By Adam Hunt
A COPA member recently asked what is the minimum ground clearance for a
propeller. He wrote “I seem to have read somewhere that a prop should be eight
to 10 inches off of the ground when the aircraft has a full forward C of G
(sitting normally on the ground). We also wonder if there is a minimum ground
clearance if the front gear or oleo is completely collapsed onto its hard stop -
say a loss of fluid or broken bungee cord.”
There is an actual answer to this question, but of course it is going to vary
depending on what category of aircraft you are talking about. Here is the answer
for each category of airplanes: Basic Ultralight - This one is easy - there are
no design standards for basic ultralights at all – that is why they are not
allowed to carry passengers.
Advanced Ultralight (The reference is Design Standards for Advanced Ultralight
Aeroplanes) – There is no specified standard for prop clearance.
Certified aircraft - (The reference is CAR 523.925):
523.925 Propeller Clearance: Unless smaller clearances are substantiated,
propeller clearances, with the aeroplane at the most adverse combination of
weight and centre of gravity, and with the propeller in the most adverse pitch
position, may not be less than the following:
(a)Ground clearance. There must be a clearance of at least seven inches (for
each aeroplane with nose wheel landing gear) or nine inches (for each aeroplane
with tail wheel landing gear) between each propeller and the ground with the
landing gear statically deflected and in the level, normal takeoff, or taxiing
attitude, whichever is most critical. In addition, for each aeroplane with
conventional landing gear struts using fluid or mechanical means for absorbing
landing shocks, there must be positive clearance between the propeller and the
ground in the level takeoff attitude with the critical tire completely deflated
and the corresponding landing gear strut bottom.
Positive clearance for aeroplanes using leaf spring struts is shown with a
deflection corresponding to 1.5g.
(b) Aft-mounted propellers. In addition to the clearances specified in paragraph
(a) of this section, an aeroplane with an aft mounted propeller must be designed
such that the propeller will not contact the runway surface when the aeroplane
is in the maximum pitch attitude attainable during normal takeoffs and landings.
(c) Water clearance. There must be a clearance of at least 18 inches between
each propeller and the water, unless compliance with 523.239 can be shown with a
lesser clearance.
(d) Structural clearance. There must be: (1) At least one inch radial clearance
between the blade tips and the aeroplane structure, plus any additional radial
clearance necessary to prevent harmful vibration;
(2) At least one-half inch longitudinal clearance between the propeller blades
or cuffs and stationary parts of the aeroplane; and
(3) Positive clearance between other rotating parts of the propeller or spinner
and stationary parts of the aeroplane.
Amateur-built Aeroplanes (The reference is CAR 507 Appendix C)
(Note: CAR 549 is not in use anymore and has been replaced under an exemption to
the CARs by CAR 507 Appendix C.) CAR 507 Appendix C does not specify a ground
clearance for props but only requires that:
3) Prior to issuance of a flight authority, an amateur-built aircraft shall be
inspected in accordance with a schedule acceptable to the Minister: a) for
workmanship and general serviceability;
b) to detect apparent and obvious unsafe features; and
c) to provide reasonable confidence of safe operation.
I would suspect if an amateur-built aircraft voluntarily met the standard for
prop clearance specified in CAR 523.925 for certified aircraft that it would be
hard for the inspector to reject the aircraft at final inspection time.