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Pietenpol/Riblett Airfoil Comparison

The Barnwell Project

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The BHP airfoil and the Riblett airfoil  Barnwell S.C. face-off!

 

Way back when, Bernard Pietenpol tried a number of different airfoil wing shapes.  Some didn’t work well at all, others worked better.  Finally, he hit upon one that seemed best for the Ford powered home-built, so he used it on most of his future airplanes.  When pressed to give it a name (Goettingen, Eiffel, Clark, etc.) he dubbed it the FC-10 (for French Curve the 10th try).

 

Over the years, there have been discussions among Piet people as to whether a more modern airfoil could bring even better results.  One of the major contenders was one of the Riblett designs.  Out of all this discussion our friend P.F. Beck of Barnwell South Carolina stepped up to help solve the problem.  Several years ago, P.F. had built a new Piet, using the BHP airfoil.  His successful Corvair-powered Air Camper has flown hundreds of passengers.  When his friend and neighbor, Don Harper, decided to build his own Air Camper, they decided to build an almost exact copy of P.F.’s airplane – except for one item.  Don’s airplane would have a new Riblett airfoil.  Then, the two airplanes would have a fly-off.   Who took off shortest – who had the best rate of climb – who had the fastest cruise speed – and more.

 

Recently, Don’s Piet was completed and certificated by the FAA.  P.F. did the first flights on Don’s airplane, and soon he was comfortable enough that he could start to collect real flight data (at least with a pencil and paper).  The result?  So far, early tests show almost no significant difference in performance, but testing will continue.  As soon as compiled, more complete test results will be published here in the BPA News and on the Matronics chat list.  Knowing both owners, we know that the testing results will be accurate and fair and very understandable.  We salute Don and P.F. for actually doing something to test the armchair theories.

 

The prelimary report is summarized above.  Below are the final(?) results on this project from P.F. Beck and Don Harper.

 

 

 

 

Latest Update on Comparison Test of Pietepol Airfoil ad Riblett Airfoil.

by P.F. Beck and Don Harper as per Matronics list

 

In January, we posted a side-by-side description of two Corvair powered Pietenpols.  Both are approximately the same configuration except that one has the Pietenpol airfoil and the other has the Riblett 612 airfoil.  The planes were flown by the same pilot each time to get both the performance numbers first posted and recent numbers listed below.  If you read the first posting carefully, you will remember that we said that the performance numbers posted were preliminary and updated numbers would be posted later as we gathered more data.

 

Many people commented on what we first posted.  Most were positive because they understood what we were trying to convey, which was...”Here is what we noted in our testing... form your own opinion”.  Some few comments were along the lines of “That’s not what I expected the performance numbers to be, so your umbers absolutely have to be wrong”.  Or, “Why didn’t you”.

 

After making some adjustments on rigging, adding a tab on rudder, replacing the entire pitot/static system, replacing the tailwheel assembly, adding stronger compression springs on tailwheel, adding a bungee trim on the control stick, sealing the gap on elevator/horiz stab and rudder, Don’s plane now handles and flies better.

 

We decided to repeat the takeoff and climb tests on Don’s plane, and then do some side-by-side flights, at different engine rpm settings, to determine indicated airspeed.

 

Takeoff was measured from the same starting point on the runway to liftoff point noted by two or more observers.  Rate of climb was measured from liftoff to 1000 ft indicated on two altimeters using a stop watch and converting to ft per minute.

 

Weather information was obtained from AWOS on the airport.

•  Wind – 334 degrees  @ 3-4 mph

•  Temperature – 61 degrees F

•  Takeoff roll was 386 ft (avg of three takeoffs)

•  Rate of climb to 1000 ft was 486 ft per minute (avg of three different climb outs holding as close to 55 mph as possible.)

 

Next, we elected to start at about 2600 rpm and then work up to max rpm in 100 rpm increments to check airspeed.

 

                                My plane                   Don’s plane

Indicated rpm        * Indicated speed        * Indicated speed

2600 rpm                      68 mph                           68 mph

2700 rpm                      72 mph                           72 mph

2800 rpm                      73 mph                           74 mph

2900 rpm                      77 mph                           78 mph

3000 rpm                      79 mph                            80 mph

3100 rpm                 82-83 mph                            81 mph

* Airspeed readings taken from airspeed indicators installed in planes.

 

Numbers are as accurate as we could read them, and are so close as to be meaningless differences between the two planes/airfoils.  Both planes “feel” best at 2700-2750 rpm and indicating about 72 mph.

 

Summary:

 

Keeping in mind that we had no preconceived opinion as to performance figures, the numbers posted here are actually what we saw.

 

Out of many comments received after first posting, some few “armchair experts” indicated that they expected the Riblett airfoil to fly “rings around the Pietenpol airfoil”.  Sorry to disappoint those who had already made up their minds, but the facts do not support that theory.

 

The bottom line is, as has been said many many times, Bernard Pietenpol knew what he was doing and his airfoil has been proven for more than 84 years.  Which airfoil do we recommend?  That’s the builder’s choice.  Either one will work.

 

We do intend to check indicated airspeed readings against our gps, when we have time and as weather permits, to try and verify accuracy of indicated readings just for our own knowledge.

 

Will we continue experimenting?  Certainly, but we don’t plan to post any more comments right now.  Maybe later if we learn something that may help others.

 

All the above information is freely shared.  If you disagree with any part of the testing we have done so far, then please do your own testing and share it with the Piet community.

 

Hope to see you all at Brodhead.

 

P.F. Beck and Don Harper

Barnwell, SC

 

(Ed. Note:  P.F. and Don will be presenting a forum on the Pietenpol/Riblett airfoils at the 2013 Brodhead Pietenpol Gathering on Saturday, July 27th – see you there.)

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