Wingloading and flight characteristics

[philscho]

What is primary difference in the flight characteristis of a light wing load plane like a Stearmen PT-17 Biplane and a heavier wing loaded plane like a clipped wing Cub J-3 of the same wingspan ( about 50")?

Phil

[Larry3215]

You cant really compare such radically different plan forms that way based on a single criteria like wing loading.

Wing loading is only one of hundreds of different factors that go into how a plane flys and no where near the top of the list as far as importance.

If you took two absolutely identical planes and added weight to one of them to increase the wing loading - and changed nothing else - then you could make a few general guesses on how they would perform differently.

The heavier plane would have a slightly higher stall speed. It would need to take off and land slightly faster. You would need to fly slightly faster in turns to avoid accelerated stalls. It would also use more power inflight for the same speed or climb rate and give you a slightly shorter flight time. That last due to increased drag.

As soon as you start talking about two different types of planes then all that goes out the window.

[hayofstacks]

The wing loading gives you an pretty good idea how fast it is going to fly. You can guess pretty well by seeing what type of airfoil it has, wing area, how much dyhedrial and incidence are in the wing and its size an weight will tell you 90% of what you need to know.

I don't know much, but My dad can look at a glance, tell you what size gas motor the plane was designed for, and makes it much easier for me to figure what type of motor I would run on it and how it will fly.

The wing loading is a direct corelation to the weight and wing area. The more weight you add, bigger batteries, motor, radio gear, the lower(numberically) wing loading. The faster you will have to fly to get the same lift to overcome the extra weight.

[JetPlaneFlyer]

In almost all respect the lighter the wing loading the better. Light wing loading gives the plane more manoeuvrability, better rate of climb, higher speed in level flight, better low speed handing, shorter take off and landing distance and slower stall speed.

The only advantage of heavier wing loading is that the model doesn't get buffeted around to much in blustery conditions.

Having said that, a Clipped Wing Cub isn't a plane I would think of as usually having high wing loading. WWII Warbirds and jets are where you usually get into 'lead-sled' territory.

[kyleservicetech]

Originally Posted by philscho

What is primary difference in the flight characteristis of a light wing load plane like a Stearmen PT-17 Biplane and a heavier wing loaded plane like a clipped wing Cub J-3 of the same wingspan ( about 50")?

Phil

Programs such as www.motocalc.com (free for 30 days, then $39) will give good clues on what happens as you vary the model weight with the same power system and wing area. Just change the model weight and look at the projected rate of climb, stalling speed, and so on.

Very small indoor flying models must have a very low wing loading on the order of an ounce per square foot in order to stay in the air. Get into the large giant scale models, and wing loadings can easily get into the 30 or 40 ounces (or more, a lot more) per square foot. As an example, your Stearman model built with a 20 inch wingspan would require a very much lower wing loading than that same Stearman model built with a 100 inch wingspan.

As others have indicated, optimum wing loading is highly dependent on the type of model you will be flying. In sailplanes they even have the ability to change wing loading in the same airplane, for different weather and wind conditions.

Someone once figured out the wing loading of a full scale military jet. That wing loading came out to be similar to the wing loading of a cast iron man hole cover tossed as a frisbee!

[Air-on]

I am thankful for being able to reach the following conclusions on which basic attributes one needs to think of in order to properly design a plane. Please help me make this a “benevolent hijacking of a thread that started very badly, as my next comment will tell. Meanwhile, these are the parameters I was able to come up with:
a- What kind of plane design: a basic trainer, an aileron trainer, an aerobatic trainer, aerobatic, etc.
b- What type of airfoil will be used: flat bottom, symmetric, semi- symmetric, bend-over-backwards bluefoam method (check proper name), etc.
c- Whether or not dihedral will be used; if yes, which gradation;
d- Whether or not the wings will have any incidence and, if yes, which gradation;
e- Which power system/ propeller, given the expected performance;
f- Any other project requirements (tks, hayofstacks!) the designer is able to set beforehand considering the previous answers, among others, in no specific order:
I – Wingspan
II- Overall weight
III-Chord (?)
IV- Max speed
V – Cruise speed
VI- Stall speed
VII-Right/ down incidence?
VIII( ) please add your contribution to this “design check list”. No roman numerals required!

• Notes on “Wing Load”:
  1- Very small indoor flying models must have a very low wing loading on the order of an ounce per square foot in order to stay in the air. Get into the large giant scale models, and wing loadings can easily get into the 30 or 40 ounces (or more, a lot more) per square foot. (by Dennis, TY! )
• 2- In almost all respects the lighter the wing loading the better. [Explanation: Light wing loading gives the plane more maneuverability, better rate of climb, higher speed in level flight, better low speed handling, shorter take off and landing distance and slower stall speed.]
  [The only advantage of heavier wing loading is that the model doesn't get buffeted around to much in blustery conditions.] (by Jetplaneflyer, tks big time!)
• 3- Priceless quote: “WWII Warbirds and jets are where you usually get into 'lead-sled' territory” (by Jetplaneflyer).
  Yes, I see lots of F15’s taking off, and the gush of kerogas they drench their afterburners with probably makes them – considering the actual payload - as wasteful as a Saturn V rocket (remember them giants? J I remember the Caravelle and the Constellation better, though!)

This check-list was the result of constructive replies, and will greatly help me on my current Piper Cub build. Now please read on.

[hayofstacks]

Iny opinion, weight and wing area give you wing loading. Type of airfoil tells you the rest. A flat bottom or high lift wing will lift up with speed. This will make the plane self right so to speak. A symetrical wing should stay the same. Again, the same goes with incidence ect.

[JetPlaneFlyer]

Originally Posted by hayofstacks

....Type of airfoil tells you the rest. A flat bottom or high lift wing will lift up with speed. This will make the plane self right so to speak. A symetrical wing should stay the same. ......

Wellllll not exactly.....

The flat bottom wing lifting (pitching?) up with speed is one I see talked about a lot but it's incorrect, in fact quite the opposite is true. A conventional airfoil with a flat bottom actually pitches down with speed. The technical term is negative pitching moment and it's the reason you wont find those airfoils used on 'plank' style tailless planes, if you used then the wing would just tuck nose down.
A symmetrical airfoil has zero pitching moment, so you have that bit right.
The only airfoil type with positive (nose-up) pitching moment are reflexed airfoils (i.e. with the trailing edge curved upward) as used on tailless flying wing type planes.

On a normal plane with a tail it's the tail that gives the plane it's nose up pitch with speed tendency. The relation of the zero lift angle of the tail to the zero lift angle of the wing (aka decalage), the size and position of the tail, and the location of the CG all interact to produce the nose up pitch that makes a plane 'balloon up' with speed and/or recover automatically from a dive. The airfoil has some effect on this pitch up/pitch down but, as described above, it's actually opposite to what 'common wisdom' on the subject suggests.

Some further reading:
http://www.desktop.aero/appliedaero/.../tailless.html

[Rodneh]

A better way to determine how well a model will fly is to use the "wing volume loading" formulae.

[hayofstacks]

I am going to have to disagree. On my alpha 450 sport, I can set a ruler on my tail and line it up with the bottom of the wing. Sliding the bsttery back and forth has cery little effect. No amount of downthrust really changes the self righting carachteristic. The incidence in the wing would also have the exact same effect as the tail being offset or angled, so I don't see how my ecplenation was wrong there either?

I guess I'll have to look more carefully at the wing tail relationship of my alpha.

Taking all of the incodence out of the wing o9f my slow stick, it does not behave as badly, but still baloons up with speed. Its tail is perfectly level if the wing is level.

[JetPlaneFlyer]

Originally Posted by hayofstacks

I am going to have to disagree. On my alpha 450 sport, I can set a ruler on my tail and line it up with the bottom of the wing.

Note i was very careful in my previous post to refer to the 'zero lift angle'. What most overlook is that flat bottom airfoils still make lots of lift even when the flat bottom of the airfoil is at zero angle of attack. To make no lift a flat bottom airfoil typically needs to be as about 3-5 degrees negative angle of attack. So when you have the tail set level with the flat bottom there is in fact about 4 degrees aerodynamic difference between the two surfaces (4 degrees 'decalage').

If you adjust the tail to about 4 degrees positive (LE up), so that zero lift axis are aligned then move the CG back to suit (do it carefully) you will end up with a plane that behaves just like your symmetrical wing model where the wing and tail are set zero-zero, the self righting and ballooning under power will be gone.

Alternatively set your symmetrical wing to about 4 degrees positive (plus move CG forward to trim) and you will find your plane suddenly becomes even more 'self righting' and 'balloony' than the flat bottom airfoil.

So again, it's the difference between the zero lift angle of wing and tail that produces this 'self righting' tendency, not the flat bottom airfoil.

[kyleservicetech]

Originally Posted by JetPlaneFlyer

If you adjust the tail to about 4 degrees positive (LE up), so that zero lift axis are aligned then move the CG back to suit (do it carefully) you will end up with a plane that behaves just like your symmetrical wing model where the wing and tail are set zero-zero, the self righting and ballooning under power will be gone.

Good information!

Out of curiosity, what happens when you fly inverted with this setup?

[hayofstacks]

I think I need to pick up some washers, and see if I can shim my tail differently. It I can get it to not putch up with speed, I would consider it a very capable plane.

[JetPlaneFlyer]

Originally Posted by kyleservicetech

Good information!

Out of curiosity, what happens when you fly inverted with this setup?

The need to add down elevator when inverted will be much reduced.

The negative side of it is that you lose the self correcting stability that you would want in trainer type models.

[JetPlaneFlyer]

Originally Posted by hayofstacks

I think I need to pick up some washers, and see if I can shim my tail differently. It I can get it to not putch up with speed, I would consider it a very capable plane.

Remember that to see the change you will also have to move the CG back. If you leave the CG where it is and only shim the tail you will find that all that happens is you have to trim in loads of up elevator to stop it diving. Move the CG and shim the tail carefully in small steps because bad things can happen if you move the CG back too far.

[hayofstacks]

Right now I hardly add any elevator while inverted, unless I have more then about 1/2 throttle.

I have moved my cg back and all it has done is made the plane twitchier. Right now I have about 10 clicks of down in my trim, which would lead me to believe you are right. If I could shim the tail to be more level and move my cg back a touch and that could get rid of the balooning I have experianced, I would be very happy. Right now it flys very well, but under a level stall it feels like it is keeping its nose high, but it drops nice and level as long as the wings stay level.

[Larry3215]

Originally Posted by hayofstacks

Right now I hardly add any elevator while inverted, unless I have more then about 1/2 throttle.

I have moved my cg back and all it has done is made the plane twitchier. Right now I have about 10 clicks of down in my trim, which would lead me to believe you are right. If I could shim the tail to be more level and move my cg back a touch and that could get rid of the balooning I have experianced, I would be very happy. Right now it flys very well, but under a level stall it feels like it is keeping its nose high, but it drops nice and level as long as the wings stay level.

As JetPlaneFlyer has said, decolage and CG need to be adjusted together to get the results you're after. In your case it sounds like you may also need to work on thrust angles. Sounds to me like you may need more down thrust.

Actually, if you want to really "trim out" your model you need to go through a series of steps adjusting CG, decolage, thrust angles, reflex and lateral balance.

One of the many "trimming guides" available is this one.

http://www.palosrc.com/index.php?opt...1:ic&Itemid=50

Keep in mind this isnt a one time through type of thing.

You will likely need to go through these steps multiple times adjusting things one at a time.

As JetPlaneFlyer said - when you change the decolage, you then need to change the CG. When you change the CG you may need to go back and change the decolage again. Changing thrust angles is the same way. Once you change it, you may find that a different cg or decolage works better.

I have to admit Ive never actually gone through these steps more than 3 or 4 times with any model. I usually get 'close enough' and start goofing off again

Check out the Pattern flyers web sites and or the IMAAC web site for a slightly different take on "trimming" but its essentially the same steps. Your end goals may be different depending on how you like to fly and the type of model. You will want a trainer to be more stable and self correcting and you will want a pattern plane to be much more neutral.

[hayofstacks]

I had over 15 degree's of down thrust in it, started little by little. It made no differance at all. If anything it made it pitch up worse. Right now I am waiting for a trip to the hobby show to see of I can replace my nose wheel plastic firewall mount before I can fly again.

After reading through that page(good link btw), the only constant I see is cg too far forward. Which I could both see as possible, and fix easily. I guess I will have to start playing with this. I have my cg fairly close to the eflight reccomendation though. If it is just a bit nose heavy, why do I have down elevator in it to maintain level flight?

Right now I have it trimed it for level slow flight. Any amount of throttle makes it pich up, as does speed. I think I am just a hair nose heavy, and I am going to try to shim the main wing, rather then washerson the bolt in tail.

[JetPlaneFlyer]

Sounds like it's VERY nose heavy if you have all that downthrust and it still pitches up. Regarding the down elevator.. it's not unknown for designs to have their decalage angles all wrong right out of the factors, also some misalignment might have crept in if the plane has been crashed and repaired.

Having said this, high wing planes do tend to need more downthrust due to the thrustline being well below the wing, so even with the CG optimised you might still need a fair bit of downthrust but nothing like 15 deg.

Good online CG calcs can be a big help in getting your CG close, then just fine tune by flight test. This one is good for planes with simple geometry: http://adamone.rchomepage.com/cg_calc.htm
Use a 5% static margin as the starting point for aft CG limit. You may get a bit further back but be careful.

[hayofstacks]

If I remember right I am actually behind eflights recommendations, meaning more tail heavy then they reccomend. I slid the 1800 battery from the front of the tray all the way back, right now its about 1" from the firewall.

It can't be THAT nose heavy. Takes off and lands wonderfully, knife edges well, the only thing it does is pitch up hard, andits directly poportinal to my speed. Coasting throttle off, it drops nice and level in a full stall. Some times one wingtip will drop, but generally I attribute this to the wind. The nose does not drop, or pitch at all in a full level stall. I just don't see how my cg could possibly be that far off. I can also hover it for a couple seconds, and again, one of the wing tips drops, but if I try to correct it, it trades wing tips.

[Air-on]

Dear JetPlaneFlyer, I am scratch-building my first depron plane, a "piper cub-ish" trainer –after the wonderful Sterling plans from their kit from the 50’s, enlarged to reach a wingspan of about 110 cm.
The idea so far was to make it flat bottom, but you got me thinking. See, I am building this Piper as a high-winged trainer, to regain my thumb/index dexterity, rusted since the 80’s huge (180 cm WS) Ugly Sticks with those messy .60 engines (no love lost, really). My plan has always been to take the finished plane to the field and have an experienced flier to test and trim it for me before I make “my” maiden flight.
That being said, would it not make a lot of sense to make it symmetrical already (keeping the dihedral angle)? And without any wing deCAlage or propeller down/ side thrust?
Thanks for your help!
Aharon

[JetPlaneFlyer]

Originally Posted by hayofstacks

If I remember right I am actually behind eflights recommendations, meaning more tail heavy then they reccomend.

Did you try plugging the dimensions into the online CG calculator?

Alternativly.. Trim the model carefully to fly hands-off straight and level at normal flying speed. From high altitude push the nose hard down into a 45 degree (or steeper) dive.


Hands off the sticks what does it do?

• If it continues in a 45 degree dive without either recovering or the dive getting any steeper then you have neutral stability. This represents the furtherst back CG position you would ever want to use, probably a bit too far back for most purposes but maybe desirable for an aerobatic/3D model.
• If it tends to recover from the dive gradually then that is the ideal CG position for most general purposes.
• It it recovers from the dive quickly and maybe rears up into a stall or even loops on it's own, then that indicates excessive stability, CG too far forward.
• If the dive gets steeper and maybe even 'tucks-under' into inverted flight then that is an unstable plane, very dangerous, CG too far back.

Once you have found the proper CG position as above by adjusting the CG and elevator trim then you can shim the wing and/or stabiliser to allow the elevator to be set 'straight'. You will almost certainly then find that you can remove most of your downthrust.


Steve

[JetPlaneFlyer]

Originally Posted by Air-on

Dear JetPlaneFlyer, I am scratch-building my first depron plane, a "piper cub-ish" trainer –after the wonderful Sterling plans from their kit from the 50’s,

Aharon,

Sounds like an interesting project. for your purposes I'm sure a flat bottom airfoil with the tail set level with the bottom surface will be good enough to get you in the air. If in the design you allow for the stabiliser to be adjustable then you can fine trim the model ance you get if flying using the procedure outlined above.. Or if it flies fine then dont even worry about it.

[Air-on]

Originally Posted by JetPlaneFlyer

Aharon,

Sounds like an interesting project. for your purposes I'm sure a flat bottom airfoil with the tail set level with the bottom surface will be good enough to get you in the air.

Thank you. In this case, should I keep the plans' wing deCAlage and motor down/right thrust?

If in the design you allow for the stabiliser to be adjustable then you can fine trim the model ance you get if flying using the procedure outlined above.. Or if it flies fine then dont even worry about it.

That is the kind of hint I love most: the "doable" type!

Your advice on CG is very clear, thank you again. I will try to attach a picture, but I should man up and start the build thread. Do I really need a "clean sheet" to show, tile for pdf and all that? I mean, why would someone... never mind the defeatist.
Bye, Aharon

[JetPlaneFlyer]

Originally Posted by Air-on

Thank you. In this case, should I keep the plans' wing deCAlage and motor down/right thrust?

looks like a feather light build.. Yes, I'd stick with the plan rigging angles and only change if it proves to need it.