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Old 07-07-2011, 04:15 AM   #1
payne9999
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Default Building light vs. durability & Wing Loading vs. Stability

I am building some scale civilian aircraft in the 50-54" range. They are quite light by design weighing in at 17-20 oz. They are stick construction like the typical Comet balsa models of days gone by but much better designed and thought out.

The plans are masterfully designed but are so light that I am breaking a lot of stringers and other fuselage and tail parts just handling it during the build. No disrespect intended to the designer because his work is incredible. However, it occurred to me that if the plane is that delicate now how will it ever survive one of my landings. I typically land quite nicely but every 8-10 landings is kind of ugly and it is often due to having unpredictable winds, cross wind landings or just dumb thumbs.

After building the fuse and stab, elevator fin and rudder I wondered: Should I rebuild and make the more critical structural pieces from light Engelman spruce (in this case 1/8" square and 1/16X1/8" bracing) or would that make the weight a critical issue?

The planes will be covered in Doculam with a light coating of acrylic or thinned latex.

Also, I have a wood and Monokote covered ARF that is the same size that is about 36oz. that flies nice and is reasonable on wing loading although I really pay when it comes to flight times. I am comparing it to my Mountain Models L-4 that is 17-19 oz. The L-4 is quite bouncy and less stable especially in a light breeze but gets incredible flight times.

So, I am wondering about the trade offs between weight and strength as well as weight and stability/wind penetration.

Tonight I started building a second fuse with some light Engelman spruce to check the difference in actual weight. I know it will be stronger and the Titebond glue joints will be a little better that brittle CA.

Also, I am ordering a pack of Paulownia wood. I will re-saw it into model size strips with a thin kerf table saw blade. Paulownia appears to have about twice the strength to weight ratio of balsa and is only 15-25% heavier.

I am curious about other builders out there and your trade-offs with weight vs. strength and material choices. I have built hundreds of balsa/tissue models and many glow planes but these ultra light electric models are fairly new to me.

Thanks,

Dave
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Old 07-07-2011, 05:08 AM   #2
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It depends a lot on how important "scale" flight is to you. The other day I was flying my DC-3 when a real DC-3 flew over. Looking at them both in the the sky together it was clear that mine appeared to fly several times faster. A real DC-3 looks like its floating along! Now mine is 60" WS and about 17 oz/sqft which is pretty good for a twin. It's fiberglass covered foam and has maybe 150 flights on it. It has withstood tremendous abuse and other than rebuilding the landing gear a few times I've yet to break it. If I had a DC-3 designed by say, Pat Trittle, it might have 1/2 the wing loading or less. It would fly at a much more scale like speed.

However, I doubt it would still be in one piece the way I fly and where I fly. Also, I routinely fly mine in winds of 15mph with no problems. A lightly loaded plane is more affected by turbulence. So it depends on what you are looking for in a plane. Lightly built stick and tissue planes do look more "scale" in flight but they don't look scale when they are buffeted by turbulence. They are less crash resistant too. So I fly those in calm winds in a nice scale like pattern.

The plane that you describe could probably gain 20-25% in weight and still be a nice airplane to fly. I'd beef up the areas that broke first on similar planes. No sense beefing up structure that doesn't need it.

They say build to fly, not to crash but want my planes to last as long as it took to build them.....

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Old 07-07-2011, 05:16 AM   #3
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Originally Posted by payne9999 View Post
I am building some scale civilian aircraft in the 50-54" range. They are quite light by design weighing in at 17-20 oz. They are stick construction like the typical Comet balsa models of days gone by but much better designed and thought out.

The plans are masterfully designed but are so light that I am breaking a lot of stringers and other fuselage and tail parts just handling it during the build. No disrespect intended to the designer because his work is incredible. However, it occurred to me that if the plane is that delicate now how will it ever survive one of my landings. I typically land quite nicely but every 8-10 landings is kind of ugly and it is often due to having unpredictable winds, cross wind landings or just dumb thumbs.

After building the fuse and stab, elevator fin and rudder I wondered: Should I rebuild and make the more critical structural pieces from light Engelman spruce (in this case 1/8" square and 1/16X1/8" bracing) or would that make the weight a critical issue?

The planes will be covered in Doculam with a light coating of acrylic or thinned latex.

Also, I have a wood and Monokote covered ARF that is the same size that is about 36oz. that flies nice and is reasonable on wing loading although I really pay when it comes to flight times. I am comparing it to my Mountain Models L-4 that is 17-19 oz. The L-4 is quite bouncy and less stable especially in a light breeze but gets incredible flight times.

So, I am wondering about the trade offs between weight and strength as well as weight and stability/wind penetration.

Tonight I started building a second fuse with some light Engelman spruce to check the difference in actual weight. I know it will be stronger and the Titebond glue joints will be a little better that brittle CA.

Also, I am ordering a pack of Paulownia wood. I will re-saw it into model size strips with a thin kerf table saw blade. Paulownia appears to have about twice the strength to weight ratio of balsa and is only 15-25% heavier.

I am curious about other builders out there and your trade-offs with weight vs. strength and material choices. I have built hundreds of balsa/tissue models and many glow planes but these ultra light electric models are fairly new to me.

Thanks,

Dave
I have one of those super light weight models that floats all over the place while flying. Its not a scale model, just a model designed for "3D".

Last time I flew it, after getting blown all over the sky in 5 MPH winds, got it back on the ground on a very good landing. While doing a taxi run back to the pit area, a 7 MPH wind gust got under the wing, and blew it over on its back. Busted the fuse in several places.

It's back together, but its so flimsy it won't be flying again anytime soon.

Another problem with those super light models is the battery and motor system. Those items are a dedicated boat anchor in a very light weight model. Any sudden high "G" forces that can happen on a bad landing, and those items can come loose, busting everything in between them and the ground. Or, the battery/motor can stay in place, and the landing gear tears loose and smashes through everything. (Had that happen on what looked like very gentle landings)

What ever you do, that landing gear support system is one of the most highly stressed parts of your model. Make certain it can't come loose, because if it does, it will do a lot of secondary damage.

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Old 07-07-2011, 05:20 AM   #4
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Hi Dave
Welcome aboard
Pleased to meet you
I build as lightly and strong as possible for the particular model and how agressively and what style of flight i would fly her
If the structure needs reinforcing and adds weight i add more power to offset
Take care
Yours Hank

"When wild the head-wind beat,Thy sovereign Will commanding, Bring them who dare to fly, To a safe landing."
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Old 07-07-2011, 09:12 PM   #5
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Hi Dave
Go along with you - the cheapest performance upgrade is a lighter model!

However, you can go too far. On a structure like you describe, I'd use 1/8" square spruce for the fuselage longerons and wing main spars, with 1/8" square balsa for the fuselage uprights, diagonals and cross-members. The spruce I use is all bought at either hobby stores, craft stores or mail order - I don't have the equipment or skill to make up my own.

Reckon you can go too far with lightness, as above. Wind is a fact of life and if your model is more tossed around by it than flying through it, you're probably not having a lot of fun. I tend to go out less in wind as years pass, but I've moved to Chicago and have learned that it's not called the 'Windy City' just because of its blow-hard politicians. My latest 9.5oz 36" span model is not going to get out flying all that much, I fear...

The photographs are of my 51" 'Great Big Sportster' that I designed by scaling up a 25.5" span rubber powered design by Chuck Wenlock and putting a minimally altered rubber model style framework in the outlines.

The model weighed 25oz with rudder/ele/motor control, a small outrunner and a 3S LiPo around 1200Ma. The majority of the framework is 1/8" square spruce and balsa, as above, some minimal 1/8" balsa sheet in the fuselage nose, 1/16" ribs, a minimum of ply - firewall, UC and wing mount with the thickest thing on the model being the 3/16" strip wing LE. The wing TE looks hefty, but I used two pieces of 1/16" balsa sheet in a 'V' arrangement, so it looks like the carved from solid sheet TE the 'full sized' version had, but was far lighter.

It was a handy enough model for any wind I'd be flying it in, but was a solid and resilient structure that was no bother in use.

Hope that helps

Dereck


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Old 07-07-2011, 09:16 PM   #6
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Originally Posted by Dereck View Post
Hi Dave
Go along with you - the cheapest performance upgrade is a lighter model!

However, you can go too far. On a structure like you describe, I'd use 1/8" square spruce for the fuselage longerons and wing main spars, with 1/8" square balsa for the fuselage uprights, diagonals and cross-members. The spruce I use is all bought at either hobby stores, craft stores or mail order - I don't have the equipment or skill to make up my own.

Reckon you can go too far with lightness, as above. Wind is a fact of life and if your model is more tossed around by it than flying through it, you're probably not having a lot of fun. I tend to go out less in wind as years pass, but I've moved to Chicago and have learned that it's not called the 'Windy City' just because of its blow-hard politicians. My latest 9.5oz 36" span model is not going to get out flying all that much, I fear...

The photographs are of my 51" 'Great Big Sportster' that I designed by scaling up a 25.5" span rubber powered design by Chuck Wenlock and putting a minimally altered rubber model style framework in the outlines.

The model weighed 25oz with rudder/ele/motor control, a small outrunner and a 3S LiPo around 1200Ma. The majority of the framework is 1/8" square spruce and balsa, as above, some minimal 1/8" balsa sheet in the fuselage nose, 1/16" ribs, a minimum of ply - firewall, UC and wing mount with the thickest thing on the model being the 3/16" strip wing LE. The wing TE looks hefty, but I used two pieces of 1/16" balsa sheet in a 'V' arrangement, so it looks like the carved from solid sheet TE the 'full sized' version had, but was far lighter.

It was a handy enough model for any wind I'd be flying it in, but was a solid and resilient structure that was no bother in use.

Hope that helps

Dereck
Hi Dereck
Outstanding
Shes marvelous quite a looker
Thanks ever so much for sharing her with us
Take care
Yours Hank

"When wild the head-wind beat,Thy sovereign Will commanding, Bring them who dare to fly, To a safe landing."
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Old 07-07-2011, 11:07 PM   #7
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It's hard to judge how strong an airframe is without seeing a picture of it.

I have a 36 in Comet Sparky that weighs 7 ounces and it flies great and is a lot tougher than it looks. Trust me I know.

I have a Rocket that is a bit larger, around 40 inch wing and has a flying weight of 12 ounces. It bounces pretty good too.

I smacked the ground with a 7 pound Aeromaster and did a lot of damage.

The more the mass goes up the faster they have to fly and the harder they hit.
So I wouldn't say it is fragile just because it's light.
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Old 07-08-2011, 02:48 AM   #8
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Hi Hank
Well, thank you very much. It was a fun model, both in building and flying. I built her a couple of years back, but when the move from MD to Chicago loomed, I sold off pretty much my entire fleet rather than have them messed up by the long move, and GBS was one that was passed on to a clubmate.

However, if you can dig up a copy of the February 2011 'Flying Models' magazine, you can see a fair bit more...

The aim was a model that looks like the rubber powered FF models I never really had the skills to master. It mostly flew around in circles like the 'full size', but landed in a field I choose, not one somewhere downwind.

Regards

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Old 07-08-2011, 03:19 AM   #9
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Thanks all for the input!

I think I will go for a compromise by just using alternate materials where I think it is critical and try to keep as close to the original plan as possible.

I like what I am reading about paulownia wood and I think I will try to use it in critical areas to increase strength. Spruce does not respond well to CA. I think it is some oils that hinder curing of the CA. I am hoping the paulownia will respond to CA better.

In the current fuse and tail parts I am using some spruce and I will weigh the finished assembly to see what the impact was. Next week when the paulownia comes I will build an assembly with it but because it is so light I may go for building mostly if not all from it.

As far as scale flight goes it has always been interesting to me how scale effects are not linear. I learned to fly a full scale Aeronca Champ. If I use that plane as an example it weighs about 850 lbs. and has a Vmax of about 100 mph.

So, if I built a 1/8 scale champ it should weigh about 100 lbs. but cruise at about 12 mph. Pretty crazy. A 1/8th scale champ that weighs even 5 lbs. will likely fly way to fast to appear scale due to high wing loading. The ultra light electric planes really do look scale on a really calm day but add a 5-7mph wind and they look quite un-scale due to the response to turbulence.

Anyway with my current fleet of 2 scale planes I only use full throttle on some takeoffs and rarely fly over 1/3 throttle in full flight.

Maybe I can find a happy medium at about 27-30 oz. for a 53" span.....

Thanks again,

Dave
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Old 07-08-2011, 03:34 AM   #10
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[QUOTE=payne9999;821315
So, if I built a 1/8 scale champ it should weigh about 100 lbs. but cruise at about 12 mph. Pretty crazy. A 1/8th scale champ that weighs even 5 lbs. will likely fly way to fast to appear scale due to high wing loading. The ultra light electric planes really do look scale on a really calm day but add a 5-7mph wind and they look quite un-scale due to the response to turbulence.

Anyway with my current fleet of 2 scale planes I only use full throttle on some takeoffs and rarely fly over 1/3 throttle in full flight.

Maybe I can find a happy medium at about 27-30 oz. for a 53" span.....

Thanks again,

Dave[/QUOTE]

In the late 1990's I built a 1/4 scale Piper Cub from a kit. That model weighed in at 16 pounds, with 38 Nicad cells for battery power. It looked to just hang around in the sky while flying, but ground checks showed that it was actually flying at about 45 Miles Per Hour.

I ran your numbers of a 53 inch wingspan, and about 600 square inches through www.motocalc.com, playing with the motor/battery to get 30 ounces model weight. Motocalc predicts that your stalling speed with the 30 ounce weight will be about 13 Mph, so your flying speed would likely be in the range of 25-30 MPH.

So if you tried to fly that model in even 5 MPH winds, it might be a real handfull. I'm by no means an expert in aerodynamics but something called Renolds Effect has a lot to do with scaling down a full sized airplane down to a model size. What it means is if your full size airplane flys at 80 MPH, a 1/4 scale model might not stay in the air at 20 MPH.

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Old 07-08-2011, 03:56 AM   #11
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My current PA-12 Super Cruiser ARF (50") weighs about 36 oz. with a 3s 2100 battery and park 400 size motor. It has pretty nice characteristics but to really slow down the landing speed I have to dial in flaperons. Without flaperons it is a little hard to setup landings in a small park (given soccer goals, backstops, lots of trees and the occasional person or dog) but it is not impossible. At 1/3 throttle it looks quite scale in the air but handles a breeze way better than my 17 oz. (48") Mountain Models L-4.

The two planes I am currently building have dedicated flaps. My main interest is park flyers that are about 2.25 lbs. or lighter.

I am not taking anything away from the MM L-4, it is a really great plane. I would highly recommend it for Cub fans.

Dave
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Old 07-08-2011, 05:22 AM   #12
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You might also look at the balse being used. Crazy as it may sound, some pieces are stronger than others. Dried balsa is the vegetable kingdom's version of foam, that's why it is so light, and so soft. So when you are building the sections of the plane that are going to be contributing to its structural strength, use the densest pieces of balsa you have.

How do you tell? Denser wood is heavier and harder. So if you lack the means of telling which piece weighs more for the same size, then use the fingernail test. The balsa that dents easier is usually the less dense, and therefore the weaker piece.

The first thing I do when building a plane is to take all the sticks and seperate all the same size pieces (for example, 1/8" x 1/8") into two piles; one hard and the other soft. The hard pieces I use for what I can see from the plans are going to require the most strength, like spars, and the softer, lighter weaker piece for "non load bearing" members.

Obviously with the precut parts you're pretty much stuck with what they give you, unless you want to go to the hobby shop, choose your own wood and cut the pieces yourself, which is what a good scratch builders do.

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Old 07-08-2011, 05:41 AM   #13
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For these builds I did select the sheet balsa which was cut into strips with a slitting tool. However, the selection of balsa is not so great, so finding a soft, medium or hard sheet isn't always possible.

This is one reason I ordered some paulownia. Has anyone else here heard of it or tried it? It seems like a perfect choice for using with balsa in a build.

http://www.worldpaulownia.com/html/p...warehouse.html

"Lightweight
Paulownia is about 2/3 the weight of the lightest commercial wood grown in the US. It weighs an average of 14 to 19 lbs per cubic foot. Paulownia is almost 1/3 the weight of Oak (44 lbs p/cubic ft) and half the weight of Pine (30 lbs p/cubic ft).
The specific gravity of Paulownia ranges between 0.23 to 0.30 (23 to 30% of the density of water).
Strength
Paulownia has one of the highest strength to weight ratios of any wood.
Strength modus of rupture MOR (psi) of Paulownia is 5740".

Balsa has a modulas of about 3200 and this means paulownia is about twice the strength.

http://www.paulowniasupply.com/paulownia_vs_balsa.htm
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Old 07-08-2011, 10:47 PM   #14
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Have heard of Pawlonia, but that's all. If you can cut it down to your required sizes, it should be good stuff for what you're aiming to build. I use spruce for the likes of model I mentioned because it's not too hard to buy and offers advantages in strength.

By the time I've scratted through balsa looking for suitably hard, straight and consistent stock, I'd have had spruce delivered from a MO source or found some in a local craft shop - both are usually consistent enough that little gets tossed due to warping, twisting knots or suchlike.

Balsa's big snag, barring the chances of actually finding stock to do the job in strip/stick sizes is there's good chance of finding soft spots in the middle of otherwise hard stock.

Every sheet of balsa I buy is weighed and marked, plus the (in)famous 'quarter grain' stock is pulled aside, marked and stored seperately for making wing ribs. have found that, over the years, remarkably little wood doesn't find a home in some model eventually, so I can't be accused of wasting it

If anyone has doubts as to the strength of a strip-built lightweight fuselage, find a 'Lazy Bee' plan and look carefully at how Andy Clancy designed it. I've done some horrid things to Lazy Bees over the years and, without doubt, Andy's fuselage framework is not only the strongest I've come across, it is also very light in weight.

Okay, most of it is made from large holes with minimal strip wood trimming round the outer edges! But all aircraft are a compromise - between light weight and strength here.

D
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Old 07-09-2011, 12:34 AM   #15
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I'll echo Dereck. I buy my balsa in bulk from folks like Balsa USA, then sort through it for density & grain. "Iron" balsa, for example, I use for spars and high-stress places.

Most of what I build comes in around 15-20 ounces, 252 to 288 sq. inches, and 80-100 watts. I use a lot of 1/8" sq. balsa in my fuses, usually with 1/16 or 1/32 sheeting for the fuse sides from the wing TE forwards. Strength isn't an issue, as the planes don't hit hard enough nor mass enough to damage their structures.

We don't care to fly in the wind. We used to fly glow, up to .61 in size, but our philosophy hasn't changed (I've been flying R/C for 40 years and my wife's been flying for 37). Even when we flew heavy glow ships, we didn't fly when it was windy, because it just wasn't fun. And, this is a hobby, which we do for fun. When it stops being fun, it stops being worth doing.

payne9999, have you tried slipping the plane in for landings? We have several Piper-type models, and all of them do great slips. That's another reason we don't care to fly in the wind; we like to do our slips!

CD
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Old 07-09-2011, 02:00 AM   #16
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Yes, I have started practicing slips. I have been flying rubber powered free flight and RC for over 30 years but I always flew at a large facility where it wasn't necessary to slip the plane in. I have been away from RC for about 10 years and so I am rusty and need to keep practicing to get enough coordination to slip the plane properly.

It is effective if you don't get too fast a descent rate. When there is a cross breeze it would be handy to crab the plane just enough to deal with the breeze. Lately I have been going to the park that is 2 minutes away at 5:15am cause there is no wind at all till about 9am.

I have been flying about 4-5 mornings a week shooting touch and goes for an hour or so.

Dave
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Old 07-09-2011, 02:11 AM   #17
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You've been getting a lot more stick time than I have, that's for sure!

Like you, we prefer to go flying early in the morning or in the evening. Our morning field is on a desert-like hilltop, so the winds pick up around 0900 these days, and the heat becomes really obnoxious by 1000 (Can't put up a shelter, as it's an active full-scale airfield). Our evening field (we just joined the club) is grassy and more comfy, but it's been raining every evening since last Sunday (Morning and evening are also dictated by the Sun's position).

Sounds like you have the situation well in hand!

CD
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Old 07-09-2011, 03:57 AM   #18
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What are "slips"? Besides women's lingere, I mean.

"Give a man a plane and he'll fly for a day.
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Old 07-09-2011, 05:40 AM   #19
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I always balk at using heavy wood, ie, spruce 1/8" sq instead of balsa 1/8" sq. My mentor showed me that one can carefully choose a length of 1/4" sq that weighs less than the spruce 1/8" sq, ie increasing the SECTION (and strength) by 4 and reducing the weight! I built a Bristol Scout using that philosophy, and the thing was feather-light, and very, very durable (I used it as a trainer!).

Remember it ain't the crash the destroys the airplane, it is when the tail catches up and stomps all over the fuselage. One CAN build light, but strong.

I have built a few Brit kits; their access to quality wood is limited, so they design for lots of space in between components, and space don't weigh anything!
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Old 07-09-2011, 08:09 AM   #20
payne9999
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Originally Posted by FlyWheel View Post
What are "slips"? Besides women's lingere, I mean.
It is when you peel off altitude and to some degree speed to make a short approach to landing by holding some opposite rudder to aileron....ie left rudder with some right aileron. It makes the airplane crab or come in turned in relation to the direction of travel. It creates a high drag situation while increasing sink rate.

normally if you come in steep on short final the plane will have too much speed and especially a cub like plane will then balloon and try to keep floating along defeating the short approach. At the last minute you have to straighten the plane out to avoid smoking the tires or going into a nasty ground loop.

Here is a gorgeous example in an L-4 Cub doing a really short approach using the "forward slip":

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Old 07-09-2011, 02:52 PM   #21
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Originally Posted by FlyWheel View Post
What are "slips"? Besides women's lingere, I mean.
There are two kinds of slips, forward slip. and side slip.
Both cases require you to deflect the rudder the full amount.
This angles the fuselage to the direction of flight causing enormous drag.

I said move the rudder to full extreme position, that gives the most drag and the most altitude is lost , rudder can be reduced to control the descent.

Direction control is then maintained using opposite aileron and by varying the aileron deflection.

If the plane maintains a straight ground track along the extended centerline of the runway, you have a forward slip.

If the plane travels sideways while slipping, track is either left or right and forward, you have a side slip.

While slipping, the airplane cannot stall as long as you can hold the wing down with ailerons.
If the wing comes up, all bets are off.

Throttle should not be on while slipping, it's like driving a car with the brakes on and giving it the gas.
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Old 07-09-2011, 10:03 PM   #22
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The cute part of asking a half-dozen aeromodellers how to perform any aspect of building is that you'll get around eight different answers.

By the time the last one has offered their suggestion, the first two to answer will have had a re-think and come up with new ways to tackle the issue

D
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Old 07-09-2011, 10:57 PM   #23
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Originally Posted by Dereck View Post
The cute part of asking a half-dozen aeromodellers how to perform any aspect of building is that you'll get around eight different answers.

By the time the last one has offered their suggestion, the first two to answer will have had a re-think and come up with new ways to tackle the issue

D
And.....by the time I get done with this build I will have 3 Super Cubs: One built from mostly balsa, one built from spruce/balsa and one built from paulownia/balsa.

One for no wind (when I am feeling over confident), one for light wind and one for steady breezes.....

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Old 07-09-2011, 11:50 PM   #24
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Not a bad idea, different weight planes for different wind conditions. Then again, you can always buil light and if you need a heavier plane just add some lead. Or a camera, or a larger battery pack...

Oh, and thanks for the explination of slip. I think I get it. Kind of like side skidding, right?

"Give a man a plane and he'll fly for a day.
Teach a man to build a plane and he'll fly for a lifetime"
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Old 07-10-2011, 12:50 AM   #25
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Kinda....By turning the wings at a radical angle to the direction of travel you are reducing lift. The effective or usable wing span is reduced by the tangent of the angle (I think? insert trigonometry here____) giving the plane a steep short landing path but by cranking the rudder hard the fuselage and fin etc. are being pushed into the wind sideways creating a slow draggy steep descent.

In the case of my 50" span Cub the effective wingspan in a really hard slip would be 35" or so which would kill a lot of lift.

The guy with the L-4 in the video had the plane turned almost 45 degrees to the flight direction which means the wing lift is reduced to about 70% (tan of 45 degrees=.707) of the original span. The actual effect on wing area and actual lift is likely much more complicated. It is like having spoilers and drag brakes extended. He is using the whole side of the fuselage, vertical fin and rudder as a giant speed brake.


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