Calculation question: for a wide cockpit is it correct to consider the wingspan only the sum of the wings lenght?

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Protech Racing

Well-Known Member
Joined
Jul 10, 2020
Messages
441
For your situation and planform. You can model CG with scale models very well. The WIG CG will not be the same as at altitude for the same craft . Model it as a glider as accurate as you can and compare the results to the know good values and CG estimator equations. Some use 25-27 % of total chord or more complicated methods . Easy with a straight wing and less so with large taper of course,
 

cblink.007

Well-Known Member
Supporting Member
Joined
Jul 7, 2014
Messages
989
Location
K2W6, Maryland, USA
Billski... Class! Thanks for the detailed explanation!
I would also recommend the book "Simplified Aircraft Design for Homebuilders" by Dan Raymer:


It will not walk you to "the answer" (that is a journey that you must trek on your own) but is a great read that will start you in the right direction, especially with respect to the sequence of conceptualization & design, and talks alot about sizing (he even offers a spreadsheet to help you out!). Obviously there are numerous other more advanced texts on aircraft design out there, but if you are new to the practice, this book is a great start!
 
Last edited:

Norman

Well-Known Member
Joined
Nov 28, 2003
Messages
3,309
Location
Grand Junction, Colorado
Please explain how the CG changes with altitude.

Thanks,


BJC
The aerodynamic center of the wing moves aft in chord dominated ground effect ie less than 1 chord length above the surface. This is the so-called "cushion of air" that WIGs fly on because it increases lift. Obviously this doesn't move the center of gravity but it does change the CG position required to balance the aircraft with neutral trim.
 

BJC

Well-Known Member
Supporting Member
Joined
Oct 7, 2013
Messages
15,882
Location
97FL, Florida, USA
The aerodynamic center of the wing moves aft in chord dominated ground effect ie less than 1 chord length above the surface. Obviously this doesn't move the center of gravity but it does change the CG position required to balance the aircraft with neutral trim.
Yup, but the claim was per my question.


BJC
 

radfordc

Well-Known Member
Joined
Feb 5, 2008
Messages
1,577
Yes! I know 150m is far off WIG effect, what I meant is the craft will be capable of sustained flight way beyond WIG effect, but I'm not willing to fly up to 1.000 feet. The normal "cruise altitude will be around 1 to 2 meters.

I'm not sure it would hurt more to fall from 1000ft than 450ft? As soon as you are at 20ft AGL you're an airplane. Like this....

 

Wespa

Member
Joined
Apr 9, 2022
Messages
8
My question raised after many spreadsheet calculations considering both. I'm drawing a WIG craft with capabilites of flying up to 150 metres high. The cockpit will be built around a fletcher 12ft speedboat hull wich is 1.55 meters wide (beam). I'm calculating the wing load with just the wing lenght solely. For a 8 meters wingspan (without cockpit and 1.8m chord) I'm achieving 50.59 Kg/M² which is my limit for wing loading, but if I include the cockpit in the wingspan this value falls to 42 kg/m², but does nt seems correct to include 1.55m of non lifting surface in the calculations. What I'm trying to do is to make the wingspan as short as possible but keeping the load under 50kg/m² to permit the takeoff from the water. Any consideration in the above will be appreciated.
Would a flat underside fuselage, with a slightly higher tail to clear the fuselage flow and raise the nose a bit do it?
 

Arnaldojrbr

Active Member
Joined
Jun 9, 2022
Messages
37
Location
UK

radfordc


Exactly THIS i'm talking about! If a guy can make a junkyard fly, with all his drag and mess... Why I can't? It's ugly, terrible drag, have a plastic chair as a seat... but it flies! Amazing isn't it?
 

Arnaldojrbr

Active Member
Joined
Jun 9, 2022
Messages
37
Location
UK

Wespa


I'm not willing to build a hull from zero. Time consuming, knowledge about hidrodynamics, weight... several factors. It's lots easier use a speedboat hull which have all the desired characteristics on water. the top fuselage structure will be my only concern.
 

Arnaldojrbr

Active Member
Joined
Jun 9, 2022
Messages
37
Location
UK
It's crude! I know! And I'm still in the learning curve of 3d design... but this is my sketch

Now you can have a laugh!

Still working in the fuselage surfaces, will take lots longer than I expected

1660342319381.png
 
Top