Model Sailplane Design Spreadsheet User's Guide
by Adam Till, aktill(AT)telusplanet(DOT)net

Last update: August 23, 2004

Download it here: SpreadsheetV81.zip

Well, after probably 100 requests in the last six months, I’ve decided to host the spreadsheet on this website again. As much as I enjoy hearing from everyone, I sometimes miss emails when they come in. Doesn’t mean you should stop emailing me about your latest projects, however – I enjoy learning about them!

First off - only the boxes that are coloured need to be changed. If a box is uncoloured, it's a calculated value or has been carried over from the input page. Changing these uncoloured boxes will interfere with the operation of this spreadsheet.

Please realize that this sheet was built for my own personal use, and as a result it’s not particularly user-friendly unless you understand all the terminology. I’ve tried to explain some of it below, but you may have to do a quick web search on some terms for an explanation.

Please be aware that there are a lot of interdependent entires on the sheet, so you may have to fill out most of it in order to get relevant results.

Thanks go out to:

John Hazel for "LiftRoll", which adds the Lift Distribution Diagram and Diagram of Local Cl

Warren Man-Son-Hing for his "Stability Spreadsheet", which addresses the Stability Parameters on the Main Input Sheet

Joe Wurts for his "Layup Spreadsheet", which is used for designing stressed-skin models and has its own sheet.

Peter Thannhauser for "Pylon Analysis", which formed the basis for the Turn Analysis section. Beware that sharp turns push into the realm of transient aerodynamics, so your results from this sheet will be fairly inaccurate for those cases.

...and Lee Estingoy for "Sparsize", which adds the spar stress calculations in SparSize.

Also, by popular demand, I’ll start trying to keep an updated revision list for the sheet. Also helps me remember when I added what.

Cheers,
Adam

Revision History for Sailplane Design Spreadsheet

Revision 8.0

- Added direct readout and compellation of Cm

Revision 8.1

- Added stabilizer required Cl for design speed

- Fixed Planform Efficiency Factor

- Added stabilizer required alpha for design speed

- Added airfoil data entry of stab and fin airfoil, based on SMAC/FMAC and area

- Added induced drag readout for stabilizer and fin

- Added drag balance for wing, stab, fin

- Fixed Local Cl output bug on vertical axis, added graph axis labels

Main Input Sheet

This is where the bulk of the data gets entered, so start here first. Most entries here will carry over into the other spreadsheets, which can be accessed using the tabs at the bottom of your excel window. If you see a prompt to update the document with changes made to another workbook when the sheet loads, just say no. I'll fix that eventually.

(Update as of 8.1 – haven’t updated the instructions, which are out of date. Hopefully everything still makes sense…email me if it doesn’t).

Wing Dimensions

Each wing is specified in four panels, so an eight panel wing is possible. A "panel" in this case is only a section with constant taper.

If you want to make a constant chord wing, just enter the same chord length in each blue box, and divide the half-span by four for each purple panel length box.

Likewise, if you want to do a two panel wing, enter the root and tip chords in their blue boxes, and the midspan chord in the "Break 2" chord length box. For Break 1, take the average of Root and Break 2 ( [Root+Break2]/2 = Break 1), and for Break 3, take the average of Break 2 and the Tip. Decide how long the mid panel will be, and enter half of this amount into the purple Root -> Break 1 box and into the Break 1 -> Break 2 box. Now repear the process for the outboard section.

Triple taper wings are a little trickier, and I'll it up to you how you want to specify those.

Chord	- the distance from the leading edge of the wing to the trailing edge at a particular point. Specify this in inches.
Panel Length	- the horizontal or spanwise distance between two chord locations. Specify this in inches.
Panel Area	- (Calculated Value) the area of a panel between two adjacent chord locations. This is output in square inches.
TE Sweep	- if you draw a horizontal line which passes through the trailing edge of the wing root, TE Sweep is the distance from this line to the trailing edge of any particular chord length. A postive value means that the panel is swept forward relative to the wing root, and a negative value means that it's been swept backwards. Sweep will affect your MAC, center of gravity, and stability parameters, so keep this in mind. Specify this in inches.
Twist	- this is used to modify the flight/stall behaviour of a wing. A positive value here will mean the the panel has wash-in, where the trailing edge of the root is higher than the trailing edge of the tip. A negative value here will mean the the panel has wash-out, where the trailing edge of the tip is higher than the trailing edge of the root. This is specified in degrees of twist. The affects of panel twist can be seen in the Lift Distribution Diagram and the Diagram of Local Cl. Jeb Bushell has done a good job of explaining how to use the stability aspects of LiftRoll, and you can find his information at http://www.geocities.com/jebbushell/COOKBOOK.htm
Dihedral Angle	- this is the angle made by each panel compared to a horizontal reference line. Since it's calculated on a per-wing basis, a joiner rod with a 5 degree bend would be simulated by a 2.5 degree entry inthe speadsheet. Poly breaks should be added to the root dihedral angle, so a double taper model with a 5 degree joiner rod and an additional 2 degrees of dihdreal in the tips would get "2.5" input into the light blue "Dihedral Angle" boxes on Root -> Break1 and Break1-> Break2, and "4" in the next two blue boxes. Again, this entry is specified in degrees.
Semi-Span Station	- (Calculated Value) see http://www.rc-soar.com/tech/spiral_eda.htm
Moment Fraction	- (Calculated Value) see http://www.rc-soar.com/tech/spiral_eda.htm
Landing/Cruise/Max Re	- Reynolds Number at a particular airspeed, specified under the "Operational Parameters" on this sheet. This is a non-dimension number that is a function of chord length and airspeed, and can be used to size airfoils for a particular part of the wing.

Does this model have a v-tail? - by entering either "1" (yes) or "0" (no) into this field, the spreadsheet will change a couple of boxes to make working with each style of tail a little easier. The calculations that are performed for on stability parameters are adjusted slightly to compensate for a vtail, if "1" is input here. This takes into account the article posted by Dr. Mark Drela at http://www.charlesriverrc.org/articles/design/markdrela_vtailsizing.htm

If yes to the previous question, V-tail interior angle (deg) = this is pretty self explanitory, and is just the number of degrees that seperates the two section of v-tail (interior angle). Most sailplanes use 110 degrees, some aerobatic/F3F models use 107 degrees, some pylon racers use as little as 100 degrees. If you play with this angle a little bit, you can get the correct balance between vertical and horizontal tail areas. To do this, check to make sure that the V(v) and V(h) values (see Stability Parameters) stay in the proper ranges.

Does the stab have an airfoil? (1=yes, 0=flat plate) - this affects the calculation for "Wing Aerodynamic Center" in the same way that "Stabilizer Efficiency" does (see Stability Parameters).

If no to v-tail question, Tailboom mounting? (1=base, 0=elsewhere) - this entry accounts for the fact that not all fins are mounted at the base (eg. Fin areas on a DLG could occur above and below a tailboom in an x-tail). Basically, it only affects the formatting of the "Vertical Fin Dimensions" section.

Horizontal Stabilizer Dimensions

This section is only active if the v-tail question is answered with a "no". Works the same way that the wing section does, only the stab can only be specified as two-panels per half. Stab area, total span and aspect ratio (ratio of mean chord length to span) are output below for reference.

V-tail Dimensions

This section is only active if the v-tail question is answered with a "yes". If you're designing a model with a v-tail, input the values as if the stabilizer had no angle to it. The spreadsheet will calculate the effects of whatever interior angle you tell it to use. When this is active, the entry for "Stab Area" becomes "Effective Stab Area", indicating that a correction factor has been applied to account for the fact that you cannot simply use the projected area of a v-tail in sizing one properly. This new area is used in the stability calculations. The Drela correction has been used here.

Vertical Fin Dimensions

This section is only active if the v-tail question is answered with a "no". Parameters change slightly with the results of the "Tailboom Mounting" question. Fin area and span are given below, works the same way that the wing section does, except you're obviously specifying the dimension of the whole fin ("half a fin" doesn't really make sense). If the model is a v-tail, "Vertical Stabilizer Area" becomes the effective area, with the same corrections outlined above.

Wing Parameters

All the values in this section are outputs, so no user input is required.

Wing 1/2 Span	- this is the length of one wing. Useful for knowing if you have a long enough table to bag that whole wing! Output in inches.
Wing Span	- this is the full wingspan of the model. Note that it doens't account for span inside the fuselage. Output in inches
Wing Area	- the area of the full wing, not per half. Output in square inches.
Wing Loading	- this is the loading per unit area for the wing. Divides the total wing area by the total model weight specified in "Structural Parameters". Output is oz/square foot.
Aspect Ratio	- the ratio of "Mean Aerodynamic Chord" to span. The higher the number, the skinnier you wing is.
Taper Ratio	- a relative measure of how much the wing tapers over its span
MAC	- Mean Aerodynamic Chord. A weighted measure of what the average chord of the wing is.
1/4 MAC Setback	- a very conservative measure of where to set your center of gravity for first flights is at 1/4 of the MAC (not root chord, as some might think). This is output in inches, refering to how many inches from the leading edge of the root. Setting your center of gravity at 1/4 of the root chord can get you into trouble on highly swept wings. The spreadsheet accounts for multi-panel sweep when calculating this value.
1/4 MAC %	- same as above, as a percentage of the root chord, referenced to the leading edge of the wing.

Airfoil Parameters

Airfoil Thickness	- descibes the thickness of an airfoil at its highest point as a percentage of its chord length. This value is important in sizing spars, as the spar is usually added at the point of largest section thickness. This is also used in calculations of panel volume. This is assumed to be uniform for the whole wing, which may or may not be correct depending on the model in question.
Cl (max)	- maximum lift coefficient generated by the main wing's airfoil section. Available in literature, though most thermal airfoils will be around 1-1.1
Cm	- airfoil pitching moment coeffiicient. Same as above
Airfoil Form Factor	- usually around 0.6 for thermal airfoil (MH32, SD7037) or 0.7 for scale section (HQ sections).

Operational Parameters

Fairly straightforward inputs, these describe the expected flight regime for the model. They have an effect on Reynolds Number calculations, and help to dictate spar design. All should be given in mph.

Structural Parameters

Total Model Weight	- input in oz.
Fuselage Weight	- input in oz. Should account for vertical tail surfaces as well.
G-loading	- not strictly required for those who don't know it. This design parameter can be used for designing slope models or pylon racers, since "line tension" isn't really relevant in these cases.
Max Launch Line Tension	- input in lbs. Maximum expected line tension for thermal models . Suggested values include:  F3B Launch tension = 200 (lbs) XC Launch tension = 150 (lbs) Thermal launch tension = 100-140 (lbs)
Wing Bending Moment (G-Load)	- (Calculated Value) bending moment exerted at wing root as a result of the g-loading from the fuselage. Neglects weight of wing. Use this value in the "Design Root Bending Moment" entry when designing powered models or slope racers.
Wing Bending Moment (V-Max)	- (Calculated Value) bending moment exerted at wing root when the wing is operating at its maximum velocity. Use this value in the "Design Root Bending Moment" entry when designing very high-speed models.
Wing Bending Moment (Launch)	- (Calculated Value) bending moment exerted at wing root in the launch condition. Use this value in the "Design Root Bending Moment" entry when designing for high-stress launches.
Design Root Bending Moment	- input in lbs. Follow the guidelines given above. If all situations are applicable, choose the highest loading condition to guarantee safety.

Stability Parameters

B	- (Calculated Value) Spiral Stability Coefficient, or an attempt to put an empirical measurement on stability. Neutral spiral stability found at about B = 5. Design a polyhedral model for B=5. Typical values for most aileron models have B~=0.5-1.5 For further reading, see http://www.rc-soar.com/tech/spiral.htm
V(v)	- (Calculated Value) Vertical Tail Volume Coefficient: Normal designs should have V(v) = [0.015-0.02] with the larger values being more stable and the smaller ones more responsive, though some sailplanes may use V(v) as low as 0.012. This value is affected by tail length and fin area, wing span and area, and is adjusted for wing and fin sweep.
V(h)	- (Calculated Value) Horizontal Tail Volume Coefficient: Normal designs should have V(h) = [0.40-0.60], with the larger values being more stable and the smaller ones more responsive. This value is affected by stab area, wing area, MAC, and tail length, and is adjusted for wing and stab sweep.
Effective Tail Length (h)	- (Calculated Value) - distance from the main wing's 1/4 MAC station to the equivilant point on the horizontal stabilizer
Stab to Wing Separation	- distance from the trailing edge of the wing to the leading edge of the horizontal stabilizer. Specify this in inches.
SMAC	- (Calculated Value) - Stabilizer Mean Aerodynamic Chord. Given in inches.
Effective Tail Length (v)	- (Calculated Value) - distance from the main wing's 1/4 MAC station to the equivilant point on the vertical fin. Given in inches, distance measured from the leading edge of the horizontal stabilizer at the root.
Fin to Wing Separation	- distance from the trailing edge of the wing to the leading edge of the vertical fin. Specify this in inches.
FMAC	- (Calculated Value) - Fin Mean Aerodynamic Chord. Given in inches, distance measured from the leading edge of the fin at the root.
Wing Aerodynamic Center	- (Calculated Value) - a calculated attempt at finding the aerodynamic center of the wing.
Stabilizer Efficiency	- a correction factor designed to account for the differing aerodynamic efficiencies resulting from difrerent stab locations. In this context, it only affects the "Wing Aerodynamic Center."  0.6 = Standard Tails 0.9 = T-Tails
a	- (Calculated Value) - internal value
a1	- (Calculated Value) - internal value
de/da	- (Calculated Value) - internal value
Neutral Point	- (Calculated Value) - Theoretically the point of neutral stability, and the ideal balance point of the wing. Given in inches, distance measured from the leading edge of the wing at the root. Use with extreme caution.
NP%	- (Calculated Value) - as above, expressed as a percentage of the root chord from the LE

Future upgrade - reintroduce winglet and tip shape calcs lost from previous version.