Questions To Ask Before Designing An Airplane (#1-3)

What we’ll do in this series is ask the most important questions a designer should be asking before building an aircraft. To assist in the process, each time a question is asked, we’ll apply it to a group of fictional aircraft that are literally being made up as the articles are being written. This is to show that regardless of the powerplant, size or speed, the basics apply to every successful and safe aircraft.

The payload, range and cruise speed requirements directly affect how big an aircraft is.

 

 

 

 

 

 

The 3 most important questions are:

1. What do I want to carry?
2. How far do I want to carry it?
3. How much time should it take to carry it.

Simple right?

Well, yes and no. These 3 questions open the pandora’s box of compromises, adjustments, and late-night napkin sketching that plague engineers. Since I don’t have a degree in engineering however, I’m not smart enough to be deterred the prospect of spending the wee hours with a stack of Marcal and a Sharpie (I actually enjoy it, most likely because my career isn’t at stake if I come up 300lbs overweight and $500,000 overbudget). Everytime one problem is solved, it is likely to create several others. Only by setting your performance limits early can the tradeoffs be minimized.

So what’s the simple part? The fact that you have full control over what you want your aircraft to do. If it can perform the mission or not within the bounds of Newtonian physics and the laws of Bernoulli is another story. But knowing that all aircraft performance figures stem from the initial 3 questions will make your life much easier when it comes time to run a weight reduction program. In fact, you probably won’t need to run the program at all.

Okay, onto dissecting the questions.

Question 1 simply asked what did you want to carry. This is the place where you’d write down what or who you want in the aircraft with you (I’m assuming that a pilot would be flying it, although if you’re building a UAV, the same principle applies). Once the basic paylod figure is created, the aircraft will start to take shape via raw numbers, not yet by appearance. Be realistic with the weights you assign to each passenger/crew. The FAA standardsare pretty ridiculous when one considers that the weights are supposed to include

How much stuff will you carry on a regular basis? Will you be able to trade passengers for cargo? Or cargo for fuel? Or fuel for a travel-sized guitar?

5-10lbs for clothing and 16lbs for personal items. Anyone who has seen the tremendous amount of carry-on luggage knows that this is way underbudget. For aircraft with fewer than 5 seats, the pilot-in-command can throw the passengers and their bags on a scale if they are concerned about being overweight or out of CG range since that is far less degrading to everyone involved.

For our purposes, we’ll assume 230lbs per passenger. This will represent a 200lb person with 30lbs of baggage. Yes it sounds high but if the actual passengers turn out to be lighter, you’ll end up with more performance, even though they’ll probably have extra baggage which negates the bodyweight advantage in the first place.

Aircraft A: 4 people. 230lbs x 4 = 920 lbs

Aircraft B: 50 people. 230lbs x 50 = 11,500lbs

Aircraft C: 40,000lbs of cargo. (230lbs x 2) + 40,000lbs = 40,460lbs

Note for Aircraft B that it is 50 people total, not 50 passengers or the number would have been 53 to account for flight and cabin crew. Aircraft C has presumably a 2 person flight crew.

So now you know how much weight you want to move. Now, where do you want to move it?

Question 2 asks how far did you want to carry the payload. This can be any range you want. If you are unconcerned with practicality or marketability there is no reason not to build a 2 seat piston single that can fly 4,000nm. There will be tradeoffs but it can be done. You could alternately fly 150 passengers 250nm via a turbofan twin. Your flight path will look like a giant parabola, but again it can be done.

Because you'll never hear "Now boarding Continental flight 179, nonstop service to anyplace that's 2163 miles away."

People don’t get in airplanes to tell others “I flew 1867nm today!”, they get in planes to go places. That 1867nm translates to leaving Dallas and ending up in Bermuda. It can also translate into taking off from Nashville and landing in Venezuela. Play the travel game. Pick your departure point and think of places you’d like to go to, or have to go to. It definitely helps if you end up facing a room full of non-aviation investors to say “Direct from Miami to Las Vegas.” rather than “1900nm plus IFR reserves.” If the room is full of aviation-savvy investors by all means, wow them with your knowledge of how NBAA reserves are calculated.

Airplane A: Dallas to Panama City Beach. 595nm

Airplane B: New York to Los Angeles. 2124nm

Airplane C: Charleston to Toulouse. 3746nm

The 3rd and final question for this post is how much time should it take to be moved the required distance. This is nothing more than division. Take the time you feel comfortable sitting (or think your passengers would tolerate being in one spot) and divide it by the distance to be covered and the result is the required cruise speed. If your range requirement was 4000nm over the course of 15hrs (maritime patrol missions often cover long distances and have attendant high endurance times) then you can set a normal cruise of 267knots. If on the other hand you picked 1700nm and wanted to spend no more than 2 hrs in cruise, you will come up with a cruise speed of 850knots. It’s completely possible to supercruise, but you will likely run into a fair amount of friction both from the aerodynamics and the government regulators who decreed that Thou Shalt Cruise No Faster Than Mach 0.95.

Airplane A: 3hrs to travel 595nm = 198knots

Airplane B: 3hrs 45min to travel 2124nm = 566knots

Airplane C: 8hrs 30 min to travel 3746nm = 440knots.

With only 3 questions you’ve already figured out what you want to carry, where you want to take it and how long it should take you to carry it. This was the easy part (I keep saying that). The next installment will feature our friends The Tradeoffs and The Compromises. They aren’t so bad once you get to know them.