BD-5A/B factory engines

I have a long technical fascination with the BD-5 homebuilt aircraft. The engines were one of the main reasons for its demise, but I found the history of the factory engines confusing and vey difficult to understand. The four profile drawings that I had seen, but did not understand, triggered an in-depth study. Slowly, the history surfaced, that it included some small surprises too.

1/72 BD-5J under construction 1/72 BD-5J N3038V finished BD-5J census BD-5A/B factory engines

Engines used in the Bede prototypes

Five different engines were used in the four prototypes that were flown during the BD-5A/B development:

Engine data

The following engines were either flown or projected during the BD-5A/B development:

Manufacturer Model Capacity Power Cilinders Type Ignition Carburetor(s) Weight Comments
Polaris 32 hp (Bede)
36 hp (Bede)
2 2 stroke two a 1970 Polaris brochure lists 292, 335, 398,
436, 488, 744 and 792 cc engines
Kiekhaefer Aeromarine 440Aero 433 cc 40 hp (Bede folder)
40 hp (Kiekhaefer folder)
2 2 stroke single one 72 lbs (Bede folder)
72 lbs (Kiekhaefer folder)
525Aero 523 cc 55 hp (Bede folder)
50 hp (Kiekhaefer folder)
2 2 stroke single one 74 lbs (Bede folder)
72 lbs (Kiekhaefer folder)
575Aero 573 cc 70 hp (Bede folder)
60 hp (Kiekhaefer folder)
2 2 stroke single one 76 lbs (Bede folder)
72 lbs (Kiekhaefer folder)
Hirth ? 440 cc 40 hp (Bede folder) 2 2 stroke double ?
F20 650 cc 55 hp (Bede folder)
52.5 hp (Jane’s 74-75)
45 hp (Bede POH)
2 2 stroke double two 87 lbs dry (Jane’s 74-75) full model designation could be F20BA1. F20 designation
was never mentioned in period articles, only 650cc.
However, Jane's 1974-1975 does use the F20 designation.
F21 720 cc 70 hp (Bede folder)
60 hp (Jane’s 74-75)
2 2 stroke double Jane’s 74-75 says 718 cc. Electric starter. F21 designation
was never mentioned in period articles, only 720cc.
However, Jane's 1974-1975 does use the F21 designation.
Xenoah G72C-C
726 cc 65 hp
60 hp
70 hp
3 2 stroke double three 157 lbs


Engine photos

Shown below is my best understanding of the engines, but nothing is set in stone! Note that the engines with and without cooling shrouds look vastly different. An effort is underway to credit all the photo sources.

caption suggests
Kiekhaefer Aeromarine
flat caption says Hirth
Oshkosh 8 August 1972
Hirth 440 cc none found so far
Hirth F20
on its side

photo upside down?
Hirth F20
Hirth F21 none found so far
Xenoah G72C-C with accessories:
later models?:

Engines in profile drawings

1. Polaris most likely. Butterfly tail. Exhaust is not correct, it came out of the side on N500BD. Note fuel tank behind pilot.

Source: Ebay. Can also be found in the October 1971 issue of 'Science and Mechanics'.
2. Most likely the initial Hirth installation (on its side), on its inlet side. The cooling air exits from the bottom. Exhaust routing unclear - are the two 'wing profile' shapes the exhaust ports in the side of the fuselage? N502BD had one such exhaust port.

Source: Ebay
3. Hirth most likely, upright installation. Two exhaust ports, correct cooling air exit.

Source: Ebay
4. TRS-18. With bell-mouth inlet, best suited to a plenum chamber-type air inlet.  

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