Biplane Center of Gravity Limits on MAC (Mean Aerodynamic Chord)
                 **as applicable to older aircraft for which no specific limits are listed
                                                      by Richard W. Fraser


The double asterisk sentence above relates to a direct quotation from part of a title within a very
important section from the book “AIRCRAFT WEIGHT & BALANCE CONTROL” by
Henri G. D ’estout, 1959 edition, published by AERO PUBLISHERS, INC., LOS ANGELES 26, CALIFORNIA
(no longer in business), Figure “C”, page 116 in the Appendix to be exact.

It is evident that most of the early aircraft that were developed, back then (back then means those of you
reading this were not born yet), did not contain the fore and aft C.G. limits. Mr. D ‘estout had 33 years
aircraft experience, as stated in his book, which means that prior to the 1959 edition, less his 33 years
aircraft experience, would make the date 1926 when he started in the business.

Mr. Mignet finished his first HM 1-1 Monoplane glider in 1912 and his HM-8 in 1928. That puts Henri in the
same “forgetful league” as the rest of the early designer / builders. Forgetful only because some of us
today are now demanding fore and aft C.G. limits where it is a pretty good chance that this was not the
requirement in that part of the century.  Remember the highlighted statement above.

Anyway, I have worked out some geometry for extreme staggered Biplanes, ala tandem wings or Flying
Fleas, whatever you may want to call them, based upon Mr. D ‘estout’s book.
Please refer to Figure 1.

Before you try to figure out Figure 1, you should familiarize yourself with the other pages copied from the
book, mainly the books page 116, Figure “C” for the biplane C.G. Limits.

Now pay attention: In the book the MAC is shown not in the correct location. In fact, it is not the MAC. The
chord shown at a 75% gap position, however is what you use. And, it also works well on highly
staggered tandem wings. Now that’s amazing? Not much guess work here.





























Limits laid out by using the total wing chord “C” length (from the leading edge of the forward wing to the
trailing edge of the aft wing) as shown in Figure 1 work better than when using a no horizontal gap as
shown in Figure 2. Position 22% of C and 33% of C on the chord line at the 75% gap position. There
should be your safe C.G. limits in the correct position for both examples.

Stay with the Figure 1 and Figure 2 examples with horizontal wing positions and the 75%G with total
wing chord measurements as shown and you should have a well balanced airplane in flight

It is also important to remember that even though these limits are listed it is only a good place to start.
Limits always requires testing the aircraft to verify this, even with the same design that was built using
the very same set of plans because there are always building differences between individuals.





























Analytically, the above limits should be safe as they were developed from experience some time ago. An
experienced test pilot should be able to tell you if they are comfortable limits. Unfortunately, very few
home builders have an opportunity to have an experienced test pilot to rely on. That is why there is
something called Production Aircraft. Government agencies demand it.

Figures 3 through 7 follow. They are copies of pages from the quoted book of experience.
As errors are found during our sampling, we will continue to revise this unique C.G. relationship.

Sincerely,

Richard Fraser

































          
                                     Figure 3 - Book Cover






























              
                                    Figure 4 - Book Title Page






























        
      
            


                                    Figure 5 - Book appendix page 116




































                                       Figure 6 - Book appendix page 117
        


































                                    Figure 7 - Book appendix page 118, but extrapolated for
                                    higher “K” values          





That’s all folks!

































                   
C.G. Limits for a Flying Flea Design
Figure 1
Figure 2
Second Revision July 23, 2010
thanks to Roy De Stadler
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