Thermal plank 2.4m, foam - Page 2 - RC Groups
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Aug 12, 2016, 11:54 AM
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Quote:
Originally Posted by HerkS
Chris -- with a model flying wing of any kind, it's unlikely that the wing Cl max will ever be much above about 0.6. With relatively low AR it's likely to be even less. Cl 1.1 for a test airfoil section in a wind tunnel might be achievable at these RNs, but a full wing cannot develop anything like that much.
Herk,
You're correct as I did get a little over zealous with my "3d" max "CL" versus the "2d" max "Cl".
When I'm looking at airfoil selection I like to stress test the airplane. In essence to base the airfoils low speed geometry on the low end of the scale worst case this plane could ever see scenario; very tight turn, inside wing.
What I'm attempting to establish is a low end Reynolds number that gets me in the ballpark for evaluation of a candidate airfoil.
I think a max CL above .6 is achievable with a swept planform it just depends on how much sweep and twist we have in the mix. Generally I don't design airfoils on a swept planform below a CL of .9. I once had a project with an Eagle Tree data logger with an airspeed pitot probe onboard. I verified that the airspeed data I was getting was accurate and it was then easy to determine what CL my "airplane" (a modern day XF5U-1) was achieving.
BTW the "pancake" I was flying would make a great topic for the History thread unless it is not considered tailless.
For Marcos project I took into consideration a plank planform with little or no sweep (sweep forward?) and thus none of the losses associated with sweepback. Granted the aspect ratio is pretty low for a glider so this means we have to fly at a higher alpha per degree CL than a higher aspect ratio but we can more easily achieve higher alpha than a higher aspect ratio. The airfoils I'm looking at are despite pretty wide wing chord operating at fairly low Rn's a result of "lighter than air" projected wing loading. Never the less the airfoils should achieve a Cl of no less than 1.1 and possibly higher despite being cambered (reflexed) in the wrong direction. If we guesstimate fairly good span efficiency (no sweepback-no twist) then I think a span efficiency of .89 shouldn't be far off base. So can we count on a max CL for the wing of (1.1 X .89) of .98?
This subject we're discussing might be good in the aerodynamics thread.
I'd appreciate your feedback.
Thanks
Last edited by drive320; Aug 12, 2016 at 02:54 PM.
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Aug 13, 2016, 09:21 AM
Herk
HerkS's Avatar
feedback:
Twist or reflex plus CG position determine a trim setting - depending on the purpose of the design. Because the craft requires positive longitudinal static stability the CG is located ahead of the neutral point of the aircraft. That creates a nose down moment that is countered by the reflex or twist at the design cruise point. To slow below that (i.e. moving toward Cl max) additional up trim is required to overcome the CG moment. This is provided by control surfaces which by their deflection reduce the Cl available from the wing. Whether it is an elevator at the center or on the wingtips of a plank, or the elevons on the outer wing of a swept design they each add more and more reflex to that area as it slows - reshaping the lift distribution in a way that makes less Cl possible for the wing.

If you want to determine the reasonable stable max Cl of a flying wing model (assuming that you actually have one) you can easily measure it's flying speed by timing a measured course in calm conditions. The calculation of its Cl is then very simple math.
Aug 13, 2016, 12:32 PM
Registered User
Herk,
Taking into consideration all of the factors you list that in a way makes less CL for a wing than the Cl of a two dimensional airfoil your experience is a model flying wing of any kind the wing CL max will unlikely ever be much above about 0.6.
If what your glide testing of your planks reveals this is valuable to know when attempting to predict how a proposed plank will perform.
For illustration I'll stick to a tailless plank with no sweepback or washout but does have control deflections (reflexed) to increase the angle of attack of the wing.
If this plank has an elevator that is centrally located that affects say 30% of the span the remaining 70% of the span is without controls reflexed greater than what is geometrically in the airfoil chosen.
I'm going at all this on the premise I do not have a plane to test, I'm attempting to establish performance beforehand.
I'm going to assume that the wing planform I've chosen and the chord widths are reasonable for good span efficiency as there are 3d losses compared to 2d data.
Let's say my chart for XYZ airfoil at a relevant Rn indicates a 2d Cl of 1.2. If I guess a span efficiency of (95 %?) then do I estimate 5% in losses that lower my max Cl of the 2d data from 1.2 Cl to a 3d wings 1.14 CL?
Going further can I reduce the total sum that the wings CL by the 30% of reflexed wingspan so my 1.14 CL is now .8 CL?
A .8 CL is 33% greater than your .6 CL rule of thumb so is this reasonable?
Is there any method you use rather than a rule of thumb .6 max CL for any type of tailless? What steps do you take for airfoil selection and have at least a ballpark guess of the Reynolds numbers the airplane should reasonably see?
I have done many glide tests from a fixed height, marking the distance traveled under calm conditions with a stopwatch to determine as accurately as possible the CL and L/D of several airplanes from hand chuck to RC gliders. Adjusting for "scatter" I agree it is a useful method to determine real world performance.
I also use a stopwatch and electronic altimeter again meeting all of the criteria above to determine sink rate.
Thanks for your feedback.
Chris
Last edited by drive320; Aug 13, 2016 at 04:11 PM.
Aug 14, 2016, 05:22 PM
Plank addicted
Marcos Quito's Avatar
Sorry pushing you guys , but I have stopped building, waiting the airfoil suggestion.
Which options would you consider ?
Aug 14, 2016, 06:25 PM
Registered User
Quote:
Originally Posted by Marcos Quito
Sorry pushing you guys , but I have stopped building, waiting the airfoil suggestion.
Which options would you consider ?
Marcos,
I was just finishing up when I saw your post.
Whatever airfoil you choose the Pioneer airfoil is not a good fit.
This airfoil is more suitable for your goal of a floater.
It is 8% thick with a generous trailing edge thickness. It is not a fast airfoil it is for "floating."
It is a plank airfoil so it gives up some L/d yet is pitch positive at all angles of attack and at low speeds.
Once you fly it you may be able to "un-reflex" the trailing edge which will improve your L/d.
Hinge your wing at 25%, do not shorten your control surfaces.
Best of luck
Chris
P.S> your not pushing I just got busy thanks for your patience
Last edited by drive320; Aug 15, 2016 at 09:48 AM.
Aug 15, 2016, 06:05 AM
Plank addicted
Marcos Quito's Avatar
Hello Chris,

Thank you very much !
Do you have more data around this airfoil ? (graphics ?)

I like it !!
I had difficulties to plot it due to unit standards ( , vs . decimal separator).
But here it is !

Chris can I go to 10% thickness, in order to stiff the wing ?
Aug 15, 2016, 09:53 AM
Registered User
Quote:
Originally Posted by Marcos Quito
Hello Chris,

Thank you very much !
Do you have more data around this airfoil ? (graphics ?)

I like it !!
I had difficulties to plot it due to unit standards ( , vs . decimal separator).
But here it is !

Chris can I go to 10% thickness, in order to stiff the wing ?
Marcos,
I am working on it now.
For whatever reason the file I posted was altered somewhat and I will re-post it shortly.
By graphics I suppose you mean charts for L/d and such.
I will do so comparing to the PW-75 which is an excellent airfoil and one of the airfoils I used for comparison.
I apologize in that the only format I am able to export from Xfoil to Nurflugel is in the .txt format.
Chris
Last edited by drive320; Aug 15, 2016 at 09:59 AM.
Aug 15, 2016, 12:56 PM
Registered User
Marcos,
There are no graphs for comparison of the Pioneer II airfoil as it just doesn't work at your "scale". It may have been suitable for full scale but is not at this scale.
One of the airfoils I used as a benchmark is Peter Wicks PW75 airfoil.
The PW75 is a very good airfoil and is superior to my airfoil in L/d. However my goals were a higher Cl ("floatier") and attempting to maintain a positive Cm throughout the angle of attack range. More than likely the PW75 is designed for higher Rn's.
If Peter see's this post possibly he would be so kind to give his input.
The PW75 is thicker than the PP 22308 airfoil at @ 9% so a more direct comparison is with the PP 22309 airfoil.
If you desire a fast plank then Peter's airfoil is superior.
I going to make some suggestions I hope you will adhere to.
Use the PP 22310 at root, PP 22309 at the first break and the PP 22308-TIP at the tip.
As your planform is nearly a straight (un-tapered) the progression of airfoils will bring it closer to an "elliptical" lift distribution
I worked quite hard to achieve a greater stall angle for the PP 22308-TIP.
With the mild taper in your tip panel it operates at slightly lower Rn than the root airfoil thus it is a struggle to get the root and tip airfoils to stall at the same angle.
The spar will taper in height and this will help keep the entire wing lighter and more importantly the tip panel light. A spar the same height for the entire span as it is not required for the bending moments.
Do not bring the aileron all the way to the tip, I would stop short @ 50 to 75 mm of the tip. Stopping the aileron short of the tip improves tip stall behavior and does not mess up the vortex at the tip.
These are my opinions you may have your own and it's your airplane so it will be your decision.
Best,
Chris
Aug 15, 2016, 05:59 PM
Plank addicted
Marcos Quito's Avatar
Hi Chris,

Uaw, such a lot of data !!!

Thank you very much.
Only a few doubts : (sorry if they are stupid)

1. I´m not sure I can make such small differences (root airfoil, central, tip). I´ll hand cut the ribs.

2. Trailing edge : did you left it not finishing on "zero", on purpose ? Do you mean, better I try to keep it that way, or the closer to zero the better ?

3. I need the thicker one and I really like your airfoil. Once again, may I make it 10% thick ?
Aug 15, 2016, 08:50 PM
Registered User
Quote:
Originally Posted by Marcos Quito
Hi Chris,

Uaw, such a lot of data !!!

Thank you very much.
Only a few doubts : (sorry if they are stupid)

1. I´m not sure I can make such small differences (root airfoil, central, tip). I´ll hand cut the ribs.

2. Trailing edge : did you left it not finishing on "zero", on purpose ? Do you mean, better I try to keep it that way, or the closer to zero the better ?

3. I need the thicker one and I really like your airfoil. Once again, may I make it 10% thick ?
Marcos,
I ask stupid questions all the time.
1. It is not any more work than using the same airfoil full span. For two panels per half span you'll need three templates. How are you shaping the foam?
Are you shaping by hand such as a long sanding board or are you hot wiring?
2. I leave the trailing edge thicker purposely to make it easier to fabricate the trailing edge. Leaving the trailing edge some thickness makes it less prone to damage. By better if you mean performance wise there isn't much difference just make them as thin as you are comfortable with.
3. If you mean "may I make the planes entire wing 10% thick" by all means it's your airplane.
I'm just trying to help you get better performance. Make it 10% thick full span it will still fly.
Get rid of the doubt's you can do it.
Best of luck and post some pictures.
Chris
Last edited by drive320; Aug 16, 2016 at 05:12 AM. Reason: foam
Aug 16, 2016, 02:56 PM
Plank addicted
Marcos Quito's Avatar
Thank you again Chris.

As I've told you the materials I'll use are the cheapest.
In the city I live there's no balsa wood. That wood available in Brazil comes from Ecuador.

The airfoil profile will be on hard foam - depron sheet.

As soon as I have pictures I'll post here.
Aug 17, 2016, 05:40 AM
Plank addicted
Marcos Quito's Avatar
Only one more doubt : hinge on top or bottom ?
Aug 19, 2016, 01:51 PM
Registered User
Quote:
Originally Posted by Marcos Quito
Only one more doubt : hinge on top or bottom ?
Marcos,
Put he hinge on the bottom.
On the upper surface of the wing airflow (the boundary layer) is likely turbulent by the time it gets back to a gap that is 25% from the trailing edge.
On the lower surface the boundary layer is(likely) laminar back to the trailing edge so a gap on the bottom of the wing is probably worse than on top.
As to airfoils consider using the 10% thick airfoil until the first break then go to the 8% at the tip.
You can use the 10% from root to tip if you must.
Chris
Sep 04, 2016, 08:51 PM
Plank addicted
Marcos Quito's Avatar
Starting to build :
Again, these are cheap materials.
The best one is the "depron" sheet, 5mm.
All the rest can be found in any "office supplier".
Follow photos :
1. Printing and cutting the airfoil : cannot use directed printed airfoil, because the "foam" skin is 5mm thick, so I have to manually draw this thickness in the airfoil.
2. Drawing the airfoil in the depron sheet.
3. Cuttin the airfoils.
4. Sanding the airfoils.
Sep 04, 2016, 09:04 PM
Plank addicted
Marcos Quito's Avatar
I have tested a prototipe of this type of construction in a short sample, and the result was very good.
Follow the test photos.

Of course 2400mm is a very daring project for such a simple construction, but considering the light weight of the ship, I think it will workout.


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