Project Estimating and Decision Making

Project Estimating and Decision Making Using Denny Dart II

by Bill Kuhl

Introduction

My last model aviation web page, "Building Model Airplanes From Plans and Materials" has been very popular. In trying to expand on that and make a practical exercise for students, I came up with the idea, "have the students compute the materials needed for building a bunch of model airplanes." This should be a practical exercise in project estimating, a skill valuable in many careers and everyone's personal life. To further add to the realism and challenge of this exercise, two different sources for balsa wood and two options for the type of balsa are considered. This exposes the student to concepts such as minimum order amounts, shipping charges, additional equipment needed, waste material, and sales tax.

Notes:

refer to website "Building Model Airplanes From Plans and Materials" for more details and pictures.
The solutions I created for this exercise are available on a separate website. In attempt to receive more feedback on my model aviation web pages, I will send the link to the solution in an email if you tell me a little about how you are using model airplanes in education or any comments you might have on my web pages. Note, I hate spam as much as everyone else and never collect email addresses for such purposes. Although my websites receive heavy traffic, feedback has dwindled to almost none. I can only assume everyone is afraid that they will be added to some list, this is not the case with me.

Email: bkuhl@luminet.net Subject: Solution to Estimating

The Exercise

Compute the Total Cost for balsa for 80 airplanes given the bill of materials for one airplane and the price list below. Break down your calculations for the two sources (mail order and hobby shop) and the two balsa types; sheet and stick. Allow for 10% waste on sheet wood and purchase one balsa stripper. Wing hold downs are made from the allowance for waste of the motor sticks (sheet wood only). For sticks allow two extra sticks for each type except 1/8" x 1/8" (wing hold down) only allow one stick extra. To make the $20 minimum for mail order, add CA glue at $3.00 and CA Accelerator at $3.15, if order is still under $20, add second CA glue.
Calculate the number of packages of propellers needed for 80 planes, 35 propellers in a package. Make sure there are extra propellers but less than 35 extra. Add up all costs.
Calculate how many feet of 3/32" rubber strip is in a one pound box. Make sure there is enough for 3 motors for each student (80 students), each motor 2 feet long.
Give your observations on the completed calculations, give advantages / disadvantages for the different options for purchase of balsa.

Bill of Materials Required for One Airplane

2 – 1/16" x 1/8" x 36" balsa strip

1 – 3/8" x 1/8" x 12" balsa motor stick

1 – 1/8" x 1/8" x 4" balsa wing hold down

1 – 6" plastic propeller assembly

3 – 24" lengths of 3/32" FAI Super Sport rubber

Price Lists

Balsa Price List

Balsa in 3" Sheets

Size - Description	Mail Order Price	Hobby Shop Price
1/16” x 3” x 36”	.46 *	1.50 **
1/8” x 3” x 36”	.66 *	2.10 **

Pre-cut Sticks

Size - Description	Mail Order Price	Hobby Shop Price
1/16” x 1/8” x 36”	.10 *	.35 **
1/8” x 3/8” x 36”	.18 *	.70 **
1/8" x 1/8" x 36"	.11 *	.45 **

* Additional Items Needed When Using Mail Order

Balsa Stripper $7.00
Shipping $8.00
Minimum Order $ 20.00

Note: If you use only sticks, the balsa stripper is not needed. This will save time as well.

** Purchasing From Hobby Shop, Assume Sales Tax

For this exercise assume 6% sales tax, this will vary between states, some states have none.

Propeller Price

Each plane needs a plastic propeller assembly that must be ordered in packages of 35. $10.56 per 35 plus $6.50 shipping. Shipping Note: there is only one shipping charge, so ordering more propellers the shipping is a smaller portion of the total cost.

Rubber Price

Each plane uses a loop of rubber that is a 24" loop of rubber strip including extra for the knot. Each student needs three of these motors because of breakage. Compute how many motors would come out of one pound box of rubber. Use .0596 gram per inch as the weight for 3/32" rubber. There are 28.35 grams in one ounce.

Calculations will be for mail order only, as finding rubber strip in the correct size in a hobby shop is difficult and very expensive.

First calculate the length of rubber in feet in a one pound box knowing that rubber weighs .0596 grams per inch for 3/32" width rubber that you will be using.

Price for one pound of 3/32" rubber is $23.00 + $5.00 shipping

Hints on How to Approach the Exercise

Balsa

To compute the total cost for the different options, first compute the quantity of materials needed allowing for waste or broken pieces.

With the requirements, create tables of quantities needed of each material, cost per piece, calculations, and extra charges.

For the balsa in sheet wood, the width of the sheet (3") must be divided by the width of the strips needed; either 1/8" or 3/8". Remember to allow 10% waste.

Divide the number of pieces in each sheet into the total pieces needed to arrive at the number of sheets needed.

Three motor sticks can be cut from each 36" strip.

Rubber

No doubt there are different methods to solve this, but I chose to convert grams to ounces and one pound to 16 ounces. Divide Ounces in a Pound by Ounce per Inch. Convert to feet.

Propellers

Only need to make sure there are more propellers than needed when ordering propellers in packages of 35.

My Solutions: Ok, I give in, no one wants to email me. The solutions I came up with can be found at:

http://www.maxconrad.com/prjsol.htm

Bill Kuhl