I hate to even post about this. Cajun has been proclaiming for years that the sharp leading edge causes a plane to hunt in pitch. I've built well over a hundred of these things, and 90% of them had sharp leading edges, and out of those, i've only had a couple that actually hunted in pitch. I cured both of them by increasing the area of the hor. stab back up to 18% of the wing area.

Have any of you aerodynamic guru's ever personally experienced a plane with a sharp leading edge hunting in pitch, and curing it by changing to a rounded leading edge?

Chris, you beat me to this. Same question. or...besides x-foil, is there something out there (even anyone with some access to some real foil plotting and testing design s/w) that could test this.?? Sharp Leading Edges and flight performance

Also....speed and engine weight/plane size ratio I believe would be a factor??



I want real numbers yo, Im tired of this Cajun, Chris thing

P.S. As far as the airfoil shape, I'm sure that someone here can determine what the basic SPAD airfoil shape is in general terms of things. Tattoo didn't you find that info. out before?

L8rs

!S

RCHAWAII Out!

um, also is it me or......isn't there a difference between a jet being PUSHED VERY HARD through the air, as compared to a plane being pulled through the air at a much reduced speed comparitively speaking? Re: Sharp Leading Edges and flight performance.?

I found a couple experts online from Nasa and related, I am going to invite them to this thread... Hope thats okay.

First one is from the Glenn L. Martin Wind Tunnel..Dr. Jewel Barlow, Director

They are directly involved with wind tunnels and testing.

In addition, you should all check this info out:



http://ldaps.arc.nasa.gov/Curriculum/tunnel.html


Some awesome links in there...see bottom of page

Maybe you could ask them if they could set up a testing program at NASA to help us out here...

(just kiddin' )

It crossed my mind if they would test something for us, but of course thats a tough call, since they probably want mills to test something.

In any case, I also emailed this address:

[email protected]

At Ask an engineer and also ponted them to this thread.

TY

!s
RCHAWAII Out!

I've never learned or researched anything more on Spad SAP (simple as possible) airfoils other than going to the field and having a blast with my planes. Also...just so everyone knows, Cajun is a very good friend of ours, so any dissagreements on airfoils etc. between us is always done with a smile...very much like buddies arguing over which football team is better

I knew that, always enjoy a good point of argument...

On that note, enjoy Cajun:

Hi Chuck,

At 12:16 AM 1/29/2003 -0500, you ([email protected]) wrote:
>Question:
>
>Sharp leading edges.
>
>Do they result in pitch instability and/or "hunting" If so, is it due to
>slower flying speeds, weight/power ratio? If not , can you explain why?

Sharp leading edges are an interesting topic. And it has to do with boundary layers and boundary layer separation. Right next to any moving surface is a very thin layer of air called the boundary layer where the velocity changes very rapidly from the surface to the free stream. The flow in the boundary layer has very little energy .. so it is easily moved and modified by changes in pressure. Near a sharp leading edge, the pressure changes very rapidly with distance. So near a sharp leading edge (at very moderate angles of attack) the boundary layer will separate from the surface. When this happens, the effective shape of the surface changes (since the outer flow follows the edge of the boundary layer .. not the physical surface). If it changes to a nice smooth contour, the pressure gradient goes away and the boundary layer re-attaches .. only to be re-separated .. it's a very
unsteady phenomenon. It's actually the same physical process that produces a wing stall on any airfoil at higher angles of attack. But it is localized on a sharp leading edge. This would produce pitch instabilities, since the pressure profile across the entire wing is affected by separations near the leading edge.

A very special Thanks to Tom for this information..

Tom Benson
Engine Systems Technology Branch
NASA Glenn Research Center, MS 5-11 "Turn up the signal ..
Cleveland, OH, 44135 Wipe out the noise"

Hey Tattoo!

Your wings SUCK man! They suck the streamers right off of the other guys combat aircraft!!! Wooohoooo!

Yes, inverted flight is really quite good with these wings on my HOR. And it does not "hunt", it is very stable in flight from (moderately) high speed to very slow dead stick landings like us combat guys always have to do.

They also do not snap out as easy as some other wings I have flown... even on high rates and tying knots in the sky the plane with eventually just "mush" but never snap out of a turn. I love being able to out-turn just about every other aircraft in a combat meet... hehehe.

Now, I do have a confession to make. Micster and I have cut some (secret) S____g airfoils out of foam and I flew one on my HOR, and he built a HOR with one as well. They have a much higher top-end speed but they do snap out if you let them get too slow and pull a little too hard... much more CG sensitive than the coro-wings also. We are gonna fly them in Open-B this weekend to see how they do. My open-A and SSC planes are still running coro and will continue to do so. I know, I am kinda taking the "S" outta SPAD, but...

Chuck,

>Is it your opinion, or is it fact or not, that speed affects this
>phenomenon with regard to sharp leading edges?

Yep, speed would affect it big time, since speed affects the ratio of viscous forces to inertial forces (Reynolds number) which determines whether the flow over the leading edge separates or not.

Tom

Enjoy fellas, I hope you find this as interesting as I do.


You already have a response from someone at NASA mentioning the fact that the details of the flow separation are dependent on the way the "boundary layer" develops on differing shapes. Sharp leading edges lead to separation at lower angles of attack than happens for leading edges with larger radii.

The result on airfoil properties, at least on lift can be seen in the
attached material which shows the lift versus angle of attack for a NACA 0012 airfoil in forward and reversed flow (figure 14 on page 2-8) and similarly for NACA 63 series airfoils and a double wedge (figure 2 on page 4-2). These are from Fluid Dynamic Lift by Hoerner. The lift versus angle of attack breaks very sharply at a relatively small angle of attack of around 6 degrees. This will lead to a very strong change in the slope of the pitching moment versus angle of attack curve of an airplane using such a sharp leading edge airfoil. The pitch stiffness will be very different at angles of attack above and below the break point. It is also very likely that the flow will be quiet unsteady at angles in the vicinity of this break point and may lead to the phenomena referred to as "hunting". An airplane that has such a sharp leading edge but always operates below this critical angle of attack will behave
conventionally.

The drag will rise quickly for angles of attack above the stall point. This will affect performance, but will not directly affect the pitch behavior of the aircraft.

I also noticed several posts about "bound vortices" with various
comments about this object. The term "bound vortex" in the standard aerodynamic literature refers to an effective vortex associated with a lifting surface. It is not an actual vortex identifiable by a flow pattern in the fluid as in the case of a wing tip vortex. The concept arises from a set of theorems known as the Helmholtz Vortex theorems. One of them asserts that a vortex filament cannot begin or end within the fluid volume. A vortex filament must end at a solid surface, extend to infinity, or form a closed loop. In the standard wing case, the vortex filaments making up the wing tip vortices begin at the wing
surface and extend to infinity. Their imaginary extensions within the wing that connect the vortex filaments attached to the left wing to symmetrically located vortex filaments attached to the right wing make up the "bound vortex".

The "votex" that will usually exist just downstream of a sudden step in a solid surface is an example of a "trapped vortex". It is not in any sense analagous to the "bound vortex" concept. A trapped vortex is an actual pattern in the flow just as is the case for a trailing tip vortex.

Best regards,

Jewel B. Barlow
Director, Glenn L. Martin Wind Tunnel
University of Maryland

NONE OF THIS POST SHOULD BE COPIED OR OTHERWISE DUPLICATED WITHOUT EXPRESS WRITTEN PERMISSION FORM THE AUTHOR!

[email protected] wrote:
> Hello,
>
> Nice website, and great work. My name is Chuck Roseberrry. I am a radio
> Controlled airplane hobbyist. We ghave a debate I'd like to get your
> feedback on and have you join the discussion if/when you have the time
> located at the below link.
>
> The discussion is how sharp leadeing edges affect flight performance.
>
>
> http://www.rcuniverse.com/showthread...504#post640158
> <http://www.rcuniverse.com/showthread...504#post640158>
>
> Thank you for your generous time.
>
> Chuck
> admin/webmaster
> Spadworld.nett

Thanks to everyone who participated in this spirited little debate. A special thanks to RCH for keeping it going and attracting the professional opinions.

It is friendly debates like this that hopefully get some of the young guys interested in carreers in aeronautics and aviation.

I will confess I learned a lot of new stuff about wings in my own research on this subject.

I have been involved with Spad since the very early days, and would never allow a difference of opinion to jeopardize my friendship with Tattoo, Kraut, Chris or any of the other Spad guys. I have been referred to as "opinionated" over the years, but at my age I see no reason to change. So, be prepared for more debate topics in the future.

Cajun

I can't thank you guys enough for the response to this, it has been VERY interesting! Wow...even got NASA involved...so Cajun...what's the next one?

I do have another issue that I have been told is an aerodynamic no-no...and thats putting the Horizontal stab in the same plain as the the wing. I've been told by "field critics" that the air off the wing would disturb the airflow across the tail and make the elevator less effective, to the point of loosing elevator control. I didn't know this was supposed to happen, so along with a sharp leading edge, and the horizontal stab in line with the wing, on a very short coupled airplane...we built the "Dogfighter" which is one of the most popular combat Spads, and will turn and burn with the best of them...I guess sometimes ignorance is bliss...if I listened to guys who profess to all this knowledge, I never would have tried it...and it's fun to say "oh yeah? would you like to engage in a little dogfight with my plane that's not supposed to fly?"

TATTOO, the first thing I would do would be ask them "where would you place it?" You probably wouldn't get an answer. :stupid:

In pure aerodynamic theory that's probably not the best place for it, as it's operating in the wing wash and wing downdraft (dirty air), BUT as with the sharp leading edge thing, all theories and rules are not absolute, and minor problems can be trimmed out.

Every Spad we have ever built have had the hor. stab on top or or bottom of the fuse and they all flew.

I personally enjoy skull sessions on aerodynamic theory, but this one is too complicated for my little brain to fully comprehend.

High T tails are the preferred solution, but I doubt we would want those on a combat ship.



Cajun

Tattoo & Cajun,

There certainly can be problems with horizontal stabilizers being 'blanketed', but there are plenty of planes with stabilizers in various positions, some closely coupled, that work fine. Off the top of my head, I'd make two observations: 1) you want elevator authority for pitch in both directions, so you probably want the horizontal stabilizer near the thrust line, rather than much above or below 2) since you fly at fairly high speed and high throttle (thrust), you probably get pretty good airflow over your tailfeathers, even if their location is not optimal. Incidentally, I would say that most planes, combat or not, have the horizontal stabilizers pretty close to the same plane as the wing. Pathological situations, like blanketing of the tail, are not always real obvious in advance, even when professionals are doing the designing. When they show up, they get fixed. You don't seem to be seeing a problem, and I wouldn't really have expected one from looking at your plane, notwithstanding the warnings from the guys at your field. Time to go flyin'.

It's not really correct to say that the 'pure aerodynamic theory' is not absolute. What is more accurate is to say that it can't be applied too broadly. For example, if one were to infer that the 'theory people' were saying that sharp leading edges are bad, or don't work, that would be an overly broad interpretation, and would be faulty. The same would go for stabilizer placement. Theory is like any other tool: you should use it where it works, and not where it doesn't. Unfortunately, erroneous conventional wisdom can come to be known as 'theory' ( like flat-bottom wings being more stable than symmetric ones, or blunt trailing edges preventing flutter ), and incorrect decisions can be made. You shouldn't blame 'theory' for that.

Just my two cents.

banktoturn

Tattoo, Cajun, and anyone interested in this information, please download it. It is a PDF file from Jewel B. Barlow
Director, Glenn L. Martin Wind Tunnel
University of Maryland - all rights reserved.

http://spadworld.net/downloads/SharpLE-Effect.pdf

I found it very enlightening.

Banktoturn.......... AMEN!

To the others I want to make a general observation.

In my limited use of aerodynamics as an aero eng in industry I usually had a set of reference books that filled up a 3 foot shelf next to my desk. They were carefully selected as there was rarely room for more in our crowded conditions. I personally barely knew where to look to get the answers for my problems. I was able to determine that the absolutely mind numbing mathematically precise and rigiorous description of aerodynamics and fluid mechanics made it a well defined science.

The thing is that most of us need to run wind tunnel tests to have even the faintest clue as to what the math means. Our lack of understanding does not change the purity of the math or theory. It is there and is quite precise and elegant. We just don't understand it all the time.

This unfortunately leads to witch doctorery, black magic and, finally, common knowledge.

However learn to use the tools that are available to most modelers. In engineering when we were presented with a problem the first thing we did was a literature search. Why repeat the same work again and again? If I wanted to determine if the horizontal stab placement makes a big romping effect of the flying qualities of a model I would do a search of the literature (model magazines). What I would find is, not surprisingly, most stabs are aft of the wing and all over the ball park in fore, aft, up and down positioning.

Conclusion - If the airplane has flown successfully then the stab must be in a good position.

If I focus my search to precision aerobatics I find that most of the stabs are in line with the wing so it must not be too bad of a placement. It would seem to be the best if I wanted the airplane to fly the same upside down as right side up. Actual use in winning airplanes finds this to be the case.

If I look at the airplanes with stabs on the tip of the vertical tail I tend to find high performance gliders and some big transports but no sport or aerobatic models. I may not know why but I would probably not put the stab on the tip of the vertical in a model like yours. But inline with the wing, sure.

Look at what has worked before and use that as a guide line. Then evaluate the results and see if Common Knowledge is right. Sometimes it is. But most of the time it is like most gossip, stretched a lot because of uncontrolled hot air.

Speaking of horizontals on top of verticals, a couple years back on the MilForum on Compuserve we were musing about that.. C-5 vs An-124..
It seems to me the An is a better design for something that can be shot at..
The high horizontal on the C-5 adds a lot of load to the vertical all by itself. Any signifcant hole(s) can cause the whole thing to fall off, including the horizontal.. Oops!
The An-124 OTOH could probably suvive those same holes in the vertical..unless they're really lucky shots.

Not a high performance aerobat, but the Seamaster has the horizontal on top of the vert. (very practical reasons on a seaplane).

Ross

banktoturn, Now that's a THEORY I can live with.

Cajun

I've mostly skipped the theory part...I just build it. With Spads, that only takes a few hours and a few bucks...then go fly it. If I like it, it's a keeper, If I don't...I have a spare flight pack for my next experiment...and that's a fact!

This thread brewed up an idea for a very simple wing...but now it's raining so it hasn't flown yet...any predictions?


Spad-30"
Chord-16"
wingloading-14.54
Weight-2 pounds 15.5 ounces
fuselage length-32"
engine-Norvel .25
build time-1.5 hours
Channels-4

You have much courage. Do you have any way of keeping the upper LE and lower LE surfaces separated to keep the bending moment strength of the wing going. You have it at the tip but it might need something at other spanwise stations. Otherwise the wing might not have enough strength.

It is a great experiment.

Yes...the fuselage is strategically shishkabobed through the wing, and that's what holds it's shape. The original idea was to actually have an intragal spar folded into the wing...but the wing came out so stiff, I eliminated it. I feel comfortable doing this because guys are flying pizza boxes the size of this wing, just a single sheet of coroplast, and they can handle it! This wing in theroy with the LE fold, should be more than twice the strength of the pizza boxes. My scientific test was to pick the plane up by the wing tips and shake the daylights out of it...and it passed my test easy!

In case anyone is interested, I flew this plane today...and it ROCKS!!! And the airfoil REALLY WORKS!!! Solid as a rock and suprisingly stable at full speed. Not a click...repeat...not a single click of trim was needed!!! Loops are so hard it chases it's tail, and actually retains forward motion while on it's back (Full pull). With conservitive stick movements, it flies great conventionally, and no hint whatsoever of any flex or flutter. Inverted is also very conventional. Where this plane really shines is slow flight. With throttle managment it can be slowed down to walking speed, and brought in for a perfect 3 point nose high Harrier landing with no forward motion or roll out. What a blast! I can't thank you guys enough (especially for the "stepped airfoil" link) for giving me the ideas for the funest plane I've ever flown. At a cost of less than $10 and an hour and a half to design and build it, the effort "in" compared to fun in return should be illegal! What a blast! If anyone's interested, here's some more on the plane

http://members.cox.net/spad/pics/QHOR01.jpg
http://members.cox.net/spad/pics/QHOR02.jpg
http://members.cox.net/spad/pics/QHOR03.jpg
http://members.cox.net/spad/pics/QHOR04.jpg
http://members.cox.net/spad/pics/QHOR05.jpg
http://members.cox.net/spad/pics/QHOR06.jpg
http://members.cox.net/spad/pics/QHOR07.jpg
http://members.cox.net/spad/pics/QHOR08.jpg
http://members.cox.net/spad/pics/QHOR09.jpg
http://members.cox.net/spad/pics/QHOR10.jpg