SENSENICH PROPELLER MANUFACTURING COMPANY, INC.


Area Code 717 14 Citation Lane
Phone 569-0435 Lititz, PA
Fax 560-3725 17543

June 26, 1997

Univair Aircraft Corporation
ATTN: Mr. Keith M. Stehman
Engineering R&D Manager
2500 Himalaya Rd.
Aurora, CO 80011

SUBJECT: Sensenich Propeller for Franklin 6A4-150/165-B4 Engines on Stinson 108 Series.

REF.: Your fax dated April 3, 1997

Dear Mr. Stehman,

Based on the following information, we foresee no difficulties in adding the Sensenich 74DC propeller series to the Stinson type certificate #A-767 for installation onto the Franklin 6A4-165-B4 engine:

  1. Currently the Sensenich 74DR propeller is listed as an optional propeller for the 108, 108-1, 108-2 with the Franklin 6A4-165-B3, and 108-3. The 74DR was vibration approved for these engine installations, Sensenich Report #55-3, and has shown a trouble free service history on these aircraft/engine combinations. This is important because the 74DR is nearly identical to the 74DC propeller that you would like to be add to the Stinson type certificate.
  2. Franklin Service Letter 72-1 (See attached) states that crankshaft P/N 20769 directly replaces crankshaft P/N 18487. This crankshaft replacement changes the model designation of the 6A4-165-B3 to a 6A4-165-B4. A review of Franklin type certificate E-238-11 (See attached) verified that the 6A4-165-B3 and B4 differ only in the propeller flange bolt patterns; certified horsepower and rpm are identical.
  3. Franklin Service Letter 72-1 also states that the Sensenich model 74DC is an approved propeller for the 6A4-165-B4 engine designation. This approval was based on the approval of the 74DR propeller on the 6A4-165 series of engines, Sensenich Engineering Report #55-3.
  4. Sensenich Type Certificate Data Sheet P886 (See attached), Rev 18 dated February 4, 1997, covers the 74D series of aluminum fixed-pitch propellers.
    1. Page 1 of these sheets shows the 74DR and 74DC propellers are approved for maximum continuous 165 hp @ 2800 RPM.
    2. Note 9, page 2 and 3, lists propeller/engine vibration approvals for use on normal category single-engine tractor aircraft. This note shows that the 74DR is approved for installation onto the Franklin 6A4-165-B3 and the 74DC is approved for installation onto the Franklin 6A4-165-B4. Both propellers have a maximum diameter of 74 inches, a minimum diameter of 72 inches, and a placard stating "Avoid continuous operation between 2150 & 2250 RPM on ground and in flight".
  5. The 74DR was type certified, #P886, on April 27, 1954 and the 74DC was added to the type certificate, #P886, on April 9, 1959. Both propellers are nearly identical in blade shape, chord distribution, thickness distribution, pitch distribution, vibratory characteristics, and performance. The main difference between the two models is the hub drilling. The 74DC has a 6 bolt pattern with 1/2" bolts and the 74DR has a 8 bolt pattern with 3/8" bolts.

In summary, Sensenich model 74DC is vibration approved for the Franklin 6A4-165-B4 series of engine and will not introduce any detrimental resonant frequencies into the airframe. The 74DC will offer the same performance, if not slightly better, performance than the 74DR which is already approved for these Stinson aircraft.

If you should require any further information on the propeller installation or approvals, please do not hesitate to contact me at (717)569-0435.

Best Regards,

Kenneth C. DeGraff
Engineering Manager

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