NACA 4 digit airfoil in my new proyect. Your opinion please.

Good morning fellas,
I'm about to start with my new project in a few weeks, and am intending to use the NACA airfoils shown in the attached pic, which depicts the finished airfoil with the sheeting included.
Both of them -root and tip- have its max thickness at 30,5% of the cord.
All other dimensions of the ship are identical to the Trivial Pursuit and will carry a Saito 72. The projected weight of the plane is 65-67 oz.

Can you please give me your opinion concerning this airfoil?
Do you think that it is suitable to use it in the described plane?
Any additional considerations will be most welcome.

Many thanks,
Claudio.

[photo not recovered: 29584.jpg]

Attachment #1, (jpg file)

>All other dimensions of the ship are identical to the Trivial
>Pursuit and will carry a Saito 72. The projected weight of the
>plane is 65-67 oz.
>
>Can you please give me your opinion concerning this airfoil?
>Do you think that it is suitable to use it in the described
>plane?

I think those are excellent choice. I would doubt that they will be significantly different (better or worse) than the TP airfoils, but they will work just fine.

Brett

>
>>All other dimensions of the ship are identical to the
>Trivial
>>Pursuit and will carry a Saito 72. The projected weight of
>the
>>plane is 65-67 oz.
>>
>>Can you please give me your opinion concerning this
>airfoil?
>>Do you think that it is suitable to use it in the
>described
>>plane?
>
> I think those are excellent choice. I would doubt that they
>will be significantly different (better or worse) than the TP
>airfoils, but they will work just fine.
>
> Brett

Thanks Brett. Have you any experience with this kind of airfoils?
And let me ask you, by the way: What airfoil are you using in your "Infinity"?

Claudio.

>
>Thanks Brett. Have you any experience with this kind of
>airfoils?

Essentially all of my designs have used NACA 00 series or something very close. As long as it's reasonably blunt, reasonably smooth, and the right thickness, it will be OK.

Note that there isn't any particular aerodynamic advantage to the fact that the NACA tested them or came up with the equations to define the profile. It's easy to compute the pressure profile but unless you are attempting to develop an analytical way to determine the performance that doesn't help you very much. Drawing around your favorite shoe (as long as the basic parameters are close) is just as likely to work as anything else.

Brett

To Brett,

I read your remark about TP sections with great interest.

Can you tell me where I can find the TP sections? I presume TP refers to Transonic Pressure?

I have several C/L stunt models of my own design using the NACA 0018 symmetrical airfoil. (ex: WILDMAN 60 with Orwick 64) I don't know of any better choice. At least, some indication of efficiency is displayed during the glide to a landing. This airfoil seems to want to stay in the air forever!!

The generous L.E. radius (I am told) prevents abrupt stall and generally smooths-out the transitions in square maneuvers.

Maybe some Aero-Expert will jump in here with a better theory?

Floyd in OR

>The generous L.E. radius (I am told) prevents abrupt stall and
>generally smooths-out the transitions in square maneuvers.

>Floyd in OR

Yes Floyd. I've heard/read that several times and is the reason why I want to try with NACA 4 digit airfoils.
Have you noticed that allegedly smooth transition in squares?

Thanks for your reply.

Claudio.

>I have several C/L stunt models of my own design using the
>NACA 0018 symmetrical airfoil. (ex: WILDMAN 60 with Orwick
>64) I don't know of any better choice. At least, some
>indication of efficiency is displayed during the glide to a
>landing. This airfoil seems to want to stay in the air
>forever!!
>
>The generous L.E. radius (I am told) prevents abrupt stall and
>generally smooths-out the transitions in square maneuvers.
>
>Maybe some Aero-Expert will jump in here with a better
>theory?
>
>Floyd in OR


Hi Floyd (and Cesar),

Just a slight correction. The large LE radius does NOT prevent "abrupt" stalls. In fact it is kind of the opposite. Here's a brief discussion.

What the larger leading edge radius does do with respect to stalls is that it "delays" the the airflow separation that produces the stall to a higher angle of attack. This, of course, allows greater lift to be produced by virtue of that higher AoA. This is very helpful for stunt ships and, in particular, those with higher wing loadings such as Trivial Pursuit type designs intended to use the higher weight powertrains, larger displacement tuned pipe set-ups that require a bunch of fuel weight, all of which adds up to 60 plus oz on a relatively small area wing (650 square inches or so).

Classic type, sharper LE airfoils at this wing loading employing "traditional" powertrains would not have performed nearly as well. On lower wing loading airplanes the larger radius is less important and not likely to be required because large AoAs are seldom necessary.

The one down side to very large LE radii is the fact that when the "do" stall they stall very abruptly. Under modern power this simply isn't an issue, clear up to weights in the high 70s for a properly designed and powered airplane using the blunt LE. Under power you simply almost never get to a critical AoA in a corner tight enough to be controlled by the human pilot.

The only place this can be an issue is when trying to stretch a glide. Back when I flew the original Imitation (I probably had more flights on that airplane than all the rest of my original designs combined) I several times had that abrupt and complete stall occur while stretching the glide. Once it goes, it's done.

I must reiterate, however, that the Imitation never even suggested a stall under power even with some experimental "extreme" maneuvering with very small but recognizable maneuvers intended specifically to try to make it do so.

The lesson learned from that was that the Trivial Pursuit was designed with a slightly (repeat, slightly) sharper LE radius. The stall issue on glide has never occurred on any of my T.P., two of which weighed nearly 70 oz.

It is also worth noting, by the way, that all of my "modern" design airplanes had airfoil high points in the 22 to 25% of chord range. You state that the airfoils you show have them at or about 33%. I haven't measure them but suspect that measurement is based on the unflapped sections you drew. The addition of flaps will likely make the high point right around that 25% figure for the entire airfoil.

Airfoils should always be considered including flaps, by the way, when providing "numbers". Flaps are part of the airfoil/wing. The only thing unique about them is the fact that the can be moved to change the shape of the airfoil and produce a higher coefficient of lift than that achievable with the flap fixed.

Your airfoils look just fine by the way. Only suggestion would be with regard to the very blunt leading edge and the earlier discussion of the abrupt stall that can result as it did on the Imitation. You could very easily redraw the leading edge to produce a smaller radius which would remedy the possibility.

Ted Fancher

ooops, should have kept reading before responding. Larry has said much the same thing regarding airfoil high points with the exception that he continues to do so without regard to the flap. I still, respectfully, suggest this isn't usual protocol.

Edit 2: It's not my day - or something. I found that I had included my response to another thread in this post. I've moved it to the right one. Sorry!

I'd just add that using "Profili" (which lists in its editable library the NACA 00xx, five-, and 6-digit sections), you can easily move the high point and vary the maximum thickness of all these sections (it 'stretches' or 'squeezes' the points ahead and behind as necessary), in addition to coming pretty close to generating NACA sections of thicknesses not listed. All of this is done automatically with the mouse or plugging in figures. The program will then do its best (good enough or not) to predict aerodynamic properties at requested Reynolds numbers with "X-Foil." This may actually be closer than a actual wind tunnel data, since NACA discovered that mounting and tunnel-wall interference *greatly* affected what they came to call the "effective Reynolds number," biasing and somewhat confusing their data. I don't know how accurate the original data is. Probably flying the model is your best determination of success.

One thing to avoid is any assumption that airfoil characteristics at RN = 3 x 10^6 are what you will find at model wing RN's. The trends in behaviour with changes in thickness reverse at our lower RN's. For instance, the 12%-15% NACA 00xx sections excel for "full-sized" wings, but at our sizes, the 20+% sections have greater maximum lifts and reasonable drag.

Larry's program does much the same as Profili, and with him you have a wealth of expertise and good will in helping you along. "Profili" downloads are available free on the internet, and for a modest $10.00 you can down load a more advanced version. It will print out any airfoil you concoct and will interpolate and print out any ribs between root and tip in a straight taper or elliptical plan view. This includes skins and a limited number of spar and lightening hole choices.

Concerning what Ted said about bluntness of the l.e., I found that you can fit a parabolic leading edge to any NACA 00xx section at its high point and achieve a nice smooth transition with smaller l.e. radii and curvature continually decreasing from l.e. to point of maximum thickness ('high point'). I also liked what I got by moving the high point forward to around .25C starting with a few of the NACA laminar (6-digit) sections.

(Edit 2: Paragraph removed to other thread)

Edit: I hven't the time just now to re-edit some illustrations (Club newslatter time!), but here is a comparison of an NACA 00xx section with a parabolic l.e. to the original NACA section. Profili printed the modified section fine below, but left out data points in the comparison drawing. You should be able to see the difference though. Perhaps the modified section will like gusts better...?

[photo not recovered: 29599.jpg]

SK

Attachment #1, (jpg file)

I liked John Miller's suggestions (noted again below) about the NACA six-digit airfoils. So a few years ago I generated some with high point moved forward. The flapless plane I built using the 20% thick (at .24 chord) section illustrated flew well. Without time to edit, I'll just append the X-Foil analysis below.

Edit: 'couldn't resist: this started as an NACA 63A010 airfoil, but was modified in Profili by moving the point of maximum thickness to the 24%-chord point and doubling the thickness.

[photo not recovered: 29600.jpg]

SK

Attachment #1, (jpg file)

The two airfoils above use the same profile, but one is proportionally reduced in chord and thickness to add a stationary flap, giving it an 18.77% max thickness with the same total chord. Profili's X-Foil access then compared these two (and another) to the NACA 0020 section. It liked the stall characteristics and maximum lift of these airfoils better. Reynolds Number was 400,000. Here's the comparison:

[photo not recovered: 29601.jpg]

Now, if I haven't totally screwed up, its back to newsletter editing - Ooops, its over to the Lost Nations thread, and then....yipes!

SK

Attachment #1, (jpg file)

To Serge,

When the high point of a NACA 00 series section is transferred to the front, say 15-20 % it starts to resemble the symmetrical Eppler profiles. Rumour has it that the major effect in flight would be an increased air resistance during dive, like in the square figures.

Have you any means of verifying this? I tried to find this in Profili with the aid of Stephano Durante, but the result was very tricky or minute. However it would be a bonus !

I haven't looked, but I think that the X-Foil program would tell you whether there's an increase in profile drag coefficient with this forward movement of the high point. I would expect that, but at low speeds, the "ideal" streamlined shape may not be obvious. Anyway, this would be the indicator of whether it would be slower in a dive, which the plane sees only as a speed increase from increased downward force. Higher profile drag coefficient would mean lower terminal velocity; so I'd look at the drag at zero degrees aoa.

SK

>
>It is also worth noting, by the way, that all of my
>"modern" design airplanes had airfoil high points in
>the 22 to 25% of chord range. You state that the airfoils you
>show have them at or about 33%. I haven't measure them but
>suspect that measurement is based on the unflapped sections
>you drew. The addition of flaps will likely make the high
>point right around that 25% figure for the entire airfoil.
>
>Airfoils should always be considered including flaps, by the
>way, when providing "numbers". Flaps are part of
>the airfoil/wing. The only thing unique about them is the
>fact that the can be moved to change the shape of the airfoil
>and produce a higher coefficient of lift than that achievable
>with the flap fixed.
>
>Your airfoils look just fine by the way. Only suggestion
>would be with regard to the very blunt leading edge and the
>earlier discussion of the abrupt stall that can result as it
>did on the Imitation. You could very easily redraw the
>leading edge to produce a smaller radius which would remedy
>the possibility.
>
>Ted Fancher
>
>ooops, should have kept reading before responding. Larry has
>said much the same thing regarding airfoil high points with
>the exception that he continues to do so without regard to the
>flap. I still, respectfully, suggest this isn't usual
>protocol.

Hello Ted,
Yes, the numbers I gave regarding max thickness in your TP airfoils and the ones I've drawn where taken without considering the flaps.
I didn't know that the flaps must be included in the equation.
Many thanks for your reply and sorry...

Claudio.

Ted,
A case in point was watching the very abrupt stall with little or no warning whatsoever on my 76 oz Trivial Pursuit on the third corner of the Hourglass maneuver. Consistent but abrupt and always resulted in the airplane literally falling out of the air.
Certainly not underpowered with a RO Jett 65.

Randy C [photo not recovered: sipcof.gif]

>Ted,
>A case in point was watching the very abrupt stall with little
>or no warning whatsoever on my 76 oz Trivial Pursuit on the
>third corner of the Hourglass maneuver. Consistent but abrupt
>and always resulted in the airplane literally falling out of
>the air.
>Certainly not underpowered with a RO Jett 65.
>
>Randy C [photo not recovered: sipcof.gif]


Well, Randy, there are, of course, limits to my magic!

Ted

p.s. That, third corner stall is not unique to your airplane by the way. I've seen a number of airplanes "give it up" at that point ... some on a regular basis. You might want to investigate the use of the wire turbulators that have been discussed in several threads in the past. Uncle Jimbys appropriately name Frankinstunt would do it with just a suggestion of a corner up there. It's easy to just tape a chunk of .018 control wire (or equivalent) right around the high point on the bottom of both wings (outside corner where it gives up). This fix has proven surprisingly effective on several "challenged" airplanes.

Worth noting is the dramatic difference in density altitude between your situation in Arizona and ours here in the Bay Area. I would expect my 68 oz original to be on the ragged edge up there.

A little interesting history you may have already heard.

The original TP (the Cream colored one) was designed with a sharper leading edge than its "common design thread" predecessors, the Imitation and Excitation series. It was a very good airplane for several years until one team trials (I forget where) where it was very hot and muggy. One day during practice it was drizzling pretty heavily; just at the point you're considering stopping and getting out of the wet. The TP on consecutive flights started to give up in the last corner of the triangles and hourglasses ... a classic turn/stall/complete the turn sequence.

This is where the turbulator was first tried and it worked very well. I believe we made the team that year but I'm not certain of that. In any case it worked just fine.

Subsequent TP had a modified leading edge that was sharper than the ones on the Imitation/Excitation but noticably blunter than the one on the original. These designs flew at a couple of world champs and won a couple of nats at comparatively porky wing loadings (historically).

These designs employed on of the first ever molded leading edges which made for extremely accurate and repetitive construction.

FWIW, the original is the only one I have left now and it flies wonderfully well to this day. We ultimately retrimmed it with just a bit more flap than elevator travel and I haven't given any thought to the issue since. Same basic concept, a little more flap deflection for a given pitch request produces enough lift to carry the freight. (you might also want to give this concept a try. You might be surprised. The 2004 short lived version was improving dramatically with additional flap vice elevator movement when it met its untimely demise)

Ted

pps, Randy, you might be interested to know I've been taking voice from a classically trained opera singer in San Jose for the last six months or so. Tons of fun and, although I'm not much interested in opera per se, it has had a tremendously worthwhile effect on my technique for the stuff I do enjoy, Broadway tunes, etc.

>These designs employed on of the first ever molded leading
>edges which made for extremely accurate and repetitive
>construction.

I remember that discussion. Good idea that several others have since used to good effect. Not me, of course, I still eyeball carve it with a hand axe, etc. I think the need to get it perfect is greatly reduced if it's very blunt, which ours are compared to, say, Bobby's and Billy's. Although I would have to say that the Thunderbolt aifoil looks a lot like a NACA 0020 or Imitation in the front.

My sole contribution to the idea was to suggest the stringer at the LE to make it safer to launch. I was afraid that the unsupported sheeting would be too easy to put your finger through.

> You might be surprised.
> The 2004 short lived version was improving dramatically with
>additional flap vice elevator movement when it met its
>untimely demise)

My first thought, when it crashed, was that the newly-adjusted elevator slider had "slud" out of place. I don't know if we ever came to any particular conclusion, my guess is still that the wing gave way partially, pulled in down elevator, and bunted it into the ground. Such is life.

Brett

Ted,
The turbulator would probably be a good idea but unfortunately, since I dont run as fast as I used to, the TP met it's final destiny after that last stall a few weeks ago. My fault since I was determined to make the third corner snap like the orhers....dumb idea and yes...I knew better.
Terrific to hear that you're studying with an Opera singer. Have you actually learned any Arias. I assume you're a tenor.
I too like the "show tunes" and sometimes sing them for fun.
The new Trivial Pursuit is in work.

Randy C

>Ted,
>The turbulator would probably be a good idea but
>unfortunately, since I dont run as fast as I used to, the TP
>met it's final destiny after that last stall a few weeks ago.
>My fault since I was determined to make the third corner snap
>like the orhers....dumb idea and yes...I knew better.
>Terrific to hear that you're studying with an Opera singer.
>Have you actually learned any Arias. I assume you're a
>tenor.
>I too like the "show tunes" and sometimes sing them
>for fun.
>The new Trivial Pursuit is in work.
>
>Randy C


Oops, sorry. Didn't mean to bring up bad memories

Nope, no arias yet although Carl (the voice instructor who sort of looks like a later in his short life Mario Lanza) is talking about something really, really simple. We shall see. My Italian is substantially inferior to my "laugh along" French.

My first record ever was Mario Lanza singing La Donne e Mobile. My mom had taken me to see the Great Caruso as a very "young"ster. I came home pretending to sing the aria. Mom bought it for me (a 45RPM two sider) along with one of those old "suitcase style record players. Wore that out trying to sing along. Although a little embarrassed, I'm not entirely ashamed to say it later played a lot more Roy Orbison and Elvis than opera arias. Sang along with them as well and wore the duck tail haircut (but no black rimmed glasses).

I sang tenor for the most part when I was young but one of the things I've learned with Carl that I'm more of natural baritone and can't really carry notes strongly above a high G or the occasional A ... wouldn't even consider singing an A in public nowadays although I did a lot of them in high school and university chorales.

Carl's got about 60 students,many of whom make me want to only do tunes from Phantom at recitals so I could wear the mask and remain anonymous! Seriously good, some of whom are professionals. It's been a lot of fun and has filled sort of a void since my modeling dragged to a halt due to my shop being out of service for a number of months while the house gets remodeled/expanded.

I'm going to audition for a role (Arvide) in Guys and Dolls being produced by a respected local theater group in August. My son performs with them and I'm hoping to be able to get on stage with him again.

Ted

Oops.

Sorry Claudio, didn't mean to hijack your thread. Singing's a mutual interest of Randy's and mine.

>>Ted,
>>The turbulator would probably be a good idea but
>>unfortunately, since I dont run as fast as I used to, the
>TP
>>met it's final destiny after that last stall a few weeks
>ago.
>>My fault since I was determined to make the third corner
>snap
>>like the orhers....dumb idea and yes...I knew better.
>>Terrific to hear that you're studying with an Opera
>singer.
>>Have you actually learned any Arias. I assume you're a
>>tenor.
>>I too like the "show tunes" and sometimes sing
>them
>>for fun.
>>The new Trivial Pursuit is in work.
>>
>>Randy C
>
>
>Oops, sorry. Didn't mean to bring up bad memories
>
>Nope, no arias yet although Carl (the voice instructor who
>sort of looks like a later in his short life Mario Lanza) is
>talking about something really, really simple. We shall see.
>My Italian is substantially inferior to my "laugh
>along" French.
>
>My first record ever was Mario Lanza singing La Donne e
>Mobile. My mom had taken me to see the Great Caruso as a very
>"young"ster. I came home pretending to sing the
>aria. Mom bought it for me (a 45RPM two sider) along with one
>of those old "suitcase style record players. Wore that
>out trying to sing along. Although a little embarrassed, I'm
>not entirely ashamed to say it later played a lot more Roy
>Orbison and Elvis than opera arias. Sang along with them as
>well and wore the duck tail haircut (but no black rimmed
>glasses).
>
>I sang tenor for the most part when I was young but one of the
>things I've learned with Carl that I'm more of natural
>baritone and can't really carry notes strongly above a high G
>or the occasional A ... wouldn't even consider singing an A in
>public nowadays although I did a lot of them in high school
>and university chorales.
>
>Carl's got about 60 students,many of whom make me want to only
>do tunes from Phantom at recitals so I could wear the mask and
>remain anonymous! Seriously good, some of whom are
>professionals. It's been a lot of fun and has filled sort of
>a void since my modeling dragged to a halt due to my shop
>being out of service for a number of months while the house
>gets remodeled/expanded.
>
>I'm going to audition for a role (Arvide) in Guys and Dolls
>being produced by a respected local theater group in August.
>My son performs with them and I'm hoping to be able to get on
>stage with him again.
>
>Ted
>
>Oops.
>
>Sorry Claudio, didn't mean to hijack your thread. Singing's a
>mutual interest of Randy's and mine.

Ted I know nothing of Opera or music for that matter But the next time Bob Dixson and Dee Rice have their music thing set up. Ask Louis Rankin to sing you won't believe what comes out! Maybe both you guys could sing a Duet.

>
>Ted I know nothing of Opera or music for that matter But the
>next time Bob Dixson and Dee Rice have their music thing set
>up. Ask Louis Rankin to sing you won't believe what comes out!
>Maybe both you guys could sing a Duet.
>

Hi Sparky,

I'm really not much of a "singer". I have a strong and pleasant voice but my musical skills are pretty much limited to very structured content that I've rehearsed a lot. I greatly admire guys like Bob Hunt and Billy Werwage who can carry on a more or less extemporaneously ad-libbed concert for hours at a time around the Bar BQ at VSC. I've no doubt that Bob, Dee and Lou fit right into that mold as well.

I'm no longer surprised to hear about yet another modeler with fine musical skills/interest. One of my "light bulb" moments about the culture of stunt was learning how much it has in common with musical arts. A large part of the reason it is "special" in ways that many hobbies/sports are not. Doesn't make stunt "better" necessarily but reveals a lot about the people that embrace both for a lifetime rather than as a passing fancy.

Love your new "Continentalish" stunter. A great choice that should gather a lot more attention than one out of thirty or so P-47s when it comes to Concours and appearance points. Well done!

Ted

>Love your new "Continentalish" stunter. A great
>choice that should gather a lot more attention than one out of
>thirty or so P-47s when it comes to Concours and appearance
>points. Well done!
>
>Ted

Thanks Ted I stold the Magazine idea from you.

[photo not recovered: 29608.jpg]

Attachment #1, (jpg file)

>Oops.
>
>Sorry Claudio, didn't mean to hijack your thread. Singing's a
>mutual interest of Randy's and mine.


Don't worry Ted. If your singing skills are close to your flying abilities, you better start uploading some clips in You-Tube!


Claudio.

Hi Claudio,

I always used the NACA-00nn because I had the equation for it. ;->

I believe Ted's airfoil is similar, but has the high point moved forward. The NACA-00nn has its high point at 30% of chord, while I would guess that Ted's is closer to 25% of chord.

If you want a modified NACA00nn with a movable high point position, email me and I can give you my Stuntrib-X Excel spreadsheet, which generates output DXF files. It's very easy to use, but only does a single rib at a time, but that's not much of an obstacle, since you can interpolated chord and thickness values right in the spreadsheet.

[photo not recovered: 29585.jpg]

Here's a 25% high point airfoil it made, for comparison:

[photo not recovered: 29586.jpg]

Ted's may be closer to 20% of chord, and I make no claims that this is Ted's airfoil, of course. But you can easily experiment, and if you move the high point aft, the airfoil will start looking appropriate for a stab, perhaps.

[photo not recovered: 29334.gif]

"If you wait, it will go away." -Hellrung's Law

>Hi Claudio,
>
>I always used the NACA-00nn because I had the equation for it.
>;->
>
>I believe Ted's airfoil is similar, but has the high point
>moved forward. The NACA-00nn has its high point at 30% of
>chord, while I would guess that Ted's is closer to 25% of
>chord.
>
>If you want a modified NACA00nn with a movable high point
>position, email me and I can give you my Stuntrib-X Excel
>spreadsheet, which generates output DXF files. It's very easy
>to use, but only does a single rib at a time, but that's not
>much of an obstacle, since you can interpolated chord and
>thickness values right in the spreadsheet.
>

>
>Here's a 25% high point airfoil it made, for comparison:
>

>
>Ted's may be closer to 20% of chord, and I make no claims that
>this is Ted's airfoil, of course. But you can easily
>experiment, and if you move the high point aft, the airfoil
>will start looking appropriate for a stab, perhaps.
>

Hello Larry,
Yes, please. Send me your Excel spreadsheet. My e-mail:
[email protected]

What is the advantage (if any) of your NACA airfoil (with the high point located more forward) compared to the one I've posted?

According to your knowledge and experience, which NACA 4 digit airfoil do you think that can generate most lift and less drag?

I've measured Ted's (Trivial) airfoils and they have the max thickness at 30% of the cord (root and tip) but are quite different to the NACA 4 digit series (less LE radii and flatter from the high point to the TE)

I fly stunt (say, seriously) since 1982 and through all these years I've seen that -in the hands of a qualified pilot- almost every plane can perform perfect patterns, even with totally different airfoils (some radical, as the French planes).So, this is -at least- quite confusing to me.

Thanks for your input Larry.

Claudio.

>I fly stunt (say, seriously) since 1982 and through all these
>years I've seen that -in the hands of a qualified pilot-
>almost every plane can perform perfect patterns, even with
>totally different airfoils (some radical, as the French
>planes).So, this is -at least- quite confusing to me.
>

I would like to jump in here and join the discussion. I draw a lot of plans for others and myself, and have found a few interesting items regarding airfoils used in modern stunt ships.

As you've noticed, a competent pilot can get a very good pattern out of almost any airfoil. There is at least one design that uses 1/2 inch flat sheet for a wing, that has been reported to be a decent stunter. I personally have never seen it fly though.

I believe that at our reynolds numbers, airfoil shape is most important in the entry, or front of the foil. What happens aft of the high point is less important than the front, though there have been some concerns regarding the seperation area around the flap hinge line and the trailing edge.

NACA 4 digit airfoils are about as good for much of our designs as possible, with the following limitations, in my opinion.

As the percentage of thickness increases, the blunt entry can become a bit draggy. Penetration in less than ideal conditions can worsen, and the design can suffer from some upset in the windy-turbulent conditions.

Many designers have opted for a different style of airfoil to help with penetration. I've noticed that designers from areas that have generally more windy and turbulent air use an airfoil that seems to be derived from the NACA 63A 6 digit airfoils.

I've used this airfoil myself for several years now and have found it to be pretty darned good.

It is modified by moving the high point forward to 25 to 30%.

It has the same percentage of thickness, but the leading edge radii is smaller, though still large enough to avoid inducing a stal during manuevers.

I'll try and attache a copy of this modified NACA 63A in a few days, so you can compare it to the 4 digit airfoil.

The NACA 4-digit airfoils all have the same thickness distribution stated as ±%of chord at each station along the chord. If you look at a chart of the coordinates for a 10% 4-digit those numbers become apparent. Any other thickness can be generated by using ratios of those numbers, i.e. an 18% airfoil will have offsets from zero 1.8 times those of the 10% airfoil.

NACA 4-digit number airfoils are the simplest of those published by the NACA. The digits all mean something. The first two digits of the number refer to the camber of the chord line and the point at which that max camber occurs, in a symmetrical airfoil case, no camber, they are 00. The second two digits are the percent of chord thickness. A 6418 has 6% camber at 40% chord and a max thickness of 18% of chord. For true NACA 4-digit airfoils the max occurs at I think 30% chord. If that point is moved, you no longer have a true NACA 4-digit airfoil.

I kind of agree with one of the comments about drawing around a shoe. IMO, at the reynolds numbers our airplanes fly, the shape doesn't really make a heck of a lot of difference. Design with a nice radius on the front and anything but a sharp trailing edge and it will be just fine.

don Burke
Menifee, CA
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways - body thoroughly used up, totally worn out and screaming, WOO HOO, What a Ride!

>
>I kind of agree with one of the comments about drawing around
>a shoe. IMO, at the reynolds numbers our airplanes fly, the
>shape doesn't really make a heck of a lot of difference.
>Design with a nice radius on the front and anything but a
>sharp trailing edge and it will be just fine.
>
>don Burke


Don,

I'm interested in your final sentence regarding "...anything but a sharp trailing edge...". I've heard that from others and gave no particular merit to the use of anything other than a sharp trailing edge sub-sonically (I say sub-sonically only because I'm aware the X15 had an almost triangular cross section to the stabilizing surfaces with the sharp edge forward and the flat aft {don't recall anything about the wings, however]). All the airplanes I've flown have had the aerodynamic equivalent of a sharp trailing edge except some of the old tubing formed stab/elevator combos on planes like J-3s, etc.

What is the rationale re "anything but" sharp?

Ted

From talking to the only really knowlegeable model aize aerodynamicist I know. Has to do with the fact that the flow is separated from about the 25% point back, particularly at typical model reynolds numbers. A sharp TE on a flap has to move through the thickness of the separated boundary layer before it starts to have any effect on the flow going past. And a flapless one can be kind of unstable in pitch for the same reason.

don Burke
Menifee, CA
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways - body thoroughly used up, totally worn out and screaming, WOO HOO, What a Ride!

>From talking to the only really knowlegeable model aize
>aerodynamicist I know. Has to do with the fact that the flow
>is separated from about the 25% point back, particularly at
>typical model reynolds numbers. A sharp TE on a flap has to
>move through the thickness of the separated boundary layer
>before it starts to have any effect on the flow going past.
>And a flapless one can be kind of unstable in pitch for the
>same reason.
>
>don Burke
>Menifee, CA


Fascinating, Don.

I wonder if Frank Williams' wind tunnel tests have shown the same result? My experience with stunt ships seems to indicate that very modest amounts of control surface movement seems to have pretty positive results, even with sharp trailing edges that I find, at least aesthetically, more desirable. If the separation your friend is substantial wouldn't one expect the controls to be pretty dead around neutral? I'm unconvinced but nonetheless still open to education.

Ted

>From talking to the only really knowlegeable model aize
>aerodynamicist I know. Has to do with the fact that the flow
>is separated from about the 25% point back, particularly at
>typical model reynolds numbers. A sharp TE on a flap has to
>move through the thickness of the separated boundary layer
>before it starts to have any effect on the flow going past.
>And a flapless one can be kind of unstable in pitch for the
>same reason.

Even if it's separated, it's still doing something - it's not like the airfoil aft of the point of separation is running in a vacuum.

Brett

>Many designers have opted for a different style of airfoil to help with penetration. I've noticed that designers from areas that have generally more windy and turbulent air use an airfoil
> that seems to be derived from the NACA 63A 6 digit airfoils.

>I've used this airfoil myself for several years now and have found it to be pretty darned good.

>It is modified by moving the high point forward to 25 to 30%.

If anyone's interested, but missed it, I've just posted something along this line above.

SK

Claudio,

I'll email the spreadsheet to you. You have to enable the macros when asked. Somewhere I have the BASIC code for it, and I'll try to find it as well, just in case you wanted to see how to construct and write a DXF file.

As for the technical question, I honestly have no idea. However I know that the Trivial Pursuit airfoil works well enough to win the Nats, multiple times.

I have notions about stunt model aerodynamics, which are rooted more in gut feelings than proven facts. But I feel that in general that an overpowered ship with a wing which is thick and draggy, with a low loading and a very blunt leading edge, as well as thick convex curvature on its aft portion is de rigueur nowadays. It's not just tradition, believe me if something else worked markedly better (or better to ANY degree), some top dog competitor would be using it.

[photo not recovered: 29597.jpg]
Shark!

Take a look at the Yatsenko Shark airfoil. This ship is widely praised as a delightful flier, and is another capable example of a Nats winner. To me, that looks a lot like a NACA 00nn airfoil, but it might not be.

[photo not recovered: 29334.gif]

PS - the rumor is that the true competitive secret of Ted's Trivial Pursuit has nothing to do with its airfoil, but is its cowl:

[photo not recovered: 29598.jpg]

"He has the right to criticize who has the heart to help." -Abraham Lincoln

>
>Take a look at the Yatsenko Shark airfoil. This ship is widely
>praised as a delightful flier, and is another capable example
>of a Nats winner. To me, that looks a lot like a NACA 00nn
>airfoil, but it might not be.

Yes, it looks like NACA 00nn airfoil indeed, with a smaller LE radii, though.
>

>
>PS - the rumor is that the true competitive secret of Ted's
>Trivial Pursuit has nothing to do with its airfoil, but is its
>cowl:
>
>[photo not recovered: 29598.jpg]

Ha! That's a pic of my Trivial Pursuit Larry! Thank you for posting it.
Let me add another one:




[photo not recovered: 29606.jpg]

Attachment #1, (jpg file)

Dear all:
I want to give my utmost appreciation to each and everyone who posted a reply in this thread sharing his knowledge.
This has been for me a VERY enlightening discussion, you can bet on that.
What a Forum!!!

My best regards to you all

Claudio.
ARGENTINA

Hi.

The Shark airfoil is created by modifying the Sukhoi 26 airfoil. The surfaces after thickest point are more flat, allmost a straight line, than in the NACA foil in this thread.
I am not a specialist in these things but I think that what makes the Shark design such a delightfull flier, is:

-Good inherent stability achieved by CG very front and thick stab airfoil.
-Good turning ability caused by long tail moment, effective elevator and center of lift very close to center of gravity. Low rpm/lightweight prop helps, too.

I think that the Shark is very sensitive to the placing of CG. I have tested a lot with different cg location and with heavier carbon props. It's not good to increase sensitivity by moving CG back, in that case the tracking stability and the nice "flow" of flight suffer dramatically. A much more effective way is to seal hinge lines. That way the inherent stability does not suffer.

Lauri M

> The Shark airfoil is created by modifying the Sukhoi 26
>airfoil. The surfaces after thickest point are more flat,
>allmost a straight line, than in the NACA foil in this
>thread.

Lauri, that doesn't sound much like the airfoil shown in the picture above - in fact quite the opposite, there's a decent amount of curvature in the aft part of the airfoil.

Just as a side note, the airfoil on full-scale aerobatic airplanes are typically geared to snap rolling with abrupt stall characteristics. The aforementioned "Frankenstunt" by Uncle Jimby is the closest I have seen to a full-scale aerobatic airfoil in a model stunt plane. I can assure you from personal experience that it has very abrupt stall characteristics!

Brett

Brett,

That was the answer from Andrei when I asked about the foil.
In the picture, it's just an illusion. Because of fuselage shape, the the wing root projection is not a straight line (sorry, I cannot explain it better). I try to find abetter picture. L

> That was the answer from Andrei when I asked about the foil.

Don't get me wrong, I am not doubting your information, just trying to see where the apparent inconsistency lies.


> In the picture, it's just an illusion. Because of fuselage
>shape, the the wing root projection is not a straight line
>(sorry, I cannot explain it better). I

Not a straight line as viewed from above? That would explain the difference.

Brett

>
> Not a straight line as viewed from above?
> Brett


Exactly. I'll try to find a better image.

Lauri M

>
>> That was the answer from Andrei when I asked about the
>foil.
>
> Don't get me wrong, I am not doubting your information, just
>trying to see where the apparent inconsistency lies.
>
>
>> In the picture, it's just an illusion. Because of
>fuselage
>>shape, the the wing root projection is not a straight
>line
>>(sorry, I cannot explain it better). I
>
> Not a straight line as viewed from above? That would
>explain the difference.
>
> Brett


Brett,

Also sort of explains the vertical grain on the rib. That had me scratching my head a bit.

ted

Hi Ted.

It's so little out of flat that grain direction would not matter. The reason for vertical grain is that the rib only needs to work against shear and compression stress.


Lauri M

>
>
> Hi Ted.
>
> It's so little out of flat that grain direction would not
>matter. The reason for vertical grain is that the rib only
>needs to work against shear and compression stress.
>
>
>Lauri M

Sure, Lauri. That makes sense. The whole thing is a marvelous piece of work and engineering.

Ted

HELLO LARRY :

I am new to the forum but not to stunt,please send me your excel spread sheet.My email is [email protected] THANKS

On its way, I hope it will be useful to you.

Best regards,

[photo not recovered: 29334.gif]

A girl phoned me and said, "Come on over. There's nobody home." I went over. Nobody was home! -Rodney Dangerfield

I need to remind everyone that my spreadsheet (and the old Stuntrib program) do not actually plot (or even display) airfoils. They emit DXF files, which have to be imported into a CAD program for further manipulation and plotting.

Excel, designed by satanic programmers at Microsoft, doesn't seem to have any easy way to do it. In its supreme Clippy philosophy of "we know what you want more than you do", it tends to autonomously resize graphs, and sometimes in bizarre wats,

BTW, if anyone has some nice generic BASIC code for plotting on the printer in the Windows environment, and willing to share it, I'd be grateful for a copy, and will credit you as a contributor on any software I distribute (I never sell it..).

I've made code to display on the screen, but not to make a scale plot on the printer. One of these days..

[photo not recovered: 29334.gif]

"One of these days I'm going to cut you into little pieces.." -Pink Floyd, from the album Meddle

Here is a drawing showing the difference between the Yatsenko and the NACA rib profiles.. The Yatsenko rib was measured from a drawing supplied by Yatsenko and the NACA rib is a root rib for the Mo-Best drawn by Larry Cunningham's stuntrib program (pre-spreadsheet). I have scaled them to approx the same thickness. I think that it is clear what Lauri was saying about the relative curvature of the rear of the section.


[photo not recovered: 29710.jpg]

Here is a zip of the PDF of the above drawing for closer study

http://www.clstunt.com/htdocs/dc/user_files/29711.zip

Attachment #1, (jpg file)
[photo not recovered: zip.gif]Attachment #2, (zip file)

My brain hurts reading this. I think I'll use a French curve and a pencil when I scratch build a plane.