RE: Suggestion for moderators
 

There were some questions posted as to trim drag of aircraft. It is common practice on the large commercial jets to move fuel both fore and aft to reduce trim drag. I believe for instance, the 747 has a rather large tank in the tail for just this purpose.

In terms of static stability and speed of the aircraft, these do change with aircraft speed. An airplane can be quite a bit more tail heavy at the upper end of it’s flight speed, the problem is slowing for landing. Airplanes such as a P-38 were able to use this, since their gears retracted rearward.

Today, with the electronic stability augmentation systems like on the F-16, the airplane can be flown in what would be considered unstable. Interesting to note, that the F-16 C increased the size of the tailplane by about 10% so that the bombing load could be increased in total weight. If memory serves me correctly, I believe that they were able to add two additional 500 lb. bombs with this change due to the tail carrying more of the load. In a sense, aircraft flown in this configuration are canards flying backwards.

On model pylon racers, most have difficulty just meeting weight with the required equipment, however, I have seen one that had instrumentation onboard to measure airspeed. It would not be too difficult to measure G loading. Perhaps more important would be taking data to determine what flight path gives the fastest times. A larger radius turn results in a lower loss of speed due to lower G loads, but results in a longer path. The trade-off here is how well does the airplane accelerate after the turn. Acceleration is influenced by the propeller selection.

RE: Suggestion for moderators
 

Bill, thanks for explaining the "aft CG is faster" premise so clearly and concisely.

As for why I was moving heavy cargo, it was a passenger who needed a bit of privacy for a few moments. Upon realizing that the extra speed would get us to our destination nearly 45 minutes ahead of schedule, and he was in no way interested in taking the controls, we decided that he should stay in the back seat for the duration of cruise. He came back up front prior to descent.

RE: basic aerodynamics
 

I used to build engine powered freeflight models that balanced on the trailing edge of the wing, but they had stabs with roughly 50% of the wing area. The important thing is to keep the CG slightly ahead of the center of pressure of the entire airplane, and those old freeflights had their center of pressure behind the wing trailing edge.

A hypothetical model with zero mass would probably be unstable, since the air influenced by the airplane has mass, and moves with the airplane. I have built models with wing loadings of well under one ounce per square foot, and their CG location is just as important as with far heavier wing loadings, and seems to like to be a little further forward than that of the heavier jobs. The mass of air influenced by a wing has a significant influence on flutter calculations. Flutter analysts frequently consider the mass of air influenced by a wing or stab to be roughly equal to that of a cylinder of air with radius equal to the wing chord.

RE: basic aerodynamics
 

This entire debate that is going on in this thread has been quite humorous... It can be summed up in a VERY short saying...
Best Overall Lift/Drag ratio of plane = best speed Period...

As for Cg / NP Placement on low Wing Loading All you are doing is changing a "normal" plane into a canard and vice versa depending on whos STABILITY criteria you want to meet.

BFoote

RE: Suggestion for moderators
 

Normal-----

A normal F16?
A normal Cessna 172?
A normal Flying Flea?
The application really defines the setup -
I just did two nice .40 powered kits for an article - very basic sport designs - CG's aprox 25%of MAC
very easy to fly . Not normal models for my own interest.
A given definition of "normal"-can be a bit of a canard also.
It is certainly interesting to see the various ideas as to what will an will not fly-- especially when the "proof " is referrenced to some text book.
Some formulas should read, "open book and close mind".
(present company excepted)

RE: basic aerodynamics
 

"Best overall Lift/Drag ratio" of a plane has nothing to do with "best speed". For a given configuration, there is only one angle of attack for maximum L/D. Suppose for a second that an airplane was at its angle of attack for maximum L/D when also at its maximum straight and level airspeed. How would this airplane have reached maximum level speed? At ANY airspeed less than maximum, the airplane would have more drag than thrust. It would be hard for this airplane to accelerate toward maximum level speed while decelerating.

The point I am trying to make (and others have made it here also) is that best L/D isn't really important for top speed. Airplanes designed for high speed (fighters and racers) don't tend to look like airplanes designed for high L/D (gliders), and not just for structural reasons.

RE: Suggestion for moderators
 

How true - some of the fastest planes (closed course) had little stubby wings.
Even the "best" German fighter -prior to WW11 - was a cobbled up thing with small wings - and acheived the winning speed with an engine designed to do just that - go like hell for the contest.
Too many, equate the "best " airplane design with thermal glider design.
It keeps em happy -----

RE: Suggestion for moderators
 

Best L/D of the airplane configureation, since you guys were talking trim drag and moving "cargo" around to increase speed, I am not talking overall design of an airplane here, that is an entirely different set of requirments and criteria to be met with a set amount of thrust.

Reason F-16 uses its horizontal stabilizer to fly on is because of the huge parasite drag at high airspeeds, thus if you balanced the fighter the same as say a Cessna 172, you would have the horizontal stabilizer doing nothing but creating Parasite drag and a bit of Induced drag etc. Take that same horizontal stabilizer, load it up, decreasing the load on the main wing which decreases its needed angle of attack decreasing induced drag and pressure drag at high speeds creating an overall best L/D ratio of the entire plane with set amount of thrust from the engine.

In my previous post I was not talking about the generic soaring definition of best L/D which is what you are refering to. Best L/D of a powered airplane at said thrust is completely different because you are burning a set amount of fuel creating x amount of thrust, and the overall best L/D with said thrust will create highest airpseed.

Brian
was on my brothers account sorry...

RE: Suggestion for moderators
 

While reducing drag by shifting CG will increase top speed, I really don't think that's the reason for the F-16's CG location. The neutral point of an aircraft shifts aft when supersonic. If the neutral point shifts aft but the CG does not, static pitch stability increases. An increase in pitch stability reduces pitch authority. To preserve supersonic maneuverability, the CG of the F-16 is located such that stabilators lift upward when in straight and level subsonic flight (and the airplane is statically unstable in pitch). An F-16 is operating nowhere near its L/D max when at its highest speed.

RE: Suggestion for moderators
 

Watch pictures of the Russian fighters which will do the "cobra" maneuver and do -literally flip flops including stuff where the craft moves backward- all with pilot -NOT computer inputs.
The F16 as well as many new designs are made to require very low control surface effort- (did I say that right?)
the "normal" setups" so often referred to here - would be abnormal on this type craft.
I wa truly amazed at earlier inputs on this thread which were adamant on fixed (fixated)CG setups for "correct and normal" airframes -----

RE: Suggestion for moderators
 

Shoe: I agree wrote it a slightly diff way. Why we are even debating compressible fluid flow questions in this forum I don't know. Fighter aircraft usually operate around 50% MAC when going to supersonic conditions because lift is generated via a different pressure differential.
Mach >1 pressure gradients on the top of the wing are fairly even while in M<1 pressure gradients are highest at leading edge. Thus the Cg moves when going through the transonic range to supersonic.

Brian

RE: Suggestion for moderators
 

The CG moves where you put it, regardless of airspeed. The Center of Pressure (CP) and Neutral Point (NP) move aft as you pass through M=1.

RE: Suggestion for moderators
 

Best speed for a given thrust is with the lowest drag. I don't care what the lift to drag ratio is if I got the required lift and the lowest drag. Maybe if I was flying to the maximum altitude it would become a much larger concern.

Flying unloaded (1g) you might make the argument for a short wing span, but once you have to turn to stay on a closed course, then wing span (or rather Span Loading) become much more important. Since you want the low drag in the turns as well as the straight-a-way. That why Formula One racers have increased the wing spans from the early days to the present. The main limitation is weight, since it take more material to build a long wing.

Aircraft design is a bit like rock-paper-scissors!

RE: Suggestion for moderators
 

Interesting bit - the original layouts were for Goodyear Racing - and other closed course stuff - for spectators.
I still have a full page of planforms of all the Goodyear racers from 1949- you can see and compare at a glance, the various setups --mostly short span.
Bear in mind - these were typically budget designs - using plywood tubing and cloth--
Manytimes, design changes are based on today's technologies ; composites, high strength metals etc...
For modelers - the early setups are designed like a model
Perfect!
My Cassut -- tho I am doing a 44% scale of the 15' version -- also could be done with very thin long wing as used on the newer racers flown at Reno.
The short span model can be turned extremely tight with no fear of snapping out.
A race plane -as you note - has to match the task---
Mine will be an aerobatic setup.
I have already played with various planforms - the stubby setup is excellent for my use.

RE: Suggestion for moderators
 

LOL, I just reread what I wrote about the Cg moving... haha, sorry for muddying the waters there. You are right shoe thanks for fixing my goof. Its the Cp that moves not the Cg.

Brian

RE: Suggestion for moderators
 

When I do use a CG --I tape it solidly in place .

RE: Suggestion for moderators
 

Haha... I think this is great... humor has finally returned. :)

RE: Suggestion for moderators
 

I think my "Cg" moves cuz in my airplanes I got little aliens dancing around in the cockpit!

cheers

Brian

RE: basic aerodynamics
 

When I posted the link on September 20th last year, I never thought for a moment that it would still be generating such interest almost a year later; over 7200 hits and 217 posts!! Not many threads survive that long or generate that much interest or that many responses. I envisaged a few newbies throwing in an occasional comment after visiting and picking up a few pointers.

I see comment, and even argument on national heroes, until it's lightened up with a little humour.

Apart from a second post on the same day, I think this is my first look back here, and many of the posts surprise me; some more than others. I've long ago forgotten much aerodynamic theory, but I practise it almost every day, and it has to be one of things in life that remains true to the theory that I learnt so long ago. As a full scale jet transport pilot, I know that the principles established at Kittyhawk a hundred years ago remain as valid today as they were then. I think much of what Wilbur and Orville achieved was a mystery to them; it was accepted because it worked and they had no theory to back it, but successive generations of designers have honed the science of aerodynamics to what we know and accept today.

We can have opinions; we can fly creations that seem to defy the science of aerodynamics, but I believe that when we think we are defying the 'laws', we are missing relevant points just as the Wright brothers did back at Kittyhawk. The 'laws' of science and aerodynamics will always apply whether we believe it or not.

I agree with dick hanson though, when he says that 'more people should be into this weird stuff!!' but are we thinking about the same things dick?

I'll take another look in next year.

RE: basic aerodynamics
 

yep without shifting some mass, the CG doesnt move, well it does as fuel is consumed, but its the CP that moves forward with increased AoA

RE: Suggestion for moderators
 

Unless you take the theory to extreme ends - you are simply "stuck in the middle".
Look at the arguments on flat wings -
Lo and behold - they are efficient!
(provided you know when and where to use them)
My hero for knowing what the hell is really going on is/was Alexander Lippish.
(Who?)

RE: Suggestion for moderators
 

Alexander Lippish the Genius behind the Me-163------among other things!

RE: Suggestion for moderators
 

In the late 40's I flew an Orbit CL with a McCoy .60. I then developed "Franks First Law".

"With enough power, you can fly a brick"

RE: basic aerodynamics
 

;)OK..OK..!! FIRST, I FEEL DICK HANSON IS 100 % CORRECT..ALL OBJECTS HAVE A C/G.. SOME FLY, SOME ,DO NOT.. I AM AND OLD SCHOOL DUDE.. IF THE RESULTS DON'T MATCH THE DATA,GET NEW DATA!! DON'T ARGUE WITH RESULTS..COMPUTERS AND FORMULAS ARE COOL, BUT,
NOT THE LAST WORD.. IF THEY WERE,WE WOULD NOT NEED WIND TUNNELS OR TEST PILOTS.. LIGHT PLANES DO FLY BETTER..HAVE YOU EVER BUILT A HEAVY PLANE INTENTIONALLY?? HEAVY PLANES, ZERO MISTAKES. NO TAKE OFF, GROUND LOOPS,TAKE OFF SNAPS,STALLS,SNAPS IN HI A.O.A, PITCH CHANGE WITH SPEED,FLYING WITH THE TAIL DOWN,LANDS TOO FAST,SNAPS WHEN SLOWED DOWN,LONG GROUND RUNS TO TAKE OFF,, ..LOTS OF TROUBLES, CAUSE ,MOST LIKELY, HEAVY!!! FORMULAS AND COMP. HAVE TAKEN SOME OF THE, COMMON SENSE THINKING ,OUT OF THIS AREA..TLAR WORKS PRETTY DARN GOOD.. SOME DESIGNERS TRY TO MAKE SIMPLE THINGS INTO ,BLACK MAGIC..BEST KEPT SECRETS,, SOOO------ THEY CAN HAVE A JOB NEXT YEAR!!! WE FLY ALL THE LATEST JETS ,F-111,F-14,,F-16,,F-22,F-18,WITH,,,,,,, NO COMPUTERS!! GO FIGURE.. THROW A STONE INTO A PACK OF DOGS,THE ONE THAT YELLS, IS THE ONE YOU HIT!!!

RE: Suggestion for moderators
 

Hi everyone. this is my first post. My head is still spinning from reading all of your posts in this topic. I came to the aerodynamic thread to find out more information about dihedral.
I am scratch building my first plane soon and I'm concerned about when and why you use dihedral. I was told that dihedral is important on a low wing aircraft for stability and it is very important for planes that have rudder only steering. so if I'm building a low wing aircraft with ailerons/flaperons do I need dihedral? and if I do need it, is there a formula for the angle?
thank you,
crankypants

RE: Suggestion for moderators
 

:DHI. 2 OR 3 DEG .IS OK. RUDDER WORKS BETTER. MORE STABLE.TO MUCH WONT HURT. WILL FLY WITH ZERO, LOOKS STUPID FROM THE FRONT. THE WING ACTS AS IF IS MOUNTED HIGHER ON THE FUSE.. FALLS INTO .. "IT CANT HURT " RD

RE: Suggestion for moderators
 

Actually, too much can hurt, in that it will tend to make the ailerons less responsive. That goes for high or low wing planes. If you put the wingtips at approximately the same height as the CG( somewhere in the middle of the fuse - say around the thrust line), that'll be at least neutral in roll.

RE: Suggestion for moderators
 

[:'(]JOHN ,THIS GUY WANTS TO KEEP IT SIMPLE.. HE IS ASKING A BASIC QUISTION.. YOU MADE IT TECHNACAL ,AS ALLWAYS.. OK, JOHN, TELL HIM HOW TO FIND THE C.G AT FUSE LEVEL!! SHOULD BE VERY EASY.. RD

RE: Suggestion for moderators
 

Yeah, that's why he asked about A FORMULA. Not like he might want to learn something, being a new designer and all. Yet I didn't get all technical and give him one, even though I have it right here. I suggested putting the wingtips near the vertical CG and I gave him two references - put the wingtips about as high as;

A) Middle of the fuse
or
B) The thrust line.

Also, check the pic I've attached. It's a front view of a P-51 with a line from tip to tip that shows the relation to the thrust line. If you want aerobatic, this is how. If you want tamer, add more dihedral.

If he really wanted to find the vertical CG, it would be easy enough. Mark the fuse with a vertical line at the horizontal (normal) cg location. Then let the plane hang vertical from the prop crankshaft. I line straight down from the crank - like a plumb bob - will cross the vertical CG line at the vertical location of the CG (horizontal too).

Seems that some guys get into RC partially to talk out their behinds instead of actually learning something. And then start with the sniping when someone suggests a better way. Hey Crankypants, I think you should swap usernames with this guy.:D

RE: Suggestion for moderators
 

:eek:OH I SEE. SO A HIGH WING PLANE WOULD USE ANHEDRAL?? THIS WOULD PUT THE TIPS AT ENGINE CENTER LINE>>>>???? "NOT"..ITS JUST NOT THE CRITICAL..TLAR WORKS JUST FINE..FORMULAS ARE OK ,BUT NOT THE FINAL WORD.. IF THEY WERE,THERE WOULD BE NO NEED FOR WIND TUNNELS, OR TEST PILOTS.A BUICK TRUNK LID WILL FLY ,IF YOU MOVE IT FAST ENOUGH.FOMULAS ARE JOB SECURETY FOR GUYS WHO CANT BUILD, OR FLY.. RD

RE: Suggestion for moderators
 

OK thanks for both your inputs on dihedral. I am absorbing all the information I can about this and other topics. I've read this entire thread. and I get both sides of the whole cg debate. As a new builder of RC planes, it really helps me to understand the thinking behind all this. After reading 8 pages of this thread, I've learned at least 2 things. 1.) There are rules about aerodynamics. 2.) These rules are not set in stone and its important to think outside the box or "wing it".(sorry I could not resist)
So thank you Dick Hanson and thank you to all that opposed him.
What, as a new builder, I was expecting to read in the aerodynamics thread were things like: airfoils. body designs, wing shapes etc. I guess I got some of that. I just don't think I need to wade through 8 pages of "I'm right, no I'm right!" to get it.
I think this thread would be more useful to new builders if we could see more testimonials and proven information that has been implemented on real RC planes. Like what has worked, what has not worked, why, and what you would do differently next time.

Rapptor tells me that 2 or 3 degree dihedral has worked for him. Johng tells me that 2 much will hurt ( i don't want my ailerons to be less responsive) and gave me an example of lining up with the centerline . both posts are useful to me. More posts would be even better.

thanks
cp

RE: Suggestion for moderators
 

;) CP,YOU HAVE MADE A VERY WISE CHOICE.. IM SORRY FOR NOT BEING MORE HELP..GOOD LUCK AND MOST OF ALL,-----------HAVE FUN!!!!!!!!!! RD

RE: Suggestion for moderators
 

Just a suggestion.
I would guess that you are building this model as a relaxing pastime.
So try this:
get some poster board or scrap stiff foam -or buy a little glider like the ones they used to sell for a dime.
Learn to toss it and see what happens when the wings are bent level-up-down.
The "stability" you are after , comes from the wingdihedral presenting a difference in drag, as the rudder steers it left or right .
Use the model held in front of you and looking at the nose straight on -you will see the underside of the left or right wing as you imitate the rudder steering it.
You need a fair bit if the rudder does all the corrections.
You need very little if the ailerons are used .
Obviously if more stability goes along with more dihedral--then the stability will fight a counter command from the ailerons.
That is a very basic view of what's happening and as was mentioned, it takes very little dihedral angle to do the job if ailerons are also used to do roll positioning.
I started building controline in 49-and freeflight shortly afterward.
One type needed no roll stability to speak of and the other needed it defined perfectly.
So sketch away and if it looks right -it likely will be flyable .
The enemy of all aircraft is weight .
The horse pucky about "heavy flys more scale like " is not true.
Your real task on a scratch design, is keeping it both light and strong.
That is the job they pay people to do at the aircraft factories.
No one was ever hired to make em weak and heavy-----
tho some do--

RE: Suggestion for moderators
 

Well, in general folks build high wing if they want lots of dihedral effect But here ya go:

:D:D:D

RE: Suggestion for moderators
 

Wow! The legendary Dick Hansen replied. Thank you for your suggestion. I kinda evolved into building planes from those little radio shack rc cars. As I was getting bored with them I started taking them apart and wondered if I could use all those micro servos to make one of those 99cent gliders fly. Several proto types later (about 10 gliders) I was having no luck. Then I thought, what if I built one of those guillows 500 series gliders and use that as the air frame. As I was building it and doing research on another rc group (sorry) I concluded that to make that fly I was going to need to spend some real dough on real rc gear. The people I was talking to about the build had successfully converted and flown these planes.
Now keep in mind, at that point I had no real rc flying experience. They suggested shelving the project until I had some flight exp. I took there advice and bought an electric plane. I’ve been crashing that plane for several months now.
As usual I like to get in over my head, so I’m thinking about building a bigger plane for that ½ a contest that starts in Jan. I’m going to work with a friend of mine who has exp building and flying gas planes. I figured I’ll have him fly it, that way if the design is totally awful, his exp would keep it in the air for longer than 10 seconds.
Oh yea dihedral. What I’ve been told, is that dihedral helps a rudder only plane turn and recover from turns better. (more stable). Now if you have ailerons on you wing would dihedral be that important? My thought on it would be no. I’ve never flown a plane with ailerons so that’s why I’m asking all of you.
Oh and Dick Hansen, do the 4 basic rules apply to dihedral as well as cg?
1-If the plane is extremely light - the dihedral does not matter
2- If the plane is too heavy - it still don't matter.
3- If you got enuf power - nuthin else matters .
4-If you ain't got enough power - same thing.
Not to start another 8 pages of trouble or anything.
Thanks
Cp.

RE: Suggestion for moderators
 

:D CP. NO MATTER HOW STABLE YOUR PLANE IS,, "YOU" STILL HAVE TO FLY IT!!! ALL THE THINGS THAT ARE SUGGESTED HERE, ARE TO HELP YOU FLY, NOT MAKE IT FLY ITSELF.. BUILDING IT "LIGHTER" ,WILL HAVE MORE BENEFIT THAN DIHEDRAL.. IF YOU FEEL THAT DIHEDRAL WILL LOWER AIRLERON EFFECTIVENESS,MAKE THEM LARGER AND MORE THROW.. RD

RE: Suggestion for moderators
 

Rap:

Quit yelling.

The third key from the near side of your keyboard on the left side is marked "Caps Lock."

Press it.

Bill.

RE: Suggestion for moderators
 

well - sorta -but dihedral is very friendly - and weighs nothing- so do it .
My basic rules are simply to make you think.
If it is too heavy -it won't fly - so why bother with CG
If it weighs zero or almost zero and has very low wing loading - you can take extreme liberties with CG- we have been back past 50% of MAC- and -they are still flyable - you gotta watch em -but they still fly
enough power to overcome weight -easy one -it will fly no matter how badly the designed is bungled.
If not enough power to leave the ground - none of the other stuf matters.

RE: Suggestion for moderators
 

clarification here from cp.
I will fly the plane that I build. My friend will help with the initial test flight. I'm sure he will be training me. He has more exp than I do with flying rc planes. I'd rather have the fate of my plane in the hands of someone who has many years of flight under his belt verses my 6 months. If the plane is unstable, a person with more exp might have a better chance of landing the plane in a safer manner than I. Then he can give me sugestions on cg etc to make the plane more stable for me when I fly it solo.

Dick Hansen I understand what you are saying about cg. I threw the 4 basic rules for dihedral in there as a bad joke.

I'll keep you guys posted when I start building the plane in Jan. I'm sure I'll have many more questions then.
Thanks
cp

RE: Suggestion for moderators
 

Hi,

Since you are a beginner, I would recommend you to use some dihedral in your first planes, it will improve lateral stability.