Lighter Models Are Safer And Fly Better?

[Dai Fledermaus]

I've just been rereading Peter Miller's book Designing Model Aircraft, particularly the Chapter titled Lightness Equals Strength where he highlights the need to save weight whilst designing and building model aircraft.

Now, I understand the thinking behind it and wouldn't want to question it, but given that in the UK weather conditions are often less than favourable, particularly in this part of south East Wales, where we haven't had many days this year without a significant wind, if it's possible to have a model aircraft that is too lightly loaded as a result of weight saving.

I think the point I'm trying to make is, how do you compromise between building something which has too little a wing loading, something which will float around on a windy day as some vintage models tend to do, and a flying brick.

[The Wright Stuff]

I've often wondered the same, and would be interested in seeing others' responses.

To be honest, it really depends upon the purpose of the particular model. For example, if a low-wing trainer is built as a platform for learning to handle heavier wing loadings before moving on to warbirds, then building it too light is certainly a bad thing.

'Too light' could also imply 'too fragile'!

Having said all of that, from a building perspective (the original context of the book), the bottom line is that it is much easier to add weight as ballast than to remove it at the end!

[chris Bond - Bondaero]

A plane that handles the wind well is likely to have a symetrical wing section and a good amount of power. There is no benefit in adding weight if you have the right design so the theory hold and build light. However lots of strength in the landing gear is a good idea.

[Steve Houghton 1]

Hey Dai - I fly in SE Wales but I am strictly a glider guider flying the mostly HUGE slopes in the area. And as you said, more often than not we have significant wind. In fact, I have only been able to fly my lighter thermal soarers from a flat field about 4 or 5 times this year, these needing a maximum of about 10 mph to fly in.

But we too have different models for different wind conditions. Some light weight models for when the wind is under 10 mph, others that will fly 10 - 25 mph, and others that fly best in 20 - 40 mph.

My Polecat EPP60 racer is a good example of that. There's no point trying to chuck that off a significant hill with anything less than 20 mph as it has a fully symetrical wing and weighs 2 ¾ lbs without ballast.

And talking of ballast. A flying buddy has a 3m F3F racing glider, the Baudis Pitbull which is fully glass moulded. That model flies like a pig unless it's carrying 1kg of ballast in it, then it flies very sweetly indeed.

Steve

A470soaring

[Geoff S]

It's funny that the two models have that I'm prepared to fly in windy conditions have very low wing loadings - my foamie Riot and a plan built Limbo Dancer fun fly. With the latter model I've had more damage after landing when it got blown away! I think it might be psychological in that I don't have a lot of time/money invested in either so I'm more relaxed. Even though I'd hate to total the Limbo it wouldn't take long to make another.

Geoff

[ChrisB]

In my experience heavier models fly far better than lighter models. That said most of my models are 30-80cc.

I had a Duncan Hutson TigerMoth. It weighed 15lbs and flew ok, but was no good in winds of any kind.

I have a Flair Stearman of the same scale and size and it weighs 27lbs and copes with crosswinds and head winds superbly. Bigger models have more wing area and can fly slowly. Its a bit different for small models.

[Jon H]

From my own experience the key is to get the model to a weight appropriate to its size/wing area etc.

For example i fly a number of 80-90'' span WWII fighters and if they are around 18-24lbs they fly very well. My sea fury is the heaviest at 23lbs and 80'' and it is the best to fly while actually in the air. The weight gives it a momentum which makes it look more realistic than my slightly lighter La7 at the same span but 3lbs lighter. That said, the La7 is a breeze to land where as the Fury needs a bit more speed and is not as totally care free. That said its not exactly hard.

I also experienced a model that was too light as i flew an old mick reeves 80'' hurricane with no retracts or anything. It didnt weigh much more than 11lbs and it was awful to fly. It just felt like i was trying to fly a paper bag. We fitted a bigger engine and retracts which brought it up to 14lbs and it was much improved

Chris, your comments about the Tiger Moth are interesting as my 1/4 stampe is 14lbs and i have flown that in a stiff old breeze. That said the ailerons on the stampe are more powerful so that is probably a big help

Edited By Jon Harper - Laser Engines on 07/11/2016 16:43:45

[Stuphedd]

I dont think you can talk about heavy or light without bringing in what I feel is a more important factor and that is wing loading .

You can have a model weighing 20 odd pounds but if it got 20 sqft of wing area it will fly beautifully slow but have enough "inertia " to iron out "turbulence "

I am a believer that "lightness " is best but here I am really referring to wing loading .

My favourite , flew it yesterday in the wet and wind is a 13 oz foamy with 1.5 square feet of wing area , very very agile so you can fly it any time anywhere !

cheers

[Peter Miller]

An interesting point. A lot to do with it is wing loading and power. Obviously the "floaters" won't like wind but take models like my Jezebel and even better Gold Fever.

These are about 440 sq inches of area, wing loading 18.5 oz. per sq foot, .25 2 stroke and can be flown easily in 20 mph plus winds. Now no one could call the model heavy but it uses a semi symmetrical section and is set up as 0-0 degrees.

That wing loading is very reasonable by anyone's standards.

My first Marauder for .25 size engines with similar set up was flown in a wind gusting up to 30.

On the other hand I once saw a scale Hurricane of 46" span that had an incredible wing loading of well over 30 ounces per sq, ft. It crashed on take off far more often that it actually flew.

[ChrisB]

You're right Pete W. Wing loading is the key.

Are you still flying those Makos Pete? You love those windy days!

Cheers

CB

[Peter Miller]

These days I am not fond of fast models and wind. My Super Marauder is fast enough and handles wind very well. The small racers are geat but I don't enjoy trying to keep up with them. and I ended up letting others fly them.

[Former Member]

[This posting has been removed]

[Steve Colman]

I'm in the same camp as Geoff above.

When I consider it a tad too windy to fly, I bring out the EPP "Hummer" from HK. For those that don't know it, it's a 1m, 3D capable aerobat weighing in at about 650g. It's an absolute blast to fly when the breeze picks up; sometimes even moving backwards in relation to the ground.

And yes, While I wouldn't want to wreck it, the fact that it's cheap and easily replaced is perhaps the psychological factor that works in it's favour.

[Stuphedd]

In answer to Chris B's comments ,

Yes I am still flying those Makos , The only thing that you can pull out of a blender at 4ft ! I have managed to break off 3 TX sticks so far on the recovery ,but being light , no damage to the airframe .

Quick ?? small models look to be going very quickly , My AXI re-engined GE BEE R3 foamy thing looks mighty quick , but a pussy cat and really easy to fly , where as my mini Stryker is the reason I take a flask of coffee !Like something stronger but I have to drive !

Relaxation is a Visionaire, As you know our small site you know its for smallish "light" models .

Light is best-----wing loading I mean !!!

cheers

[TigerOC]

If you think about this; Force of impact=mass x velocity

Wood and foam will have some level of plasticity. When you exceed the plasticity it will break.

I have recently started designing based on existing designs and have a few observations. 18 months ago I designed a composite fuselage slope soarer off an existing design (Charles River Radio Control Club LS1 (La Sierra 1) also copied by Sig as the Ninja). By working formers I was able to make sure frontal shock was taken in the wing seat which is able to bulge outward on hard landings. Adding any form of bracing in that area resulted in damage before or after the former.

Last year I designed a club contest model. The design parameters were cheap, small and reasonable cost. The design is based on the Das Ugly Stik. 31" wing span with cruciform depron fuse, balsa battery box inside the fuselage sides and a depron armin wing which provides a semi-symmetric wing section. The auw weight is all of 450g and capable of speeds up to 55mph. Total cost with all the gear £50.

You would think that it would only fly in calm conditions. We fly them in wind speeds of up to 20 knots and they handle turbulence really well. There are around a dozen in the club now and they are almost indestructable. They can do almost all the aeros you want and we crash them a lot. Normal failure is just the motor mount. Glue in a new motor mount and you are good to go again. Again I think the secret is allowing the impact forces to be released in the wing seat. They are probably the most popular model in the club at the moment for fun value.

Rob

[ChrisB]

Glad its still going Pete. Word has it there will be future changes to the site nearby.

I can recommend the Radian XL Glider!

[Colin Leighfield]

I suppose there's always a balance in these things. Gust response will be lower for a heavier wing loading. A constant strength wind within reason is a stable environment for the plane to fly in. Its' motion is always relative to the air it's moving in. Most of the wind related erratic flight we see is caused by variable wind strengths and gusting. The lighter the wing loading, the sharper is the response to unstable wind behaviour.

Peter made particular reference to his Super Marauder. My Depron composite version should be significantly lighter than the original, so there's an opportunity to make a comparison, but I think the differences are predictable. Doesn't mean it will be better, just different.

[Andy Meade]

I think you mean elasticity Rob. Once you exceed the elastic limit, the material will yield and turn plastic, then this plasticity will have a limit before complete failure. Wood has no real plasticity - it will bend then break, but does not permanently deform (unless you soak it in ammonia or water, of course).

[The Wright Stuff]

Posted by TigerOC on 07/11/2016 21:42:09:

If you think about this; Force of impact=mass x velocity

If Isaac Newton was alive today, he'd be turning in his grave!

[TigerOC]

Posted by The Wright Stuff on 08/11/2016 09:47:37:

Posted by TigerOC on 07/11/2016 21:42:09:

If Isaac Newton was alive today, he'd be turning in his grave!

Do you grasp the full stupidity of that statement

[Peter Miller]

I always used to look at it this way. Inertia is the killer,

When one bit (The spinner) stops the rest want to carry on due to inertia. The lighter the other bits are the less inertia so the less desire the wing tips etc have to continue to the ground. One reason that I like a centrally mounted single servo.

[Peter Christy]

Hmmm! This is a bit of a hobby-horse of mine! I currently have two low-wing aerobatic / semi aerobatic models: An ARTF SpaceWalker powered by a Super-Tigre 45 and a KingPin (from 1963) powered by a very old Webra 61 (per-Blackhead!). Both engines provide similar power, turning 12x6 props at similar RPM.

The SpaceWalker is very lightly loaded, and while it flies beautifully on calm days, it is the very devil to land in any sort of wind. It is a real floater, and this results in two major problems. 1) In any significant wind, the stalling speed is NEGATIVE ground speed(!), and 2) on the approach, it gets bounced all over the place. You are constantly fighting to keep the wings level and the attitude suitable for touchdown. Its VERY hard work!

The KingPin came in under its design weight - primarily because it wasn't having to lug around five Bonner Duramites, a reed receiver and a 7-cell NiCad pack! (Younger readers look up some of the history of RC!) However, it still handles rough weather with aplomb. Indeed, it had its maiden flight in conditions which saw most of the rest of the club grounded!

It is VERY strong and RIGID - especially in the undercarriage area, unlike the SpaceWalker, which originally had a terribly weak u/c mounting. The KingPin has quite a blunt leading edge to the thick-ish wing, which helps smooth out the effects of turbulence no end. It was designed to fly with non-proportional radio gear, so it HAD to be able to hold its attitude on its own. The thick wing and (by modern standards) high wing loading also produce a nicely positive rate of descent, which makes approaches and landings much easier than with a lightweight "floater". Despite this, the stalling speed remains low, and it does not come into land like an express train - although it is faster than the SpaceWalker.

So there you have it: two contrasting designs of similar size and performance, but the ancient KingPin is MUCH better suited to UK climate conditions than the modern design.

We could learn a lot from those older designs!

--

Pete