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Old Dec 24, 2010, 02:31 PM
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Dick,

I certainly don't mind at all - very nice rendering!! This is a great representation of the profile I'm talking about! Nice work!

If this profile were to be built into a structure for wind tunnel testing, and from that perspective if I were to suggest any changes to your silhouette of my MH32/KF3P profile, it would be to reduce the depth of the secondary step which is located quite closely in front of the aileron hinge line. [I'm flying a ~3mm primary step, and only a 2mm secondary step on a wing with an average 9" chord.]

I'm theorizing that the shallow depth secondary step contributes to the clean and precise aileron response with minimum drag when control surfaces are deflected substantially; earlier reports indicate that deeper steps close to the aileron hinge line result in excess turbulence and substantially reduced response to substantial aileron deflection.

Merry Christmas to ALL!!!

VIKING
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Old Dec 24, 2010, 02:52 PM
just gotta mess with it!
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Quote:
Originally Posted by springer View Post

Not sure how to interpret the moment coefficient anyone have a good explanation of interepretations?
Didn't see any reference to moment coefficient, but it's a measure of how much the wing tries to pitch down. In general, the more camber an airfoil has, the higher the moment coefficient. It can lead to instability for highly cambered wings in high speed flight, ultimately overcoming the ability of the tailplane to keep the nose up - since the moment coefficient stays roughly constant as the lift coefficient reduces in faster flight. Most high speed planes therefore have low cambered airfoils. On a tailless design, reflex airfoils were traditionally used featuring near zero moment coefficients.

The maximum lift coefficients of the KF airfoils are impressive. Forcing the boundary layer to become turbulent is clearly beneficial at the low Reynolds numbers you get in models.
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Old Dec 26, 2010, 07:24 AM
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The moment coefficient (maybe easier understood as a pitching moment) of a wing is dependent upon the critical shaping of the forward 25% of the airfoil, and also is dependent upon the actual implementation & trimming of the control surfaces (ailerons or elevons.) Especially when building a wing with independent servos for each control surface, the trimming of those surfaces combined with a bit of fine-tuning of the wing incidence and balance point result in the pitching moment of the wing being able to be trimmed quite effectively on most builds. (On the first dancer wings with the single aileron servo, I found through test-flying that having ~1/16 "droop" of the trailing edge of each aileron resulted in very good handling across the entire speed range.)

I'm guessing that most foamie scratch-built wings are not shaped to close enough tolerances where the pitching moment is looked at too closely in advance- scratch-builders typically do their control surface linkage adjusting during test flights until they get the best handling they can from a given build, then start thinking about the next build. And, of course, a foam wing can be heat-formed to modify it's contours to a certain extent after the main wing build if desired.

For those making molded wing cores & precise hot-wire cut cores, selecting an airfoil profile for high speed flight tasks with a low pitching moment would be a detail to consider.... but again, with close to full-span trailing edge control surfaces, it may not be as much of an issue with anyone outside of the competition sailplane builders.

Implementations of KF stepped discontinuities in fairly simple structural elements, which result in higher lift and reduced drag, along with more precise control surface response is something that is very much of interest to foamie scratch-builders. There's still a lot to be understood as we test-fly various configurations- (the simulation software may simply not be able to adequately evaluate the affects of these stepped discontinuities on the actual in-flight performance...) and the experimentation along the way is a lot of the fun!

VIKING
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Old Dec 26, 2010, 02:17 PM
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Ok, I understand now that the "moment" is pitching moment or pitching tendency, but how do I interpret "goodness" from the charts? For instance, the attached pic is a comparison of the two kfm's versus several other airfoils found on the Worldofkrauss website. The KFM's moment coefficients are lower, mostly straight lines with a minor blip at about -.3, and about average in slope. Should I interpret that to mean that they are resistant to pitching from the low values, and more consistant than typical foils because they are straight lines? Or am I just totally confused (probably!)?

Or, maybe Viking's comments are right on, and all the theoretical/computer simulation/modeling is not accurate enough to predict performance of our size/types of wings and on field testing is required.
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Old Dec 26, 2010, 03:17 PM
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The pitching moment of the wing is what requires a tail down force from the elevator. Less moment requires less force for stability. If the pitching moment is negative, a tail up force is required.

You want some pitching moment for positive pitch stability. Too much and you waste lift countering the moment. If you have a free flight plane, you want quite a bit of moment, so the plane rights itself quickly.

This is an over simplification, and not a complete explanation, but hopefully it gives you an idea of what the moment curve is all about.

Roger
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Old Dec 26, 2010, 07:19 PM
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so, assuming the charts are correct (for discussion purposes), the larger negative valuse of the kfm's would indicate that they will require a tail up force to maintain a straight and level flight? I don't think I see that on my planes. Setting wing incidence to zero (regardless of high or low wing) It seems like I can just set the elevator at neutral and planes fly just fine, and aren't pitch sensitive. Could it be the earlier good lift (at lower angles of attack) that compensates?
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Old Dec 27, 2010, 09:50 AM
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The elevons on my KFm2 sloper wings are set at neutral and fly perfectly, right side and up side down, no reflex needed with the CG I run with. I can hover on the edge at zero ground speed, where with my non-KF'd I can't. That's with the step at 40% of the wing, not including elevons.
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Old Dec 29, 2010, 04:35 PM
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Both comments are indications of a more negative (less positive) pitching moment. A positive moment typically requires a positive incidence on the tail of a standard aircraft configuration, or reflex on a flying wing to keep the nose up.

Roger
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Old Dec 30, 2010, 01:33 AM
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Quote:
Originally Posted by springer View Post
so, assuming the charts are correct (for discussion purposes), the larger negative valuse of the kfm's would indicate that they will require a tail up force to maintain a straight and level flight? I don't think I see that on my planes.
Which only confirms that those simulation results are meaningless for the KFm airfoils.

Rick.
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Old Dec 30, 2010, 12:09 PM
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Originally Posted by maguro View Post
Both comments are indications of a more negative (less positive) pitching moment. A positive moment typically requires a positive incidence on the tail of a standard aircraft configuration, or reflex on a flying wing to keep the nose up....
??? Doesn't a positive pitching moment mean the nose pitches up?
??? Doesn't a negative pitching moment lead to the reflex on a flying wing to keep the nose up?
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Old Dec 30, 2010, 01:55 PM
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A positive pitching moment rotates the leading edge down. That's why a tail down force is needed. That's either up elevator or reflex.

Roger
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Old Dec 30, 2010, 02:30 PM
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Chord line incidence of KF3

Friends

I'm finding (especially on the recent 58" span & 62" span shaped airfoil structure MH32/KF3P wing builds) that the wings glide a bit more cleanly & efficiently if the trailing edge of the wing is shimmed up off the plane of the horizontal stabilizer. With the airfoil shaping and the resulting raised entry point of the leading edge, the wings have a fair amount of positive incidence in the resulting chord line- positive incidence relative to the wing's own bottom surface, which is flat for the aft ~80% of chord. So they simply glide more cleanly with the trailing edge raised slightly, and with the balance fine-tuned for optimum glide efficiency performance. This is something worth considering and experimenting with on some wing builds. The idea is to end up with the aircraft gliding efficiently with the elevator zeroed to the horizontal stabilizer, not trimmed out of zero alignment.

VIKING
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Old Dec 30, 2010, 04:24 PM
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Quote:
Originally Posted by maguro View Post
.... A positive moment typically requires a positive incidence on the tail of a standard aircraft configuration, or reflex on a flying wing to keep the nose up....
Quote:
Originally Posted by maguro View Post
A positive pitching moment rotates the leading edge down. That's why a tail down force is needed. That's either up elevator or reflex...
OK, but isn't "up" elevator equivalent to negative incidence in the tail of a standard aircraft?
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Old Jan 01, 2011, 11:33 AM
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A thread of interest regarding KFm Airfoils

I wanted to make everyone aware of a thread that was started on one of these RC Groups forms in Modeling Science which pertains to many questions about the KFm airfoils.

Modeling Science:
http://www.rcgroups.com/forums/showthread.php?t=1329266

A fairly large amount of information has been posted on the site which might be of interest to you. Here is a sample of some of the data...

http://www.flightglobal.com/articles...ying-wing.html
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Old Jan 09, 2011, 09:13 AM
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Optimizing the depth & contours of the shallow KF3P discontinuities

Friends,

I continue to brain-storm on how to build a foamie wing that shows the benefits of implementing stepped discontinuities in the aft 50% of the airfoil's top surface, while minimizing or eliminating the 'drag penalties' associated with deep squared step contours.

On 9-27-2010, I designed the MH32/KF3P airfoil build which uses shallower depth steps and uses a (~flat) panel running from the primary step at 48% of chord back to a shallower secondary stepped discontinuity at 70% to 75% of chord. (The relative positions of these discontinuities are tapered forward towards the wings leading edge as the wing tapers towards the wing tips.)

On 12-24-2010, I drew up the wing structure profile below, changing the contour of the front surface of the stepped discontinuity 'pocket' to a rounded profile which should allow for smoother airflow in the recirculating flow behind that rounded step. Dick Kline converted the structure drawing into the black silhouette, & I then did a bit of modifying from there to have this version show my optimized concept. (I've actually also added an aileron hinge line gap cover on the wing's lower surface to further minimize drag in that area.)

Do these shallow discontinuity structures actually trap a continuous recirculating vortex? Or are they simply very effective turbulating structures which achieve the purpose of minimizing massive boundary layer airflow separations, while also minimizing drag?

With only a ~2mm to 3mm depth to the primary step, and only 2mm max height of the secondary step structure, the recirculating vortexes themselves which are theorized to occur may not be very extensive, chord-wise, yet they may be keeping the airflow behind that shallow structure turbulated and thereby they may be effectively preventing massive airflow (lift) separations.

In the case of the second shallower discontinuity which is located within 1/2" of the aileron hinge line, the shallower depth of this step in this location seems to *improve* the aircraft's response to aileron deflection at all airspeeds... again, possibly by producing a turbulated surface layer airflow that does not easily separate from the aileron's surface. (The thin clean trailing edges of the ailerons also contribute their part to the wing's precise response to aileron deflection.)

I'll be adding these rounded contours to the stepped discontinuities on the 62" MH32/KF3P wing this morning, and hopefully get in some test flying with this wing mounted on the DANCER III sleek EPP fuselage before the next snow storm moves in later today. It's more challenging to evaluate a wing's performance during cold weather with irregular air.... it would be nice to have some good thermal or slope conditions to really do the test flying... But I'll see what I can observe today.

VIKING
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