Expert: Paul Soderman - 10/7/2008

Question
QUESTION: Well try this:  I see your discussion on props.  I have an experimental 4 place aircraft with 250 hp 6 foot prop. Damaged and needs rebuilding.   My thought and query:  If a 150 hp engine with a 30 foot blade can produce say 1500 lbs of thrust, or enough to lift a helicopter,  and this same 150 hp engine in an airplane with a 6 foot blade only produces a few hundred lbs of thrust, does it follow that longer blade lengths give more thrust?   The Wright Brothers used two 8 1/2 foot props on one  12 hp engine,  Curtiss used an 8 foot prop on 90 hp on the popular WWI trainer the Jenny.  A popular Chinese trainer visiting airport the other day, has an 8 foot prop on 285hp engine.  By going from a 6 foot prop to 8 foot prop,  I am doubling my disc, and tube of air propulsion.  Obviously I need to extend my landing gear, and reduce RPM's on my belt reduction unit (not a big deal).  Can I pick up additional thrust by going down this road, using the same engine and plane, and how much, certainly not double???  Thanx in advance,  Mo

ANSWER: Mo
Propeller performance depends on many factors such as aircraft mission (low speed vs high speed), aircraft weight, aircraft drag, horsepower available, number of blades, fixed or variable pitch, twist, blade shape, prop speed, and prop diameter.  If diameter was the deciding factor we could say that 8 ft is better than 6, 10 ft is better than 8, 12 ft is better than 10, and so on.  I suggest that a good parameter to start with is efficiency because that relates thrust to power available, which in your case is fixed.

The following website discusses efficiency and shows that a bigger diameter is better at low speed (think helicopter), but becomes less important at high speed:

http://www.jefflewis.net/aviation_theory-theo_prop_eff.html

And this is all theoretical, no evaluation of propeller weight or complexity due to size is mentioned.  That said, you can see that under certain conditions, increased diameter can help, but you need to know where you are on the curve.

Another good website is:

http://www.epi-eng.com/propeller_technology/selecting_a_propeller.htm

Here the author actually defines the recommended prop diameter for a given horsepower available and blade number.  I'm not sure how accurate it is, but it is a good calculation to consider.  I can't be more specific in my answer without knowing something about your aircraft, engine, and propeller.  But it seems there is information out there that can help.  Try google: propeller efficiency.
Paul

---------- FOLLOW-UP ----------

QUESTION: Paul,  Again thanx for info on props.  Brother Larry, (76) He worked around planes as a Marine on carriers, years back and loves airplanes.  Sometimes he keeps his questions simple enough for even me to answer. But often he goes over my head as follows:  Can you  help me?

Question from Larry:

         I have a question about air craft of which I lack any information. I am not going to tell you I am opened minded, I would like your in put. I can not quote dates. For a long time positive dihedral was in vogue and now negative dihedral is rather common. Where is the effective conversion and why? Are there craft which change dihedral in flight? The pigeon is the most common bird that seems to take advantage of this characteristic.  That is what I think I am observing.

         From the past zero dihedral seemed to get the best ton per mile fuel economy, but I think flying that type of craft was less  forgiving or required more skill.

Paul, any help here appreciated,   Mo  

Answer
Mo
Dihedral is a common technique for adding roll stability to an aircraft.  When an aircraft in level flight is rolled due to turbulence or crosswind or pilot action, the aircraft will tend to side slip.  With dihedral, the downward moving wing will see a spanwise flow that will give it lift greater than the opposite wing tending to roll the plane back to level.  This is especially effective on a low wing aircraft.  Roll stability is important to civilian aircraft where you want the plane to fly itself as much as possible.  Military fighter aircraft are designed to be maneuverable; you don't want too much stability because  you have a skilled pilot who can put the plane where he wants it. Or you are depending on a computer to control the aircraft.  So military aircraft often have anhedral, the opposite of dihedral.  Some military aircraft have both such as anhedral at the root and dihedral outboard (F4 Phantom).  This can happen if wind tunnel or flight tests indicate a change in stability is needed and it becomes easier to bend the wing than reset the wing at the root (or if wing tip up is desired for carrier operations).

High wing aircraft don't need dihedral as much because the fuselage will act like a pendulum to control roll. Also, a swept wing has inherent dihedral so anhedral may be put in the wing root.  

So your brother must be looking at a variety of aircraft type, each with a different design need.  It is not that the popularity of dihedral is changing but that aircraft types and missions evolve to satisfy new requirements.

A good source of information on dihedral can be found at:
http://en.wikipedia.org/wiki/Dihedral

NASA was working on a morphing aircraft that would change shape in flight, but that idea is still an idea.  I know of no aircraft that changes dihedral in flight other than spanwise bending due to load.
Paul