Why hollow moulded?

[davidleitch]

Why are 95% of F3F, F3J world championship contestants' models hollow moulded?

Is it strength, surface finish, airfoil reproduction accuracy, weight or looks?

Where does the true advantage of hollow moulded lie?

regards
dave

[MDM]

Originally posted by davidleitch
Is it strength, surface finish, airfoil reproduction accuracy, weight or looks?


yes^5 + ARTF

[Ollie]

The economics of hollow molded construction are interesting. The initial tooling costs are very high. The per unit labor costs are fairly low and offset the amortization of the initial tooling costs if enough units are produced. Setting a unit price that allows a profit requires walking a tight rope between high unit volume and low unit margin of profit. The producer must have a very good idea of how many more units he will sell if he lowers the price an increment.

There is considerqble risk in this market because of the high initial cost and the market's tendency to follow fads. Major contest results can make or break the market for a particular plane. It is ironic that results are riding more on the pilots skill than the objective performance of the plane. To me it is even more ironic that most of the people who buy a plane on the basis of its contest record probably won't improve their contest performance as a result of what they buy. Except at the very highest levels of competition, contest results depend far more on practise, reflexes, eyesight and pilot experience than they do on equipment selection.

One of the advantages of hollow molded construction is that the weight and strength can be varied without affecting the tooling. The same tooling can be used for versions taylored to F3B, F3J, Thermal Duration and possibly even F3F.

[davidleitch]

Ollie

As a stock broking analyst your comments on fixed and variable costs are of interest. Your comments suggest that hollow moulded plane prices would fall further if they were produced in greater quantity. Perhaps there is a market opening there.

Of course I agree with you about pilot skill and model qualities. I always use the example of the guitar. If the average guitarist tried to play, say Jimi Hendrix's, outfit nothing but horrible noise would result. You have to match the player/pilot and the instrument/model.

However I'm still not clear on what the no 1 advantage from the user perspective is.

[MikeC]

Many of the newer molded or molded/hybrid sailplanes are from Eastern Europe.

Initial costs in eastern Europe are not nearly so high when taking into consideration that buyers are paying top money in US, Canadian or Euro dollars.

A friend of mine spent several years in Lithuania setting up tooling for the production of the Genisis Sailplane. According to him, the costs of CNC machining was less than the hourly min wage here in the USA.

Building molded sailplanes with this scenerio is much less of a risk.

[Neil Stainton]

I think the advantages, in order, are:
-surface finish
-airfoil reproduction accuracy
-strength (easliy adjustable)
-torsional stiffness

When modern sailplaines moved from wood/fabric (or metal in UK & US) to glass the performance gain was dramatic, but the initial glass gliders weren't very different in shape to the conventional gliders they superceded. Their performance gain was mostly from the improved surface finish, reduction in waviness, improved accuracy, and a reduction in gaps. Lately the introduction of carbon has allowed further improvementd through higher aspect ratios.

Neil.

[Ollie]

Objectively, from a user point of view, the buyer is getting a ARF that will stand up to hard zoom launches. This is quite unlike the lower priced ARFS built with razor blade carpentry that are almost guaranteed not to stand up to a zoom launch. The molded ships have the distinct advantage of a wing without covering sag between the ribs. This sag renders the airfoil inaccurate and the wing somewhat inefficient by comparison. Molded wings also have the advantage of a better strength to weight ratio than a foam cored, vacuum bagged composite wing.

The nonobjective advantage to the customer is that he owns a plane with superior technology just like the ones the champions fly.

[davidleitch]

The modern D box wings don't have any noticeable sag (graphite, omega) and have spars that are claimed to be just as strong, yet they are still clearly less popular.

I guess it comes down to a combination of factors, mainly driven by strength:weight ratio. Should add that I have a d box (organic) and hollow moulded plane. Both are excellent. The hollow moulded wing probably has a lower strength weight ratio than the D box. It may be stronger at the trailing edge but its clearly heavier. spar strength is comparable.

I actually think that these days, being too light is not an advantage.

Ollie

for all the cynicism sm about the average customer, including me, and which I share, the real point is that the competitors, including the winners, of all these major comps will almost certainly be flying molded wings. Presumably these guys do know what they are buying and why.

By the way I checked the spelling of mold/mould. It seems that either is acceptable.

[Neil Stainton]

Why ask a question, and then ignore the answer? Maybe you will listen to direct quotes from Prof Fred Thomas's book "Fundamentals of Sailplane Design", P23:

"If the roughness is greater than Kperm the drag increases directly with the dregree of roughness"

"It is evident that surface roughness as small as 1/100mm to 1/10mm may be detrimental. Surface waviness has a similar effect."

This is for full size sailplanes, and the allowable roughness functions (there are two of them, for laminar and turbulent flow), are dependent on reynolds number.

Even for full size sailplanes the waviness/roughness has to be measured with sophisticated equipment, it is not evident to the naked eye. For model reynolds numbers the max waviness would be a lot smaller.

Another benefit I mentioned was torsional stiffness. This effectively sets the max speed the plane can be flown. The hollow moulded wing will be better than the D box wing as torsional rigidity of a shaped tube is a function of cross sectional area of the tube.

BTW how can you conclude that it comes down to strength:weight ratio, and then say that low weight is not an advantage???

Neil.

[Ollie]

Neil,

I agree with most of what you say but consider that the boundary layer thickness increases with decreasing reynolds number and the boundary layer thickens as the flow progresses from leading edge to trailing edge. It is my understanding that the affect of waviness or a bump on the surface depends on its height compared to the thickness of the boundary layer. If my understanding is correct then more waviness or bigger bumps can be tolerated on models than on full scale. Also, bigger bumbs can be tolerated near the trailing edge than near the leading edge. The problem isn't just one of two dimensional flow. The covering sag between ribs introduces bumps in the spanwise lift distribution that increases induced drag.

David,

The abrupt change in countour on the upper surface at the aft edge of a D-tube is minimised by close rib spacing, a wide D-tube and thin low cambered airfoil. The covering sag on a two or three meter sailplane model with close rib spacing, a wide D-tube and low curvature of the airfoil upper surface may be small enough that its detrimental effects will be hard to measure. This is not the case for cheap ARFS with 3 inch rib spacing.














the problem isn't just tw

[davidleitch]

Neil

I apologise.

Your response was brief and to the point and backed up with an example. I was very busy at work today and didn't read your response properly even though I kept replying. I though it was the change in airfols that made the difference between the "old" gliders and the new ones. Now I note your observation that it was the change in surface (reduced drag) that improved peformance even thoug the foil was the same.

I will pay more attention to surface finish. I know I like the fnish of molded wings and now I know why.

I am surprised that torsional (twisting) strength is that relevant.

As far as strength to weight ratio goes, I was only observing that built up D boxes have a weight advantage over most molded wings I have seen and that the weight advantange might give them a better strength:weight ratio despite lower strength.

thanks

[Neil Stainton]

David, thank you for the apology.

> I will pay more attention to surface finish. I know I like the fnish of molded wings and now I know why.

Me too! My first moulded model (an Avionik pylon racer) was also my first brushless. Its performance was outstanding, and I wondered it it was due to the Aveox 1010 motor or the "mouldedness" of it. I found out by replacing the motor with a good brushed one (Velkom 2020/20) and it still performed excellently, just not for so long! However although I had "prooved" that it was the moulded airframe that was responsible, like you I couldn't work out why. It wasn't till I read Prof Thomas's book (highly recommended) that I found out.

> better strength:weight ratio despite lower strength.

OK, now I understand.

Neil.

[RCheroske]

There is one more advantage to a molded wing. You can build a wing shape that couldn't otherwise be done with vacuum bagged or built-up wing structure.

[Doc Data]

The latest airfoils making the circuit only differ between each other by very small amount. The accuracy that can be obtained by molding a wing allows this to difference to exist.

Oh yeah, I forgot! A molded plane makes a great sound in a low, high speed pass over the field! :D

Dave

[Neil Stainton]

Originally posted by Ollie
I agree with most of what you say but consider that the boundary layer thickness increases with decreasing reynolds number

False. The bounday layer starts at zero, and increases in thickness as it progresses towards the trailing edge. Thus it increases in thickness as the Re increases. To put it another way, the boundary layer is a convex shape and is relatively bigger at lower chords/Res, but still does increase with chord/Re.

Maybe you are thinking of the drag coefficent that does indeed decrease as the Re increases.

and the boundary layer thickens as the flow progresses from leading edge to trailing edge.

True.

It is my understanding that the affect of waviness or a bump on the surface depends on its height compared to the thickness of the boundary layer.

True.

If my understanding is correct then more waviness or bigger bumps can be tolerated on models than on full scale.

False.

Also, bigger bumps can be tolerated near the trailing edge than near the leading edge. The problem isn't just one of two dimensional flow. The covering sag between ribs introduces bumps in the spanwise lift distribution that increases induced drag.

Both true.

I didn't reply earlier because I needed to check the book myself, rather rely on my memory!

Regards,

Neil.

[Phat2Wheel]

Maybe some resident aerodynamicist can comment on this. I've always thought that the turbulent BL gets tripped earlier with higher RE. However at lower RE (very low), when the turb BL get tripped, it may forever be separated. At higher RE, the BL stays attatched due to higher flow energy.



Originally posted by Ollie
I agree with most of what you say but consider that the boundary layer thickness increases with decreasing reynolds number

Originally replied by Neil Stainton
False. The bounday layer starts at zero, and increases in thickness as it progresses towards the trailing edge. Thus it increases in thickness as the Re increases. To put it another way, the boundary layer is a convex shape and is relatively bigger at lower chords/Res, but still does increase with chord/Re.

Maybe you are thinking of the drag coefficent that does indeed decrease as the Re increases.

[gwf_fly]

Davidleitch,
Advantages of hollow molded:
-very accurate
-knife edge trailing edges
-higher aspect ratios
-torsional stiffness
-tremendous energy retention
-can cover vast amounts of sky
-as of recently, one can order a molded to suit one's needs(slope, F3B, TD, etc. They are making them a bit lighter now without getting too sketchy on buckling strength.

Some disadvantages:
-more expensive - $900 on up is typical
-usually start heavier
-if you kink a wing, can be very difficult to repair
-In all-out, dead air TD, hard to beat a good bag winged job coming in at 8.3 oz/sq. Ft. when the typical molded runs close to 10 oz./sq. ft. That could cost you hitting your time, especially if you are less experienced with a molded.

Bottom line, I fly both types. My NYX for the heavier days, and my PSYCHO for the light days. Both can be ballasted for changing conditions. It depends on how aggressive I want to fly. :cool:

[bfraser]

Interesting thread! If I understand all this correctly then a plane such as an 3.1m Electric Graphite which has an all up weight of 72oz with 10 cells would not thermal as well as a Molded 3.1m Graphite which weighs 80 oz with ten cells unless there was no lift or wind. And this is because the molded wing is more accurate, smooth and has more torsional stiffness? As I plan on buying an electric Graphite shortly I sent an email to Vladimirs models (they make the Graphite) and I asked which Graphite sailplane version would be the better thermal sailplane in all round conditions. He told me that for pure sailplane (non electric) the molded version would be better but for an electric powered one either the 3.1m or 3.45m full D-box Graphite would be the better choice. After reading some of the posts people made about the lighter Graphite sailplanes being better for dead air days I'm now thinking that as there are hardly any days when I fly that there is not at least a bit of wind I should buy the molded cross tail electric Graphite but I am still a bit unsure especially considering what Vladimir said. Can anyone comment a little further on this?

[Doc Data]

I think when you are considering sailplanes of as high a caliber as we are talking, getting one plane or the other becomes far less important than the skill of the guy on the sticks!

Buy a quality plane that pleases your eye and wallet... then fly the heck out it. I don't think, unless you're a top end flier, you will ever really notice a difference!

Dave

[davidleitch]

The heavier plane will climb slower, turn slower and land faster. On the plus side it will retain more energy.

Whether a molded plane can overcome these disadvantages via reduced drag, increased torsional stiffness and higher lift (due to better airfoil reproduction) to give longer or "better" flight experience is an open issue.

Molded planes will certainly suffer less hangar rash.

Bottom line is I don't really know but its clear that for non electrics the competition pilots do go molded almost without exception. I can't see how electrics would change the situation.

The spar in my Organic and the airfoil reproduction from the carbon/kevlar molded d box looks to be just as good as the equivalents in my molded spiro. However it is easier to twist the tips of the organic (torsional strength).

In absolutely still air I think the lighter d box plane would stay up longer, but its hard to tell.

[gwf_fly]

Hi Davidleitch,
I see some responses to what I wrote about - molded versus other construction. Let me just expand on a few things, and perhaps it will help clarify what I was saying the first time around. I agree, the guy at the controls is the biggest factor. It is not a matter of one being better than the other, but what the pilot wants to accomplish.
I just finished 2 days at the Sacramento Spring Fling. Two great days and nine rounds of TD!
If you look on line at the scores for the contest, and then look at the top 20 scores, I recall that there is still a slight edge for the NON-MOLDED type of ship amongst those flyers. For example, I did not see the likes of Daryl Perkins flying one of the ready-made molded ships from the Czech republic! There are plenty of bagged winged ships among the top flyers including the likes of Fred Sage. However, at the same time, I bought my all molded NYX from whom many consider to be one of the greatest pilots around, Ben Clerx. He was designer of the Mako (a bagged wing ship). We fly the SC Squared circuit here in So. California. For those not familiar, you can pull it up on RCsoaring.com under CLUBS.
Yes, in general, less hangar rash with the molded, but if you buckle the wing half way down the span, it can be a handfull to try and repair it when compared to a bagged wing which is simple to do using by the foam wing beds. Where we fly here, often times the day starts out with low overcast at the time of the first and second round. This can be dead still, and very tricky to get a 6 min. round under those conditions. I am forced to camber my molded (HN 354 SR foil)under those conditions, and I have to be very aggressive. With the Psycho (7035 foil), it is more of a cake walk to get the time in cool, calm, overcast conditions. With either plane, one or two sloppy turns can cost you!
I know several guys I fly with who all had tried the molded ships at one time or another in the last 5 years. Some of these ships were very tempermental, and they had some really nasty stall characteristics - despite all the hype. However, the molded ships are getting better all the time, and I think it comes down to your flying style and what you want to do with the ship.
As for me, it's fun to put my NYX on a straight dive speed run from 900 feet while loaded up with another pound of ballast! Nothing sounds quite like "Moldie." I just don't normally do that with the bagged wing ship. I prefer to save that one for dropping the nose on the dime!

[davidleitch]

Very interesting commentary GWF.

I am sure we are all agreed that pilot skill comes first.

It also appears to be that in some competitions the plane is a relatively more important part of the result.

For instance Tom at www.f3x.com observed:

You will notice that most models score well in the AMA TD column. The reason is that a good pilot with a decent model can win a TD contest. The same can almost be said about F3J, The toughest part in F3J is a model that will survive a good competitive launch and hang with the competition.

The F3B and F3F tasks depend more on model performance than F3J and AMA TD._ You need a good sailplane to win here.

I guess good means purpose built. The conclusion on molded planes seems to be:

strongest and lowest drag best airfoil reproduction but typically at some weight penalty. They also seem to be the most expensive despite lower variable costs than a bagged or D box wing.

The better D Box wings are made in a mold (graphite, omega) so the difference is only behind the spar in any event. I know even less about bagged wings.

[tempest411]

I'm not an expert pilot, but I have flown a fully molded, bagged, and molded LE/built up TE planes and I think the bagged has been the best compromise for me. My NSP Edge is an easy flyer, but still super responsive, and is easy to repair...but very durable so I haven't had to do any of that yet! I think the bagged planes offer a better value as well. You just get a lot for what you pay! I don't believe molded planes, at least the Hera I got cheap, is more ding resistant than the bagged planes. I've just started trimming out the Hera, so it really too early to say a whole lot, but I can say that even thought it's within a few ounces of the Edge(about 70 oz), it really, REALLY retains a lot more energy! I haven't noticed any problems with tip stalling, but as fast as it wants to fly, I feel hesitant to slow it way down like I can the Edge. The molded LE/built up TE plane I had was the Amethyst. It flew kind of like the Edge, easy, responsive, but it wouldn't go as fast as the Edge, and definitely not as fast as the Hera, when you wanted to do so. In the end, I think a plane only needs to be good enough for you to learn from and have fun with it. Unless you're out practicing as often as some competitors do, you'll never realize the higher potential a $1700.00 Icon will allow you to achieve compared to a $600.00 Compulsion or Fusion.

Rick

[Doc Data]

I agree with most of what Rick says but I think basing his preference on a comparison of a Complusion to an Icon, while true, is a little lop-sided. The Icon is a pinnacle in design (and price) and, IMO, not worth the price for the average flyer. How about comparing the Compulsion at a little over $600 to a molded Graphite at $750? I have both these planes and love them both. My Compulsion has a SD7037 airfoil and very light making it great for light wind days. The Graphite with a MH32 airfoil is a more all-round plane and handles almost all conditions.

So, you see, in the end, the decision is obvious. BUY THEM BOTH!!!:D :D :D

Dave

[Les Horvath]

Hello All!
I have to disagree with all of you that say that molded wins are better! First of all it is ridiculous to pay such a high pricefor a TD glider! as far the accuracy is concerned I can cut (and advertise)
less than 0.005" accurate, but I can cut 0.002" or even less! I do
not want tohurt all you good peoples filings, but Feather cut and
Muller type of machines DO NOT CUT AS ACCURATE CORES than I can! With the molded wing, you like it or not stuck with an expensive glider! The good exemple is Harley Michaelis "Genie"!
He built close to 30 models with little changes and experiment that would be not possibile if He had to use CNC molds. Lot of
people fly different gliders and airfoils of thei likings! You also
would kill the satisfaction that you come up with a glider that flies better because I changed thickness, camber or what ever, and it is my creation, my secret! Molded wings will somevat kill your creativity, imagination! So all you good people you opened a can
of worms don't blame me!
Love you all:
Les