Step 3. Select the type of configuration to be designed

Given it is nearly always desirable to place the fuel, payload and empty weight C of G at the same longitudinal location. Doing this limits the C of G travel, resulting in a configuration with less wetted area, due to less need for trim control.


Using the definitions of outlined in Chapter 3.3 “Configuration Possibilities” on page 95 of part II, the following description applies given the mission statement. This aeroplane is land based, conventional, twin tractor engines mounted in wing nacelles, a cantilever low wing, zero sweep, full span flaps with droop ailerons and a tricycle retractable undercarriage.

Given this is a development of a known design; many of the choices are already made in terms of configuration layout. The fewer changes from the original design configuration, the better!

Step 2. Perform a comparative study of simular aeroplanes

I have found other examples accross the internet of light multi engine aircraft.  Unfortunatly no have been anywhere near my megar budget.


1. Company: Zenith Aircraft. (USA)

Model: Gemini CH620

Type: Kit development

Website: www.zenithair.com/gemini/gem-what.htm

Note: 2 Jabiru engines, fixed pitch and fixed gear, development is on hold.

2. Company: Tecnam. (Italy)

Model: P2006T

Type: Certified aircraft

Website: www.tecnamaircraft.com/Tecnam_P2006T.htm

Note: 2 Rotax engines, 4 seater, retractable gear. As at 2008 cost EUR $280,000 for a basic model

3. Company: Aeroprakt (Russian)

Model: A28 and A36

Type: Complete aircraft.

Website: www.aeroprakt.kiev.ua/eng_html/main.html

Note: A36 built on request and A28 still under development.  I did find the A36 listed on a US disributors web site for $175,000 USD

If anybody finds any more, please leave me a comment.

Step 1. Review Mission Statement

This step does seem straight forward.  No changes yet and work done thus far is inline with the original concept. 

Preliminary Design Sequence I

According to Part II, there are 16 steps in this sequence of design. They are referred to as class one methods. For a summary of each of these steps can be found on page 11 of chapter 2. Note these sizing methods only have an accuracy of ± 10%.   So step by step I will try and follow them and post the results here. Some of these steps look complicated, oh well...

Sizing to cruise speed requirements

From Chapter 1, part 3, page 162 thru 165 it can be seen that formula for cruise speed is;

V = 77.3{h_p(W/S)/σC_D(W/P)}^1/3                     note: V is in mph

or

V = (550SHPh_p / 0.5ρSC_D) ^1/3

	Sea Level		5000 ft
	mph	kts	mph	kts
Vcr @ 50% power =	123	107	130	113
Vcr @ 75% power =	141	123	148	129
Vcr @ 100% power =	155	135	163	142

Note: with 5 hours of usable fuel on board, at 75% horse power then range is 600 Nm at sea level.

Sizing to climb gradient requirements

From Chapter 1, part 3, page 132 that it is possible to combine equations (3.29) and (3.30) to yield the following formula for climb gradient;

CGR = 18.97h_pσ^1/2/ (W/P) (W/S)^1/2C_L^1/2-(L/D)^-1

where: L/D = C_Lclimb / C_Dclimb

            C_Lclimb = C_Lmax – 0.2

            C_Dclimb = C_D0 + C_Lclimb² / πAe

		Climb Gradient  1 as to:
	FAR 23	Sea Level	5000 ft
Take off climb AEO (All Engines Operating)	1 : 12	3.6	4.0
Balked landing climb AEO	1 : 30	4.8	5.6
Take off climb OEI (One Engine Inoperative)	~	3.6	4.0
Balked landing climb OEI	~	4.9	5.7

Sizing to rate of climb requirements

From Chapter 1, part 3, page 129 through 131, it can also be seen that rate of climb (RC) can be determined by;

RC = dh/_dt = 33,000 x RCP   (ft/min)

Where: RCP     = Rate of Climb Parameter

= [h_p / (W/P) – {(W/S)^1/2/ 19(C_L^3/2/C_D)σ ^1/2}]

To maximise RC, it is evidently necessary to make C_L^3/2/C_D as large as possible.

(C_L^3/2/C_D)_max = 1.345(Ae)^3/4/C_D0^1/4

For normally aspirated engines, assumes a reduction in sea level horse power of a factor of 0.85. (i.e. 85hp/engine * 0.85 = 72hp/engine at 5000 ft)

From Chapter 1, part 3, page 129 through 152, it can be seen the formula for best rate of climb speed is;

Vy = [2(W/S)/ρ(C_D0πAe)^1/2}]^1/2

Best rate of climb and speeds	FAR 23	Sea Level		5000 ft
	ft/min	ft/min	kts	ft/min	kts
Take off climb AEO	300	1710	82	1315	88
Balked landing climb AEO	~	1461	58	1049	63
Take off climb OEI	steady	527	79	304	85
Balked landing climb OEI	~	293	57	52	62

What is the FAR 23 Climb Requirements?

From Chapter 1, part 3, page 129 through 131, it can be seen that a twin engine aeroplane with a take off weight less than 6000 Lbs and a stall aped of less than 61 kts is required to meet the following requirements, for FAR 23 certification.

Take off climb AEO (All Engines Operating)

• Minimum climb rate of 300 ft/min at sea level

• A steady climb angle of at least 1:12

Configuration: gear up, take off flap, max cont. power all engines.

Balked landing climb AEO

• A steady climb angle of at least 1:30

Configuration: gear gown, landing flap, take-off power all engines.

Take off climb OEI (One Engine Inoperative)

For multi engine (reciprocating type) aeroplanes with a MTOW < 6,000 Lbs and with a V_so < 61 kts, the requirement is only that a steady climb rate at 5000 ft must be determined. 

Configuration: gear up, take off flap, max cont. power all engines.

Balked landing climb OEI

Note a positive climb performance is not required!

Configuration: gear gown, landing flap, take-off power all engines.

Sizing to landing distance requirements

FAR 23 certification requires an approach be made 1.3 times the stall, over a 50’ obstacle. The landing ground run (S_LG) is related to the square if the stall speed, in the landing configuration, as is the total distance required (S_L) As per Part 1, Chapter 3 page 108 as;

Ground roll (S_LG) = 0.265Vso²

Total distance over 50’ obstacle (S_L) = 0.5136Vso²

	Sea Level				5000 ft
	SL		SLG		SL		SLG
	ft	m	ft	m	ft	m	ft	m
Full flap	1394	425	719	219	1617	493	834	254
Flapless	1788	545	922	281	2078	633	1072	327