Design change ideas to the Cabin Eagle

[pramer]

I purchased the Cabin Eagle plans sometime back I got #19  I think.  I started tinkering with a few changes to the plans due to the high drag area behind the cabin.  I am posting some pics tell me what you think.  Proposed airoil is Riblett 30-418 which is high lift super forgiving airfoil with low center of lift movement compared to the current airfoil.  Same airfoil used on the bearhawk patrol but they used a 13.5% thick and this is 18% thick.  I used a thicker wing to get the spar depth used on the Cabin Eagle. Pictures on google photos  I have no interest to hijack the design of the Cabin Eagle but I have so many changes.  I havent built it yet.

[Kiwi55]

I have made several changes in my CE build. You can see in the attached pic the changes I made in the area behind the cabin.
...Paul

[pramer]

Looks like that streamlined area behind the cabin will help for sure.  My ideas were to have the flat area of the rear fuse on the top so that it generates lift instead of drag at high angles of attack.  At cruise the rear flat fuse top is not adding to drag as its straight with the direction of travel.  Trim drag is very big with the original design hence the modern RIBLETT airfoil.  Here are my Plans that I made CABIN EAGLE MODS

[dapug]

Looks like that streamlined area behind the cabin will help for sure.  My ideas were to have the flat area of the rear fuse on the top so that it generates lift instead of drag at high angles of attack.  At cruise the rear flat fuse top is not adding to drag as its straight with the direction of travel.  Trim drag is very big with the original design hence the modern RIBLETT airfoil.  Here are my Plans that I made CABIN EAGLE MODS

Nice drawings.  If I got in there tweaking, I'd be tempted to just take the geometry out completely, make it look more like the Belite UltraCub (sleek, IMO).  But then I'd have to evaluate what that does to weight.

Have you settled on a final design?

[scottiniowa]

 Trim drag is very big with the original design hence the modern RIBLETT airfoil.   

I have not heard this type of statement before in aircraft design  "trim drag"  to what is this referring?  And how does a different airfoil effect  "trim drag"?

[Dan_]

Different airfoils have corresponding differences in pitching moment.  If you have to hold pressure on the stick or change the angle on your horizontal stab you create drag.   Some reading on it...

[pramer]

I did have it finalized yes however I bought a Ultravia Pelican Club GS project and my plan is to rebuild that first so it will be some time till I get back to actually building my Cabin Eagle.  I love the Eagle aircraft because of the simplicity so I have thought hard about changes.  The Riblett airfoil has a low pitching moment and is tolerant to having the CG moved back much further so I moved the wing forward in my drawings.  Plan to start CG at 35% and move back with testing should be able to get to >40%.  Another as drawn change along with the  wing moving forward is larger tail surfaces with symetric airfoil about 6% thick.  The reason for this is the wing stall or mush happens at over 20 deg angle of attack with current wing loading this allows for some seriously slow flight but I felt the tail surfaces too small based on some of the advise Harry Riblett has in his book.  According to the numbers my spread sheet puts out cruise of close to 100 mph is possible.  Another change is the angle of incidence since the airfoil at cruise of 95 mph at zero incidence generates 1954 lbs of lift.  The center of lift at cruise is between 65% and 95% of chord.  So down trim is still needed at cruise.  If angle of incidence was at +2 deg like the current cabin eagle it would have 3359 lbs of lift @95 mph and thats more than a handful of down trim causing huge drag.  On the other end of the spectrum 3 point landings should be possible at 8 AOA at 35 mph at gross weight.  The change in airfoil makes for a deeper front spar since my design goal was to keep the rear spar the same depth. full power on stall/mush at about 21 degrees AOA @28 mph.  Disclaimer.. I am not a rocket scientist all the formulas used are from Harry Riblett's book.  More than happy to help anyone with design but the plans I make are free and meant to supplement the Cabin Eagle plans by Leonard.  I am also working on changes to make a tricycle version.  Think a little more stol capable of high AOA takeoffs with flaps.

[pramer]

Cabin Eagle Tri Gear Mods here is my idea for making a tri gear model.  Plans PDF Plans Cabin Eagle Tri Gear  Let me know what you think. As always free and made as a supplement to Cabin Eagle Plans by Leonard.

[cluttonfred]

Pramer, I don’t see any landing gear mods in your drawing.  Did I miss something?

[pramer]

Updated Plan w/ Gear Sorry the other drawing didn't show gear not sure if I would do it like Zenith does it on the CH701 or stay with the original tube style which I think would work if angled to the rear instead of forward as in the tail dragger version.  Definitely heavier to use the 701 style but way more rugged.

[cluttonfred]

Updated Plan w/ Gear Sorry the other drawing didn't show gear not sure if I would do it like Zenith does it on the CH701 or stay with the original tube style which I think would work if angled to the rear instead of forward as in the tail dragger version.  Definitely heavier to use the 701 style but way more rugged.

Thanks, pramer, that's not at all how I imagined it.  In my mind, two of the key aspects of the CE (or any of Leonard's designs) are simplicity and light weight.  I expected something along the lines of the stock gear flipped back to front (figuratively) and a simple free-swiveling nosewheel something like the Just Highlander/Escapade below.  Have you run any numbers on how much weight your modifications are likely to add to the airframe?  Cheers, Matthew

[pramer]

I think you are right to use the original gear flipped around is best.  Based on added tubing and fabric area and thicker wing I think 20-30 lbs on the tail dragger and more like 50 for the Tri Gear version.  I will put a 100hp suzuki G13BB with a gear box on it.  That engine installs around 200lbs with the light weight pulleys and alternator.  I am buying engine stuff from AIRTRIKES.net.  Installing this engine upright will reduce visibility over the nose somewhat but lets face it guys flew the aircamper with a FORD model A engine and the were looking through the radiator.  I want to fly on floats and skies.  Floats need that extra power.  I appreciate simplicity and light weight any suggestions to help maintain these are greatly appreciated.

[Dan_]

I think you are right to use the original gear flipped around is best.  

 I appreciate simplicity and light weight any suggestions to help maintain these are greatly appreciated.

This has been done with a heavily modified DE...  You may want to contact Bill Utt.  Click his user name in the thread below.

https://www.eaglersnest.com/forum/index.php?topic=510.msg2679#msg2679

[cluttonfred]

pramer, I admire your enthusiasm but do you know the loads and maximum speeds for which the CE is calculated?  If the wing is designed for +4g and you increase gross weight by 10%, then you are down to +3.6g for the same load on the wing.  Increased power means increased speed means increased drag force on the wing, and since force increases with the square of velocity, a 10% higher maximum speed means 21% increase in the drag on the wing.  If it were me, I'd do everything possible *not* to add weight including going with something like a G10 rather than a G13 to keep the weight down.  If you are looking for a high-powered bush plane on floats, the CE may not be best place to start.

[pramer]

Certainly I agree that this isn't exactly the airplane for me hence my ideas for changes.  I don't believe there is a perfect design out there just trying to build one that is closer to what I want.  I am fairly certain that the Cabin Eagle evolved from design changes to other existing designs.  I am not an engineer and haven't built the design yet so I don't know about the G loading the final design will have.  The Riblett airfoil made it necessary to increase the thickness of the wing to keep the rear spar dimension this increases the main spar height to 8 7/16" .  If I remember right that's an inch deeper spar.  Should be stronger but how much I don't know.  

Building and testing a wing panel with sand bags would be a good idea I'm sure.
I have a SPREADSHEET that I snapshot showing airfoil drag based on AOA and speed.  Considering that the drag acts over the entire length of the wing I cant see that being to much drag for the drag bracing in the Cabin Eagle plans. 

 I have a local RAA (Canadian version of the EAA) chapter here in Winnipeg that I will heavily involve before and during my build.  I have a large number of plans for other aircraft and continue to collect and study them as I believe that much can be learned from using ideas others have used.