Franklin engine and prop

[Woody]

I'm not sure I understand that the Franklin engine can only use certain props, if that is true.   I have a biplane that I will use a Franklin 90 on, so ~~~ will I have to stay in the limits of say 70x54 or can I use any prop that has the same mounting configuring on it and does it have to be wood?   Thanks

[Brett Lovett]

Woody,

I assume you are talking about a type certificated biplane.

You would need to first refer to the Type Certificate Data Sheet or Aircraft Specification for the aircraft you intend to install the engine and propeller on.  If the engine is approved for installation there, the approved propellers and limits should also be listed there.  If the engine is installed under STC or Field Approval, the propeller installation should also be specified and approved there.

Type Certificate Data Sheets and Aircraft Specifications as appropriate are available here:
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/Frameset?OpenPage

Brett

[Woody]

  I did not make myself clear.  I have a Fisher Celebrity biplane.  I also have a O time 90 hp Franklin engine that I'm going to put on it.  I was hoping that I did not have to use the same prop that the Culver uses.  I'm just now addressing this so I thought I'd ask someone that knows more than I do about it.   Thanks

[Brett Lovett]

No, you certainly wouldn't be limited to using the propellers approved for the Culver LFA, and I don't think you'd be limited to using certified propellers either.  The diameter and pitch (static RPM) requirements wouldn't apply either.  There would probably be a maximum diameter appropriate to the Fisher Celebrity. 

With a metal or composite prop, you should probably check with the propeller manufacturer to make sure the specific prop won't have any undesirable harmonic resonance characteristics in combination with the 90 hp Franklin.

[Woody]

Thank you Brett.  This is info I wanted.  It appears that I can really explore options with this.

[Paul Rule]

Woody,

I don't think you can (by looking through the TDS) find a (certified) Franklin 90 turning a metal prop.  Although the Franklin book does not specifically prohibit metal there have been (old) reports of crank failure with metal props.  The Franklin crankshaft, I have been told, has a larger inside bore then small Continentals.  I have not personally confirmed this. 

Wood absorbes power pulses better then metal.

My recomendation... use a wood prop and check that it gives aprox. same static RPM as a certified installation of similar performance.  Then in-flight it should not over speed the engine unless you are near max level flight speed or in a shallow dive.

[Woody]

Thanks Paul.  I'm looking for wood and shy away from the metal for this project.  I appreciate the help!