New Project- thoughts?

[ben_kapitan]

For about a year now I've been wanting to get into R/C aircraft, specifically their potential for uses other than just 'flying around'. Imagine what I could do if you didn't have to focus on not crashing? Tonight I finally took the time to get my ideas together, and the attached images are the result. To aid in your feedback I'll briefly summarize what I would like to accomplish:

-Build a platform that can stay airborne for long periods of time.
-All weather, day/night capabilities.
-Come home on loss of ground signal.
-Do the above without spending millions of dollars.

As a newcomer to the field, Ive got some questions for the older, more experienced users:

1. Those of you who design your own airframe - do you have a specific piece of software that you use to compute aerodynamics, or is it more of a 'trial and error' / dead reckoning process? I guess what I'm looking for is software that I can input wing dimensions, C.O.G, aircraft weight, etc and get an estimate of how it will handle in the air.

2. What materials do you prefer, as far as hull and flight control surface construction?

3. When starting a new project, what helps 'map' it all out, as far as where to begin, what to do first, etc? I have a lot I want to do - but I'm finding I'm having a hard time organizing and prioritizing it all.

4. When it comes to flight controls and flap scheduling, have you had the need to implement an air data system (pitot static, angle of sideslip, etc)? What about leading edge flaps?

5. Feedback on my drawings! I know the flaperon/aileron sizes and landing gear placement need adjusting - but other than that - ideas?

Thanks!

[Felthamg]

Hey Ben!

With respect to your first question I believe I have a few suggestions. Download a copy of Digital Datcom+ (an improved version of the original Digital Datcom that simplifies input/output). Digital Datcom is a compendium of data put together by the USAF and allows for the rough prediction of an aircrafts behavior. The program requires a geometrical representation of your aircraft as input and the flight regime, it then computes the aerodynamic derivatives (the behavior of your aircraft from a steady state disturbance).


Once you have the aerodynamic derivatives you can solve the Linearized Equations of Motion and the results will tell you if your aircraft is 1) stable or unstable both laterally and longitudinally 2)the specifics of the dynamic modes , the lateral equations have 3 dynamic modes roll,spiral,and dutch roll. The longitudinal equations have 2 modes, phugoid and short-period (for more information consult a flight dynamics textbook or wikipedia [dynamic modes for an aircraft]).

You can do the analysis by hand (somewhat difficult :) ) or if you have available a copy of MATLAB then you can easily do the analysis. I have a matlab script written that will compute the dynamics of an aircraft given the information from the Digital Datcom output. If you like, I can send you a copy for you to run , or if you dont have matlab or dont know how to use it you could send me the output of digital datcom and I would gladly do the analysis for you and send you the results.

If you get really ambitious you could download a copy of AEROSIM (a blockset for Matlab) that would allow you to make an entire simulation of your aircraft. You would need the information from Digital Datcom + information on your engine. From there you could simulate the flight of your aircraft and even visualize it using FlightGear.

The only problem I could forsee for you is your particular aircraft goemetry. Digital Datcom is designed to handle conventional aircraft geometry, so you might have to simplify your representation a bit, but give it a shot.

[Felthamg]

With repect to your "leading edge flaps" question.

Firstly I believe they are called leading edge slats, the flaps are usually located on the trailing edge.

Leading edge slats are used in aircraft to allow for high generation of lift without stall. They do this by redirecting the flow of air over the airfoil at large angles of attack. Usually an aircrafts wings would stall at high angles of attack but the leading edge slats prevent stall. At higher angles of attack more lift is generated by the wings. For example, the Beoing 747 uses this system as well as some STOL airplanes (short take-off and landing). For the STOL aircraft the leading edge slats allow the plane to take off very quickly at very high angles of attack.

However, leading edge slats do have their downfalls. A wing using slats generates more drag. I believe the Boeing 747 have retractable slats to reduce the drag during cruise. The system for retraction is complicated and I would not recommend it.

Unless you really want an aircraft that can take off quickly (shorter runway) then I would not recommend using leading edge slats.

[Felthamg]

with respect to your "flaperon" design. why?

Flaps are used in a similar manner to slats. During takeoff they can be deflected downward to alter the camber of your airfoil and lead to greater lift generation. They can also be deflected during landing to generate more drag.

I believe Flaps can be used in a control system (stabilizing lateral dynamics, say for an autopilot) but I havent seen that implemented.

I would recommend wing tip ailerons, that is ailerons located near the wingtips. The further your ailerons from the body of your aircraft the longer the moment arm and the more effective they become.

If you just want a control surface to allow for roll/yaw control then I would recommend wing tip ailerons and just forget the flaps. Flaperons are usually implemented in full-body wing designs and usually they are a combination of elevator+aileron (elevons I think).

Again, if you require short take off and landing then consider flaps but wing-tip ailerons , rudder, and elevator should be sufficient as control surfaces.

[Felthamg]

one last thing (I promise),

why did you include wingsweep? (your wings are "bent" at an angle backward). Wingsweep is usually incorperated in aircraft that fly at high Mach numbers (near the speed of sound) usually preventing shockwaves from forming on the wing and severly increasing drag. Wing sweep also changes the stall angle of attack and the lateral and longitudinal dynamics of an aircraft but thats more information than we need.

I doubt that you are planning on flying your aircraft anywhere near the speed of sound so I would suggest a straight-wing design instead. A straight wing will generate more lift at the same angle of attack and a swept wing and technically less drag.

Blended wing body designs are usually swept but I believe that has more to do with changing the location of the CG and stabilizing the configuration.

I know swept wings look "cool" but a straight wing is probably the better solution.

[d_wheel]

It appears you are making things much more difficult than they need to be. The drawing looks cool, but is impractical in many ways. My advice is to start simple. Get a trainer type aircraft and find someone to buddy box with you until you solo, then move on to this project. The knowledge gleaned from the training process will help you design an airframe that is more practical.

Later;

D.W.

[spastic]

It appears you are making things much more difficult than they need to be. The drawing looks cool, but is impractical in many ways. My advice is to start simple. Get a trainer type aircraft and find someone to buddy box with you until you solo, then move on to this project. The knowledge gleaned from the training process will help you design an airframe that is more practical.

Later;

D.W.


good post, i agree. go for low earth orbit before the moon:)

[John O'Sullivan]

You don't really have to re-invent the airplane. There are thousands of excellent flying proven designs available that will serve you well to educate you in what will and what will not work.
The designers of these proven aircraft did not achieve their proven designs by building their first "secret weapon" model, but by progressively developing their designs from existing proven designs.
Remember that just building a "cool" design without any practical appreciation of aerodynamics is a sure fire road to failure.

Initially go with a proven design and AFTER you have mastered that, you can go on to develop your own designs.
John O'Sullivan