Roberto B.

I'm looking for someone who have first hand experience about flying a giant scale model with a full scale airfoil wing.
Thanks.

speedracerntrixie

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Iassume you are refering to an aerobatic airplane like the Extra or Cap 232. Both of those alonf with the Sukhoi use the same basic airfoil. Depends on what you are looking for. It is a very forgiving airfoil when used on a model but is difficult to get good snaps and spin entries. The airfoil just dosent want to stall.

Roberto B.

Thank you for the reply.
But at the last WAC in Foligno I spoke with some pilots. And they all said that their airfoil are used just because they are easy to stall for cleaner snaps and spins. They told me also that begin the snaps with elevator for the break, than rudder, and only after the rotation has begun, give ailerons to manage the roll rate and the stop.

speedracerntrixie

My Feedback: (29)

Full scale practice rarely works for our models. The big difference is power to weight and wing loading. What is a nice predictable stall for them is something that just dosent want to stall for us. With our light loadings we need an airfoil with a more aggressive stall. If there was any benifit to using a full scale airfoil then the model manufacturers would have picked up on it long ago.

Roberto B.

On what model did you make such experience?
I ask you that because what you say is just the opposite of what they say:
Their airfoils are easier to stall than the traditional ones we use in our models.

rhall999

Hi Roberto, There can be quite a difference in how a specific airfoil reacts from a full size to a model airplane. It all has to do with something called "Reynolds Number". I am not an expert on it, but basically that has to do with the way the air works with the wing in relation to the size. Since out models are much smaller, they are a lower reynolds number, and full scale aircraft are high reynolds numbers.

Due to the complexity of how airfoils work, and how they can differ from low to high reynolds numbers, what is true for a full size Extra 300maybe quite different than a1/4 scale one. This is what speedracer is getting at. Even though the wing on a full size Extra has a fairly sharp stall, on a smaller scale, that airfoil may not want to stall at all. There is also another thing to factor into that as well. Consider that a full size Extra can weigh aroune 1400lbs empty, if we scaled that airplane down to 1/4 scale it should weigh 350 lbs, but we all know that would not fly.Realistically, our model will probably be somewhere between 10-12 lbs. The combination of our light weights, lower airspeeds, lower reynolds numbers and a speedracer maentioned, the wing loading,all contribute to an specific airfoil having completely different characteristics when scaled down.

A practical example of this is too look at a control line stunt aircraft, or perhaps on of the "fun fly" type designs of the 1990's (Sig Fazer, Sig Something Extra, the old Morris Hobbies kits,area fewexamples). These types of airplanes have the same sort of airfoil as what you see on the modern full scale aerobatic planes, quite thick, with the first 20% being the fattest, and more of a flat taper towards the trailing edge. Now in our models, these types of wings love turning really tight corners,slowermanuevering athigh angle of attacks, and generally just get mushy instead of stalling nice and clean. At the same time, you put the same kind of airfoil on the big airplanes, they are much faster, and they provide a nice clean abrupt stall for the snap rolls, tumbling, and other types of freestyle manuevers.

I hope this helps a little bit with understanding why you don't see scale models with the proper "scale" airfoil.

Roberto B.

Thanks for the info.
Assuming that what you say is true, I have a question:
If such airfoil is so stall resistant for the model size, why it is never used on the trainers?

rhall999

Hi Roberto, your welcome. I just hope I got everything reasonably accurate. Like I said, I am not an expert by any means, I've just spent a bunch of time looking into this sort of thing just for the sake of learning as much as I can.

Now, as for why they don't use airfoils like that for a trainer, well, the study of airfoils can be very complex. While an airfoil may behave on way when slow, it may be somewhat different at high speeds. Also, an airfoil that is hard to stall, may not be stable. This all comes down to something called the "pitching moment". Basically, all airfoils have a tendency to want to pitch nose down, or nose up, or some are fairly neutral on there own. What designers do, is they will pick an airfoil that will work for their design. For a trainer, you want an airfoil with a gentle stall, able to slow down, and stable. Usually they choose on with a forward pitching moment, and then put the CG a bit more forward than an aerobatic type plane. Then, they design the horizontal stabilizer to produce a down force rather tha lift in order to keep the nose from dropping. This all makes an airplane that is dynamically stable, meaning it will right itself when you get into trouble. Typically, stability sacrifices manueverability. You can experiment with these planes by moving the CG aft further and further, and you will see that it will become less and less stable, especially in pitch, but you will gain a bit of aerobatic ability.

Now, with the symetrical airfoils we talked about previously, many of these are fairly neutral. The CG is adjusted accordingly so that the aircraft is contollable without being twitchy. What this means is that the airplane will be very manueverable, but it will not be "self righting" like the last one. If you get into trouble, it will just keep pointing whever it was headed before, and will not stabilize. This is what is called neutrally stable.

There is one more thing you could run into, an airplane with such a wing that is dynamically unstable. These airplanes, if you get into trouble, it makes itself worse!!!

All of these can have the same slow flight and gentle stall, but for the trainer, it is that dynamic stability that you want. The self-righting ability to give the student more time to react and correct. The symetrical airfoils on those fun fly types do not give that enough, which is why those airplanes can do the tricks that they can.

Hope this helps a bit again

PS, if there is anyone else out there who knows this stuff better than me, or can explain it better, please jump in and help, or correct me!!! BMatthews where are you

Roberto B.

What you say is absolutely reasonable, thanks.
Anyway, it seems that non one yet on this forum has used a model using such airfoil, because what I'm expecting is some kind of first hand experience.
I'm going to set up a 3.25 meters Extra 300s, and my will is to make it a full scale replica, including the airfoils.
What kind of drawbacks can I expect from this endeavour?

Avaiojet

Roberto,

Take a couple of deep breaths.

Now don't let your imagination get away from you.

Charles

rhall999

Hi Roberto, yeah I don't imagine there is anyone who has really tried to do what you are asking about. But, here is my 2 cents on your project. 3.25 meters is getting up around 40% scale or so for an Extra, so it could actually work decent. The reason I say this is because at that sort of size, you will be operating a bit closer to the same reynolds numbers as the real one. Now, while you may not get the exact flight characteristics as the full size, it should fly just fine, and still be nicely aerobatic. If it were much smaller, I would not be too sure. I would say go for it, see what happens. At the very least you will have a great looking, decent flying airplane, but I think at that size, the scale airfoil will work alright.

No, If I may, I want to throw out another comment regarding your desire to make it "full scale". I assume by "full scale", you don't mean the full size, (otherwise the wingspan would be more than 3.25M) but what you really meant to say wasexact scale. In otherwords, the correct scale outline and planform, proper sized and shaped control surfaces, and so on. To that I say....DO IT!!!! I am getting so sick of all these "3D" aerobatic rigs that are supposed to be scale, but with the extended fuselages, long tail moments, enlarged controls, wings/tails moved up or down etc, that you have to really look close to tell what aerobatic plane they were trying to model. And even then I sometimes can't tell what they are supposed to be!! They all look the same, and believe me, I have been an aerobatics nut, going to full size airshows and contests my whole life, and I can identify any full size aerobatic plane with only a glance, but these "modified" RC jobs all look the same, and most of the time you can't tell an Extra from an Edge from an MX. With the wings and tails all being moved around an reshaped and sized, the only reason you can tell if it is a Yak or Sukhoi is the round cowl, but then you can't tell the difference between those different types. I just am getting tired of it, and I really want to see an actual scale aerobat again.

Anyhow, no that I am back from going of on a tangent, good luck with your project!!

Roberto B.

Yes, of course I'm going to make a full scale replica, not a full scale 1:1 size!! Sorry for the misunderstanding.

I couldn't agree more about what you say. It seems you have stolen the words from my mouth!
Watching the latest World Air Championship, I had the chance to look at all kind of different airplanes put close each other, and you can bet that the first comment I had to say each time was: "so, THIS is like the Sbach (or put another name you want) is like!".
I fell in love with the Sbach (I had a long talk with his designer) and the MX22, but, despite the fact there was just one in competition, I had to admit that the old Extra 300s is my favourite. In a side by side comparison, the new 330SC and the S, the SC looks ugly even tough it surely flies better because of its bigger rudder and elevator.
I already know that a full size replica will have its own defect (a lot of mixing, etc). When I asked some pilots about this problems about, for example, knife edge flying, they all answered that it is not a problem, because they don't have a spring loaded control stick as we have on our radios, so they really don't have a clue about where the stick center position is, while flying!
Now, let's take the springs out of our radios and do the same! We won't have trimming issues anymore!! - I write it down: I'm joking, of course...-
Plus, there were a lot of Sukhoi 26 (that I later knew it flies better for precision aerobatics than the 31) in the assembly/disassembly stage. It has been very fun to help the crew.

rhall999

Yup, ever since Ifirst learned about the Sbach a couple years ago, I have loved the looks of that airplane. I gotta say though, I do like the lines of the new Extra 330SC myself, but the good ole' 300S is a pretty nice looking bird. The one that flew at the WAC this year was a neat yellow, purple and white airplane I believe. I do like the color scheme he had on it.

You ever seen a real Yak-54 up close?? I saw the real Russian Thunder at a couple airshows. Most of these so called Yak-54 models only resemble the real one in the fact that it has a round cowl and longer "2 seat" canopy.

Personally, I enjoy an airplane that has a bit of control coupling, and I do not use radio programming to help me fly it. I just train my thumbs which way they need to push or pull, and then I get to fly my own airplane instead of the electronics doing it for me. My manuevers sometimes aren't as perfect as the next guys, but I can usually handle crosswinds, fly smoother, and just generally fly a bit better because my brain and thumbs have been trained to add their own "mixing" in whatever amount is needed for every different scenarion. Not just the 20% or whatever you have programmed in. But...thats just me and how I like to do it

Another neat thing about the full size guys and not needing the mixing, is that when you are sitting in the aircraft looking out the front, it is much easier to see slight deviations in the path in Knife edge or whatever, so they can make whatever slight corrections are required before the deviation is large enough to see from the ground. We don't have that option.

I have been fortunate enough to go for a ride in a full size Sukhoi 29, that was quite an experience, loops, rolls hammerheads, even a torque roll!! What a day that was. The pilot even let me fly it for a few minutes, and let me tell you, that control feel was entirely different from what I am used to in my "normal" airplanes I have flown (Cessna 172, Citabria type stuff).

speedracerntrixie

My Feedback: (29)

Being that this was posted in the IMAC section my answer was one that has flown an exact scale 33% Sukhoi. Yes it did have the scale airfoil and it had no real bad habits, flew extreemly well at slow speeds and tracked well at all speeds. It flew quite light even though it weighed 29 lbs and was powered with a DA 100. The airfoil seemed to work for everything but good clean snap rolls and spin entries. This is why I would not suggest it for an IMAC type of airplane. The airplane also had all the bad habits of the full scale, pitch and roll coupling was present. However if you just want an exact scale sport airplane it will work fine.

rhall999

Ooohh, Ibet that Sukhoi was one nice looking airplane, any pictures??? That is awesome that in 33% scale it flew very much like the real one...except for the stall and snap problem. Lends a bit of weight to my theories as to how Roberto's Extra will fly

mithrandir

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The full size Extra has a section with a very blunt leading edge... and the top and bottom are flat on the aft 60% of the cord... and thick...

on a model, typically with low wing loadings as you will likely have, this type airfoil will cause the plane to go to very high AOA's before the wing stalls.... though this can be thought to be good... for an IMAC type plane it is bad...... the plane will need to go to very high AOA's for snaps... this is a killer... it robs speed and makes holding a heading difficult/impossible.....

I built a 35% Su26 several years ago and used a section similar but thinner then the real Su26... and it did exactly what I described above.....

so if you want a plane that spins and snaps cleanly without "Digging in"... you must use a thinner section... at least a section with a smaller leading edge radius....

Roberto B.

Thanks for the info, Mithrandir.
I'm aware of what I'm going through, but I definitely want to take this chance, because I really want a true scale model.
Ps: I'm keeping on thinking about me living in Adelanto for that job..... not really a piece of cake, I suppose ;-)

speedracerntrixie

My Feedback: (29)

Roberto, it all depends on what you want to do with the model. If you just want scale appearence then I say go for it. If you want a true flying model that is capable of competing in IMAC then you will have a poor flying airplane.

Magne

True.

But if he wants a good IMAC model then a true scale model would not be the best choice anyway.

Magne

JAS

My Feedback: (2)

Having flown the original prototype CARF 3M Extra 330S with a full scale airfoil, I can tell you that myself and 4 other pilots all said the same thing... crap. It wouldn't snap or spin at all. Wing was changed to the current airfoil and flown back-to-back with the scale airfoil and after those tests, the scale airfoil was trashed and never looked back.

Roberto B.

Thanks for the info, Jason. Very helpful as usual.

Anyway, the job is going to be made so that I could always switch to a conventional wing without too much hassle.

mithrandir

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Unless the model is a Yak 55 with the highly tapered wing!!!

lol

speedracerntrixie

My Feedback: (29)

Well in this case we are talking an Extra 300S and in exact scale will be a pig. It will have poor a poor snap and stall, Will have a bad tuck with rudder application and will roll couple with rudder. For model applications, the stab is too high, the wing is too low, the airfoil is too stable at light loadings, too much dihidral, too short coupled.

Roberto B.

Ok, it's all clear to me..
The problem is that if I want an Extra that looks like an Extra, there are no shortcuts available.
If the flight qualities will override the pleasure of the look, I could always decide to have a beautiful gate guardian in front of my house...

speedracerntrixie

My Feedback: (29)

All depends on what you want. If scale is your thing then it's all good but if you intend to compete IMAC with it then you will be limited.