Sizing to take-off distance requirements

For FAR 23 certified, propeller driven aeroplanes it can be observed that the distance (S_to) required to clear a 50’ obstacle is 1.66 greater than the ground run. The ground run (S_toG) is a function of wing loading, power loading, & take off co-efficient of lift.

Utilising 2 * 85 Hp Jabiru 2200 engines, then as per part 1, chapter 3, page 95 as;

S_toG = (W/S)_to(W/P)_to/σCL_to    

where:  CL_to = CLmax/1.21 

Sea Level				5000 ft
StoG		Sto		StoG		Sto
ft	m	ft	m	ft	m	ft	m
167	51	277	84	194	59	321	98

Sizing to stall speed requirements

FAR 23 certified multiengine aeroplanes with W_to less than 6000 Lbs must have a stall speed of no more than 61 knots. The power off stall speed maybe determined from part 1, chapter 3, page 90 as:

Vs = {2(W/S)/ρC_Lmax}^½   (answer is in feet / second)

Given the following details from the single engine design are already known; Wing profile NACA 747A415, a highly laminar flow wing with low drag profile but generates poor lift at slow speeds so C_Lmax (clean)   = 1.4 and C_Lmax (flap)   = 1.8 With a Mean Aerodynamic Chord (MAC)    = 1300 mm and a proposed increase in span to 9 meters then;

(S) surface area = 120 sq.ft

(W/S) = 16.5 Lbs/sq.ft

	Sea Level	5000 ft
Stall Clean  (Vs)	59.0 kts	63.6 kts
Stall Flap     (Vso)	52.1 kts	56.1 kts

The single engine aeroplane starting point

I found my single engine staring point!

It is a homebuilt, available as plans only, no kits.  This is good for me as I will need lots of the small details that a set of plans can provide.  I have chosen the Cherry BX-2  Here is the link http://www.bx-2.de/e/html/cherry_bx-2.html

Why this aeroplane?  Well....

1. Two seats, side by side 
2. Low wing design
3. Designed low horsepower requirement (only 65 HP)
4. Tricycle retractable undercarriage
5. Detachable wings, for road transport
6. Full span flaps with drupe ailerons
7. Composite construction

In short many of the features I want in the twin have already been worked out by the designer and given the numbers of these aircraft that are flying world wide, it is safe to say that these features have proven themselves reliable.  If I do not have to re-invent the wheel then all the better, as there is less scope for me to muck things up.  

Preliminary Sizing

Using the information on the engine, for 75% power cruise fuel burn. The previously discussed payload weight and that weight range as a starting guess, we can then use the fuel fraction method as outlined in the Roskam text.  Plus using Breguet's range equation, the total weight of fuel can be estimated to be 271.2 Lbs.  This includes 45 mins reserve.  

 From this we can estimate empty weight.  As the text explains a liner relationship between log of the empty weight and the log of the take-off weight, using the regression line constants for multi engine aeroplanes of composite construction, we can find an allowable value for empty weight.  Comparison of these two methods, showed they matched within 0.04%.  Thus with some confidence I can say the following;

W take-off    =    1984 Lbs   900 kgs

W empty      =    1152 Lbs   522 kgs

W fuel          =     272 Lbs    123 kgs 

Why choose an aircraft of composite construction?  Well....  I think I have found my base model single engine aeroplane for conversion into a twin.  More about that later in the next post.

Payload

Research suggests that standard passenger weights are about 80kg per person, so lets be generous and increase that somewhat.

Pilot          =  100 kg
Passenger =  100 kg
Baggage    =  50 kg

Given most airlines around the world only allow 20 kg per passenger as a checked in baggage allowance, 50 kg for two people is also generous.

If I design the VLT to carry these weights, then the limiting factor may well become the physical size of the compartment.  This would become preferable as it would simplify things, assuming weight and balance is OK, then if you can fit it in, then you can go.

Engine choice

Looking towards using two(2) x Jabiru 2200 engines. They are however rated at 80 HP

Check out their web site https://jabiru.net.au/engines/

Why this one? oh maybe....size, weight, power, fuel consumption.  As an added bonus, I have flown behind one before, they are relatively common and I kinda would like to give Australian made a go.

So until my VLT design evolves to a point where a larger engine is required, I will set about using the specifications for this engine in the design process.  Now we can go forward from this starting point, in terms of  fuel consumption, horse power, weight and physical dimensions.

How Light is a VLT?

Running with the idea of converting a light single to a light twin.  Its almost self explanatory that the lighter the starting point the lighter the finished conversion is going to be and consequently the smaller the engines (and smaller running costs) I can use.

In the growing sport aviation and recreational markets there are a few striking efficient and surprisingly advanced designs out there.  If you go back a generation (or two) in performance, I have discovered a whole class of two seater, typically older designs that where engineered to be flown behind a 65hp continental engine.  Many of these where light enough to be classed as ultralights "back in the day".

I am thinking I need to find a low wing design (easier to convert into a twin), based upon an airframe that could be pulled through the sky by a 65 hp motor.  These aircraft often have max weights in the order 450 - 550 kg.  Allowing for the weight of an extra engine, more fuel, beefed up structure and larger control surfaces; then perhaps two 85 hp engines on an airframe with a max weight in the order of 800 - 900 kg would be possible.

How about some instructions?

The problem now is of course, the not so little problem of exactly how does one go about it? There is the aerodynamics and structural considerations.  Searching the Internet, some good reference texts and examples turn up.  I especially like the free links to resources on certain uni websites.

Many years ago when I taught flying, one of my then students was at Sydney Uni studying aeronautical engineering.  They were using a series of books collectively called "Airplane Design" by Jan Roskam.  Apparently it was common for the students, to refer to these texts as "the cook books".  It would seem that the formulas in the text were based upon the empirical data collected from groups and classes of certified aeroplanes, developing statistical norms for these groups and such the development of formulas that closely predict performance based on those norms.

My goal for the VLT, is to have stable, standardised flight characteristics, in line with every other light twin I have flown.  I do not want to end up with a design configuration that requires any level of extra special pilot ability to control.  Using the Roskam texts should encourage a conservative design, consistent with my goals.

An idea to start with

How and where to begin? That is the question, I have more ideas than actual plans at this stage.

The leading idea I have kicking around, to take a leaf out the big manufacturers handbook.  Look at Piper or Beechcraft where they used their larger single engine aeroplane models as the baseline for conversion into their small twin engine aeroplane models.  Some examples are the Piper Arrow became the Seminole, Comanche became Twin Comanche, Cherokee Six became the Seneca. The Beechcraft Bonanza  evolved into both the Travel Air and Twin Bonanza, the Musketeer became the Duchess. Interestingly Cessna who favours high wing designs in their singles, did not evolve any high wing singles into twins, Cessna appears to have had to design their light twins from scratch.

Clearly I need to find a light single engine, low wing design to convert into my small twin engine design. I need to find a design that has many of the features that I want in my VLT, but already in a single.  If the base single design already has most of the features worked out. Then there is less work for me in the designing process. Why re invent the wheel? The less I have to do the less I can muck up!

Now the search is on for my starting point, using those requirements from the mission statement as a guide, what singles out there fit the bill?