I’ve had a few queries regarding the Weight & Balance of Christen Eagles recently….

As with some other small biplanes, Eagles are “sensitive” to a Centre of Gravity (CofG) that is rearward of that designed and specified in the manufacturers “Flight Manual” - out of the “flight envelope” in other words. Some of the specification weight info is on the “Eagle Facts” page here. One key weight in the spec is the “Empty Weight 1025 lbs w/electrical, radio, and canopy“. Despite being the figure provided by the original designer and current manufacturer, I have yet to come across an Eagle as light as this - the lightest are usually around the 1070lbs mark and these have usually been built specifically with weight reduction in mind. The upper gross weight flight envelope is 1600lbs.

Anyway, if you own an Eagle, or you’re simply interested in such things, I modified a W&B spreadsheet for use with my Christen Eagle II, G-NEST (aka “Nesty”).

The above is a screen shot of the spreadsheet showing the two flight envelopes - “normal” (utility) and “aerobatic” flight. Click the image to download the Excel spreadsheet so you can play with the figures yourself!

All the figures used in it are taken directly from the official Christen Eagle II “Flight Manual” Page “1-9″, Rev “0 3-20-80″. By entering different pilot weights, fuel loads etc. into the spreadsheet, you’ll quickly see how easily these aircraft can be incorrectly loaded to place the CofG too rearward - the worst case result is to cause difficulty recovering from a spin (intentional or otherwise). Basically, if the small green marker (superimposed on the flight envelope in the image above) moves to the right of the red or blue lines (on the right of the envelope limit boundary), the aircraft balance is too “tail heavy”. Of course if the marker is anywhere outside the two flight envelopes, the aircraft shouldn’t be flown!

There was one Eagle accident caused when two “heavy” pilots were flying aerobatics in their Eagle but didn’t take account of fuel burn during the flight - the weight of fuel decreased, the CofG moved rearward - unfortunately to the point that made it impossible for them to recover from an intentional spin. Of course calculating the W&B of an aircraft should always be an integral part of the flight planning and preparation work - calculating the W&B at take-off AND landing to ensure that the aircraft remains within limits during the whole flight.

Frank Christensen, the original designer of the Christen Eagle II aerobatic biplane, knew the sensitivities of the rearward CofG of similar aircraft designs, which is (apparently) why he designed the battery to be located behind the rear seat (P1). He knew that it would provide an easy way for owners to adjust the balance of the aircraft if required - simply move the battery further forward to relieve any pressure on the rearward limits of the flight envelope! We did this with Nesty to provide a greater margin before the rearward CofG limit was encountered under different loading configurations and also took the opportunity to install a far lighter battery at the same time. We had to gain approval for the modification from the LAA which wasn’t a problem - just loads of paperwork and it did hold us up a bit. You can download the drawing I prepared for the modification application here (2.75MB pdf) or click the thumbnail above to see a larger jpg of it.

I’d be interested to hear from any builders who managed to build a Christen Eagle II to that specification 1025lbs!! Personally, I reckon it would be easier to lose 50lbs off the pilots’ weight than shave a Christen Eagle II to 1025lbs?

All for now.

Paul

When I put the previous post together this morning, I’d tried to find a decent photo of Seamus’ “on-steroids” Christen Eagle II to illustrate it. Torsten Hoff responded to an email I’d sent him and he kindly sent me this terrific shot of N540BE:

Click the above for a higher res version (2016×1916). More of Torsten’s aviation photos can be found here.

You can almost feel the power of that 540 engine (normal Christen Eagle II engine is a 360 cu in) pulling that tiny airframe into the blue! Jealous? Moi?! Gotta go - mop up my dribble!

Paul

Seamus McCaughley, owner of US registered Christen Eagle II “N540BE”, left a comment on the site that really deserves its own post. Seamus has “modified” his Christen Eagle II to “hyperbipe” standards - an awesome end result, which looks like A LOT of fun!

Here’s his comment in full:

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Hello all!

Santa Monica California checking in! I am a Christen Eagle II owner here in the US. I have owned my Eagle for 5 years and simply love it. I flew it and tried to fly the wings off of it literally and figuratively. I have flown all the Pitts airplanes and others and feel the Eagle gives me not only the best “bang for the buck”, but is the most user friendly as well as being a bit safer in the event of and unscheduled ‘step out’. This being said; I needed more!! More power, more roll authority (especially in the vertical), more rudder and an updated paint treatment. Well here is what I came up with… Google my tail number; “N540BE” or see construction photos on flickr and flying shots here (some great shots by Torsten Hoff and Doug Robertson).

I built a Hot Rod IO-540 (345hp). Shortened the fuselage 5in and rebuilt the wing using “Super Stinker” aileron airfoils. The new boards are 10in longer than the ‘C’ ailerons as well as being three hinge and better hardware. I did all the tricks to strengthen the wing structure. The rudder is squared and larger. I did all the work except for the actual spraying of the (Sherwin Williams, Jet Glow) paint and the final welds on the fuselage tubing. Engine, covering (Ceconite) and design…. All of it!! I came up with the blue and white before I found the pic of G-NUTA here on your site.

How does it fly? Unbelievable!! Some fellow builders/flyers said I would have months of sorting after the first flight. Well I hate to tell them I did not!! A poorly earthed oil-pressure sender and some small oil press and fuel adjustments and that is all. The airplane will fly flat out at 220mph pushing 32” MP at 2800rpm. (Below 1000’) I have played with cruise #s and come up with 22 squared and 12.3 GPH to give me 170 MPH (at 8500”) not to bad!! I have 23’5 gal in the (shortened) main tank and can carry 5 more in the top wing. The power is great and having a 6cyl and carbon 3 blade is heavenly smooth. The roll rate and all not only meets but also exceeds all expectations. An Eagle but better!! The only problem is managing the power when terminating the up lines and learning to fly without that 55lb gyro up front. Other then that…. Pure heaven!!

Hope you all enjoy and would love to hear from you.

Seamus McCaughley

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Nice work Seamus - keep us updated and have fun!

Paul