p3990013

Hello!

I want to modify my Cessna for flying faster. As the pictures show, I want to reduce the size of the wings (the red color is the are of the wings that will be removed). Do you think it will be succesful or will the plane be unable to fly?

Before:



After:



Also any suggestions for battery? Should I go for a 4-cell? (now I use a 2250 mAh 3-cell LiPo)

Thanks!

da Rock

Designers in real life reduced wings on regular production airplanes to make them into racing planes. But what they did was to cut the span, not what you propose. There were a number of very good reasons they did what they did. And there are a couple of good (or bad) reasons what you wish to do will cause problems.

Shorter span reduces drag a couple of ways. And that modification keeps the fore/aft aerodynamic balance unchanged. The CG locations that worked before are still going to work. There is the least "collateral damage" from just reducing span.

What you want to do is going to screw the wing''s airfoil over it''s entire span. For example, at the new tips, the profile is basically 90% ailerons, 10% "airfoil". And that airfoil is a screwed one. It''s gone from a smoothly cambered profile to a sort of wedge thing that will have a relatively large thickness to chord ratio. The result is not anywhere like a highspeed profile would be. Also, the part of the wing you''re removing is going to take quite a bit more lift capability for the area removed than simply chopping the span will take. Whacking the airfoil from root to tip is going to screw the airfoil from root to tip. The entire wing is going to lose it''s lift and probably increase it''s drag exponentially.

The original airfoil from root to tip was probably a simple cambered one. Cambered airfoils aren''t the choice for speed airplanes anyway. But start to chop the nose off a cambered airfoil and the resulting airfoil is an even worse choice for speed. The farther out the span, the more nose you chop off the profile at that point the worse that airfoil becomes.

Also, if you look at the remaining wing on your redesign, where is the MAC going to be? Where will the Center of Lift of the whole wing now? The wing is now going to be lifting quite a bit further aft than before. That is going to screw up the balance.

There is a reason the aero engineers just chopped the span. Actually there are a lot of reasons they did what they did. And even more reasons they didn''t do what you propose.

da Rock

If the electric motor will take the extra voltage, that''s certainly an excellent way to up the speed. You can look that up on the motor specifications, and also see what prop is recommended at the new voltage.

How much heavier is the new battery than the old?

crasherboy

I my opinion,doing that to your wing would be a disaster and most likely you would not fly it long if at all. Of course I could be wrong.

p3990013

thanks for the tips! yes, I assumed I would destroy the airplane in the process, that''s why I started this thread. good thing I did that!

seanreit

My Feedback: (60)

Rock, I must admit, I''m going through this right now on a different level. I have a wing panel that was 29 inches with 16 inches of sweep and took it down to about 24.5 span and 20 inches of sweep.

Original airfoil was naca 008 root 22.125 and tip naca 0012 about 10 inches.


Same root, tip naca 0010 8 inches.

Goal of course to go faster.

I don''t pretend to understand everything you wrote, but I do want to hear if I''ve done something over the top wrong. The fuse width is about 12 inches wide.

BMatthews

Also these little models are often running with as little wing area as you want in the first place. Then there''s the little issue that foam models often do not have the best reputation for stiffness. If you were to truly hotrod this thing for speed you may well find that if you manage to reach racer like speeds then the wings will flex, flutter and rip themselves off the model.

From there it''s all about the motor and prop. Chances are from the look of the prop you have this is not a good motor for flying fast. You probably would be best off to get a motor that has a Kv value in around the 2000 to 2500 range that will turn a small prop at some crazy RPM''s and result in higher propjet like airspeeds.

All in all I would suggest that if you want to go fast you just get a model that is setup for going fast. I believe that there's a version of the Stryker propject ARF foamie that comes with the right motor already. It's mostly the motor and prop that determines the speed of models in this size range. The model design merely determines if it rips itself apart in the air or not.

A couple of options would be to go and buy that Stryker or to get a Speed 400 class racer or if you want to do your own work do a Search in the 1/2A forum for "sheet wing racer" or "swr" to see what some of the guys there have managed to come up with for simple but fun high speed models. It'll still require the right motor to achieve those speeds but the model will be more suitable.

pimmnz

Of course the other way to make this thing go faster is to increase the span. Longer wing equals higher aspect ratio, which means less drag, and lowering the wing loading means that you can fly at a lower angle of attack, again less drag. And the most efficient way of going faster is to reduce drag, increasing power is a poor way of going faster, ideally you need to do both for maximum gain.
Evan, WB #12.

B.L.E.

Are you trying to say that clipped-wing Cubs fly slower than the regular version?
At high speeds, all wings operate at a very low angle of attack to provide the needed lift. This means that induced drag is low at high air speeds regardless of aspect ratio. What is the aspect ratio of a SR-71 wing anyway?
At high speeds, parasitic drag from the fusilage, engine cooling, and wing wetted area is the biggie.

Nathan King

Parasitic drag exponentially increases as a function of airspeed while induced drag tapers off at high speeds. As airspeed is doubled your drag goes up by a factor of four! Where those curves cross is your best endurance airspeed. Now, parasitic drag can be further narrowed down into form drag and skin friction drag. Form drag is caused by air coming in contact with the frontal area of the aircraft. Skin friction drag is just what the name suggests. Howard Hughes had all rivets installed flush with the body of his H-1 Racer to minimize skin friction drag. He also utilized flowing fillets at the fuselage/wing junction to reduce intereference drag.

A high aspect ratio wing is a bit of a tradeoff since they offer a reduction in reduced drag but have a more parasitic drag. This is why airplanes designed to be efficient at lower speeds typically have high aspect ratio wings. Now parasitic drag can vary by factors as minute as air density but to a much smaller extent.

Essentially, a fast airplane boils down to having as little drag as possible with gobs of power. After all, total drag determines the necessary amount of thrust to maintain a given airspeed. Specifically regarding the original question: Don''t cut the wings like that. I can guarantee you won''t like the way it flies. As Yoda would say, "A modified wing only does not a fast airplane make." It's just not that simple.

Montague

The whole "make the wing longer" vs. "make the wing smaller" thing depends on what you're starting with, and just how fast you're going.

When designing SSC combat planes, we discovered that more wing area and a longer span made the planes go faster. Faster here meant a top speed around 55-60mph. Obviously, the lighter wing loading was allowing a lower AOA and getting us to a better place on the L/D curve.

On the other hand, as I understand it, one of the reasons for cutting down a wing on a full scale racer is because the origional warbird was designed for performance at a combat weight that's a lot higher than the weight of the stripped down "a flew laps around the pylons" configuration. Less weight means that they could chop wingarea without causing too high an AOA. Someone correct me if I'm wrong on this last bit, but I do know that the "combat weight" of a P-51 is quite a bit higher than the weight of a P-51 with out guns, ammo, armor plate and drop tanks...

Anyway, when modifying a model to go faster, you need to know, did the origional designer put in too much wing, not enough wing, or just the right amount of wing for the speed you want to go. I could see a situation where adding wing makes you go slower and chopping wing makes you go slower, if the origional designer did a good job.

combatpigg

My Feedback: (3)

P399, if you would like to build a fast little plane with your existing equipment, carve a similar sized wing [to the Cessnas']out of contest grade balsa and build a minimally sized [in cross section]fuselage. Leave off the wing struts and the upper cabin, also make the vertical fin shorter than the Cessnas'. Now you will be all set with a little speedster that will be light enough and tough enough to take pretty much any power source that you can fit within the confines of the firewall.
I build and fly a lot of planes in the 8-11 oz range and have played with various shape and size wings. Try to stay with wing designs that have at least 4 inches of chord and no less than 100sq inches of area for a small speed plane of this weight.
The portions of the wing that you show [above] with the pointed tips would be nothing but dead weight and drag, contributing nothing to lift. Keep in mind that the minimum wing chord for your model is dependent on its' weight, but your model would have to be infinately light to allow that drastic of a taper.

HighPlains

My Feedback: (1)

It's a wee bit more complex than all the above.

As long as you are only flying in un-accellerated flight, span doesn't matter all that much. But most of us need to turn the airplane, so induced drag becomes an issue.

Reynolds visits some ugly truths when chord goes away.

Nobody cares about supersonic designs when designing RC models. Might as well talk about spacecraft.

The wing is the small part of the overall drag when flying level. When you turn, it dominates.

saramos

If you wanna go faster, put in a bigger motor.

B.L.E.

That only works up to a point. That point being where the engine is so big that it is the plane's entire payload, in other words, your plane is an engine with wings on it. At that point, using a bigger engine just means you have a bigger flying engine. The increase in the plane's weight cancels out the extra power.
I doubt that any engine will make a trainer go 200 mph. About the only way you could make a trainer go that fast is to check it in as baggage on a airline flight.
If you want to go faster than 100-120 mph with a model, the airframe has to cooperate with the engine by being designed for low drag.

saramos

Agreed. It was meant (mostly) in jest.[)]

Joking aside, I also think it would be better to clip the wings, rather than tapering the leading edge. To be able to maintain a proper foil shape would require foam cutting equipment, and the proper root and tip templates. Reducing that much wing would reduce a lot of lift, and may require more power to obtain flight. It may also weaken the wing quite a bit and you may have to add a new carbon spar. Much easier to clip the wings and add a little larger motor.

BMatthews

Actually putting in a bigger motor will always make it fly faster.... or at least provide the ability to fly faster if it is proped right.

Other factors may make it an ugly airplane to fly and one that'll snap roll out of a sharp turn but more power always means faster if it's used right. The provisos are needed because in this age of brushless motors that have widely varying Kv values it's possible to swap a small motor with a high Kv value for a much larger and more powerful motor with a low Kv that just won't spin the right sort of prop the right way to actually let the model fly faster.


..... life was a lot easier when we just used glow engines......

Getting back to the model that started all this I don't think there's anything at all to be gained by clipping the wings. This is already a small model. Look at the size of the blades of grass it's sitting on. Playing with the prop and motor and possibly adding another cell to the pack is all that is needed. A higher Kv motor that'll spin that prop with about 1.5 to 2 times the current and not melt down is going to make that Cessna hum like a Mustang. That's all that's needed in this case.

Granted it's a relatively dirty design for speed but the biggest gains for this thing are going to be the power system options. To make any significant gains in the model design it'll be neccessary to move on and pick out a racey style of model like a Speed 400 class racer or a smaller hotliner glider and motor those up appropriately.