How to reduce wing loading/stall speed.

[Ian Southerton 1]

Hi, hopefully not a silly question!

I've not been flying long, but have finally plucked up courage to fly my mini jazz that I built from plans. I've converted it to electric but have had to put a lot of weight in the nose to get the cog correct.

Anyway, I find it flies too fast for me, webocalc states the stall speed is 15mph.

So, as I want to persevere with the plane, i wondered whether making a set of wings 8" longer would reduce the wing loading and allow the plane to fly slower?

Ian.

[001]

Is it difficult to balance because the tail was built too heavy? 1oz. at the rear = 3 to 5 oz. in the nose. Then....

Is it not possible to locate the battery further forward?

If not, I would consider making the whole nose an inch or so longer if possible and removing some of the weight..

Making the wings longer is an option, but when i have tried it in the past it seems to not work as well as it should.

[Chris Bott - Moderator]

Ian is there any way you could make a lighter tail to replace the existing one? Save an oz there and you can take much more lead out of the nose too.

[Martin Harris - Moderator]

What's the AUW Ian?

[Ian Southerton 1]

Hi, it was my firdy build and i used heavy pushrods, solid tail and elevator which hadn't helped!

Also, I think the glow engine it was designef for was a lot heavier than the electric motor i fitted.

I could modify the fuselage, but it would be loads easier to build new wings.

Battery is against the motor bulkhead.

Auw is 1040 grams about 250 too much.

Ian.

[Simon Chaddock]

Increasing the wing area will of course reduce the wing loading and the stall speed a bit but......

It does not address the fundamental issue of having to add nose weight

It will alter the flying characteristics. You are changing the ratio of wing area to tail area.

It will increase the total drag and weight which will reduce the performance or require more power for the same performance which will add yet more weight............! The so called upward weight spiral.

 

As others have suggested rather than build a new set of wings you would do much better to remove the excess weight from the existing airframe.

Reducing weight will at the same time improve the available performance and reduce the stall speed.

Edited By Simon Chaddock on 30/07/2013 23:31:14

[Greybeard]

I should not be too difficult to make lighter push rods so that could be a good place to start. Enlarging the wing will not make much difference unless you match the amount that the model is over weight, in this case 25% and doing that will alter the flight caracteristics considerably. I think it better to correct the original problems than add to them with modifications.

[Peter Miller]

Reduce the weight in the tail, make a new, built up tailplane and lighter pushrods. This will allow you to remove lead from the nose.

One very good reason for this is the dumbell effect. If you have heavy weights out at the ends of the fuselage they will make changing the pitch much slower. Take a length of wood, hold it in the middle and and twist your wrist to make it rotate.

Now add weight (lumps of modelling clay) to the ends and try again and see what happens.

You will find it much harder to stop and start the rotation.

In passing, this is also why wing tips should be kept light to improve rolls, especialy hesitation (four point) rolls

[Andy Sephton 1]

For what it's worth, here's some maths:

Lift = 1/2 x air density x flight speed squared x wing area x lift coeficient

You can't do anyting about the 1/2, the air density or the lift coeficient (unless you change the sing section), so you are stuck with wing area and flight speed. The lift must equal the weight of the aircraft for level flight, so putting it another way:

Lift at slowest speed you can fly depends on the square of the flight speed (or stall speed in this case) and the wing area.

In practice reducing the weight or increasing the wing area will reduce the stall speed, but the effect is a function of the airspeed squared.

Put another way, if we double the weight of the aircraft we only have to increase the speed by 1.4 (the square root of 2), if we halve the weight of the aircraft, the speed for level flight reduces by about 0.7. The same relationship is true of we doouble or halve the wing area.

The point is, a small increase in wing area and/or a small decrease in weight will have a negligible effect on the stall speed of the model. In oprder to make a significant difference to stall speed, you'll have to make an even bigger diofference to wing area and/or weight.

Given your fiigures of 1040gm for the model and 250gm too much, reducing the weight by 250 gm reduces the overall weight by a factor 24%, but the stall speed would only reduce by 14%.

[Steve Hargreaves - Moderator]

"Add lightness" should be engraved on every aeromodellers soul....

As others have said surely the best way forward is to put the model on a diet...especially at the rear end....either build a new tailplane & elevator or drill/cut a few holes in the existing units to reduce their weight. Swap out the pushrods for lighter items (snakes or a closed loop system even). Every oz lost at the tail equates to about 2 or 3 more at the nose so a worthwhile overall reduction.

But don't stop there....can you fit lighter servos? How about the Rx battery..might a smaller capacity pack fit? Or even a small 2S LiFE battery perhaps....you could easily save an once or two there over AA NiMH cells....

The lighter you can make the model the better it will fly......

[kc]

What is the model covered with? Maybe recovering the tail parts with lighter material ( eg Solarfilm instead of 'tex together with lighter construction might help.
Can the servos be moved forward? Can the rudder pushrod be replaced with closed loop?

You may have to accept that this is a fast and furious model and build something more docile! Better to build another model than muck around with modifications to this one ( except weight reduction )

[Ian Southerton 1]

Thanks for all your replies.

I'm an engineer, and think I over engineered the mini jazz!

I made an extension for the motor mount as the elctric unit is shorter than the ic unit the jazz was designed for, so if I make a fuselage with an extended nose, i can move the batts forward. Also, I reckon i could move the servos forward with lighter control linkages (i'm currently runnung 2 linkages to the elevator) and one linkage for the elevator. There is an option on the plans for a made up elevator rather than the solid one I built, and with few more mods (fitted 1/32 light ply as a strengthener) I may be able to get the weight down.

Have to remove some tiles in the kitchen and then I'll see what balsa I have to start on another fuselage!

Ian.

[Steve Hargreaves - Moderator]

Surely try the other weight loss plans first including the elevators & see how it goes before building a new fuselage???

[kc]

It may be just as quick to build a new fuselage & tail than mess around modifying the old one. However consider sawing off the front end and extending it a little.
But generally i would say start on a new model of a slightly more docile type because you wont alter the fact the the Mini Jazz is small and fast all you will do is allow it to fly slower without stalling.
So leave the Mini Jazz till later and build another simple aerobatic model. Maybe Peter Millers Swamp Rat( August RCME ) converted to electric. Your experience will allow the next model to be built lighter & better. A bit bigger may be easier to fly.

[Ian Southerton 1]

As said, I think making a new lighter fuselage would be quicker than modifying the old one.

I also have a wot4 foam-e, but it looks so good on top of the book case I'm afraid to fly it! I'm waiting for a 3 axis gyro type receiver to arrive, then I'll try it, I have used one on my flying wing and on the mini jazz. Like the look of the swamp rat though!

Ian.

[Ian Southerton 1]

Well, still making the fuselage between plastering the dining room, moving rad pipes etc!

I flew the Wot4 foam and on the 3rd landing, hit the grass a bit hard and it exploded-going off foamies

Sooo, thought what the hell, if I crash the Jazz I have enough balsa to rebuild it. Wellll, I've flown it dozen times now and am getting the hang of it, landing wise, I've hit the deck pretty hard and the only damage was a slightly bent undercarriage.

Ian.