allowed the spit pilots to turn closer to the plane's limits, since as they approached stall AOA, the wing root would begin to stall first, causing buffeting and stick shaking that would clue the pilot in and he could reduce elevator input as required. The 190 and 109, like most other planes previous to washout would just abruptly stall, sometimes spinning.

The washout simply serves to reduce the AOA at the wingtips so they stall last.

There are ways, other than twisting, to achieve an aerodynamic washout too. One is by having an airfoil at the tip that is thicker (as a % of chord) than it is at the root. This is commonly used on aerobatic aircraft where warp washout is undesirable as warp only works when the plane is upright. Inverted it increases tipstall potential, while by using varying airfoil techniques it can work in both orientations.

The major penalty for washout (nothing is free!) is increased drag in all flight regimes as the whole wing can never be at the optimum AOA. Some parts will always have excessive AOA (root) and some will have less than optimal (tip), but with reasonable amounts of washout this is usually well worth the cost.

Twisted washout is especially helpful on warbirds which are known to land hot and tipstall if slowed too much.

J

Lafayette

Oh, ok thank you I remember kind of now. I have seen this washout on the fokker 90 jet I flew recently. But the problem is the airofoil I have is a NACA 2142 or something close. This airofoil has 0 lift at around 1 2 negative AoA. Since my purpose is to learn level flight and at fairly low to medium speeds, I think it is useless to have washout wing tips, but thank you anyways I will consider it in my next plane when I am confident with da sky- although my budget won't be comfortable thats for sure... .

Now... someone please answer that other question on "the shape of an rc model does not matter too much". Bmatthews told me whatever flaws there are on my model a good cg and wings and tail all level to the nose at a right angle will assure the model flies. I have been taught the hard way in books which said that any flaw could turn into a fatality. Anyways, Jaka - I would prefer you to answer this- or anyone else, please answer this: If I have a powerful engine for my model and all my surfaces such as tail and wing are at a right angle, But I have a medium sized flaw extending throughout my right wing and somwhat throughout my left (the ridge/bump), what chances are there in percent for my model.

I know yall said the chances of it not flying are quite small, but please I wanna here from some people a straight forward answer- will my model fly or not. I can take the fact that it will tend to roll on one side and all, I just wanna here if it will take off from the ground or not.

Yishht87... Thank you and I would really appreciate an answer to the question in the last paragraph.

Jburry

Washout is actually most beneficial at low speeds, near stall. It's not a bad idea, but if you have a constant-chord wing (not tapered), then it already has washout built in without warp. The effect, in this case, is caused by fuse turbulence causing the wing roots to stall first.

OK, you asked about shape and RC models. I assume you're talking about the precise shape of the airfoil.

In normal cruising flight, the shape of the airfoil isn't a big contributor to how the plane flies. This is true. A thicker airfoil will prevent large speed increases with more power or in a dive, and a thin one will speed up well. Both will carry similar loads, as it's the wing area that really matters for this. On most models, a flat plate airfoil would cruise pretty much as well as a clark-y of similar thickness. The difference is primarily in how the foils stall.

An airfoil is measured in alot of ways, and camber is one important measurement (beyond chord, thickness and such). All wings are cambered. Symmetrical wings have 0 camber, while a flat bottom wing is equivalent to some undercamber, and a genuine undercambered foil is very cambered. This affects the stall. The more cambered an airfoil is, the higher the critical AOA will be where the airflow detaches from the top of the wing, and the wing stalls. Cambered airfoils essentially present the leading edge at a lower local AOA than the wing's total AOA, if you follow.

Say we have 2 wings. One is a symmetrical, the other is flat bottomed. They are of otherwise equal numbers, same chord, thickness and span. If a line is drawn down the middle of the wing's chord, halfway from the top and bottom surfaces at each point, that line will show the wing's actual camber. On the sym wing, it'll be a straight line down the middle. On the flat bottomed wing, it'll be a curve about half as deep as the wing's top surface. If we place the wings so that they're at thier 0 lift angle, we'll see that the camber line on the symm wing is straight and level. On the flat bottom, the chord line will point down somewhat at both the leading and trailing edges. As the wings rotate up in the airstream, the symmetrical wing will stall first. Maybe it's AOA will be 15 degrees at this point (random number for discussion). The flat bottom wing will also be at 15 degrees, but locally it's leading edge will be at something less than that, maybe 10 degrees. It will require somewhat more AOA to actually stall. This is why this type of wing is used on trainers and heavy lifting craft where a late, gentle stall is needed.

Model aircraft (well, the small ones, like yours) run at pretty extreme reynolds numbers compared to the full scale stuff. Because of that, our airfoil selection is much less critical than it is for the big guys. Not something to get hung up on.

Hope this helped, and didn't just confuse.

J

Lafayette

to summarize this (don't worry I understood all of what you said thanks[8D]) you are saying that it it is not critical for my airplane being able to fly if one of my wings, the right one, has a slightly different shape than the left one and that my wings will not play a critical role in the airplane's ability to fly. Right?

Thanks, Yishht. But you are still considering my wings are perfectly similar to one another. But what I am worried about is that they are not- b ecause of the larger ridge on one of the wings. Say that one of the wings differs from the other. How will that impact my airplanes ability to fly- considering that it is small- 1.5 meter span, and an engine which will slightly overpower my plane, making sure it has enough thrust to perform anything which the actual aircraft by itself cannot do- like if the wings dont produce quite enough lift (although my wings are so long I am sure they will).

Lafayette

Which diagram do you think describes best how the airstream would behave around my wing?

Jburry

First one is closer.

Look at, read and understand the info on this link:

http://www.av8n.com/how/htm/airfoils.html

The lift force from an airfoil is about 80% from the low pressure region above the wing, and only about 20% from the pressure below it. Bumps and humps in the lower surface are bordering on irrelevant on our aircraft.

Don't worry about it. Warps you built in and haven't noticed (they're in almost every plane!) will have a larger effect than this spar bump on the lower surface of your wings. Stop worrying and start flying....

That link I provided will really help you understand how air behaves around a wing.

J

Lafayette

Thank you Bmatthews for the profili2 and xfoil but I am not going to pay for them unfortunately- I am sure no matter how good the programs are real life situations always exceed computer dynamics- because that software works on an algorythm which in many cases is not so perfect in real life. So I used my creativity and took a flat piece of plastic and glues a small cardboard and tape ridhge on the bottom. What happened was close to diagram 1 although it was diagram 2 all the same. The ridge acted as a flap, thus making all the airflow turbulent behind the ridge. I noticed that the boundary layer never detached from the wing- and that is good since that is the crucial part for producing lift. All the flow above the flap was slightly deflected upwards but all the same it was still attached to the bouydary layer.

The conclusion being diagram 2 is underdone and diagram 1 is overdone. What I got was much closer to diagram 2. My conclusion is that the ridge will not severely act on lift production, although it will induce airflow transition from laminar to turbulent a centimeter or two earlier that the natural transition. The wing will produce slightly less lift than the other one (by about 20-30 grams because lift production relies 80 percent on the upper part of the wing which is perfect) but that will not severely affect the flight considering plane weight being about 2.2 kg. Some trimming will do the job.

How did I do my test? I used a tap and some cold water. You all know hydrodynamics are the same as aerodynamics.[X(]

Do you think my conclusion is right?

Yishht87.

Lafayette

Hey all again.

I decided already. The day I come back from my holidays I will strip the bottom section and fix that ridge/notch. I just think it is pathetic to let my expensive plane fly with a potential disaster on the shoulders. The airflow could change any second in flight- a split second vortex formed under the wing could make a stall. I don't want to let that happen. If a crash is this planes fate, I wanna do everything for the crash to not be my fault.