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Author Topic: Winning with 15% Motors  (Read 1683 times)
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schnellwilli
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« on: February 10, 2009, 03:39:56 PM »

Here in the NE, the FAC mass launch events have been flown under the 15% rule for more than a decade. This rule stipulates that the motor weight must not exceed the weight of the empty airframe. The purpose of this rule was to cut down retrieval times and reduce the chance of a model flying beyond the field. In addition, with the “graying” of the competitors, long retrievals were becoming a challenge. It was an ingenious solution to a growing problem, conceived by Dave Stott, the co-founder of the FAC.

The 15% rule took me by surprise upon returning to the hobby in 1999 after a 20 year layoff. I hated it although the reasoning behind it was sound. It took me an entire season before I got the hang of it. The first models that I campaigned in the Greve and WWII events were a 27 inch wingspan Barracuda and a P-Nut Scale Chambermaid, both from my own plans. After being equipped with what I thought to be near optimum prop/motor combos, they were unbeatable. The Barracuda went on winning local events until one of the Air Marshalls in our club said, just before the last heat of a WWII event, “The only way we can beat that thing is if you break it”. At that point I decided to retire the model. Next thing off the board was a 23” WS Fiat G-55 which was built from a David G. Smith plan with a few of my own modifications. The Fiat turned out to be an even better flyer than the Barra and went on garnering Kanones for years until it was lost in a thermal a year ago.

Collectively, these three models won dozens of local events, placing less than first only about 5-6 times. The losses were mainly due to crashes or broken motors. I cannot recall a time when any of them were outflown. These models had a number of things in common, very light wing loading, flat bottom cambered stabs, long noses, propellers with the same pitch / diameter ratio and motors that would take about the same number of maximum turns. The 27” WS Barracuda weighed 34 grams, the Chambermaid P-Nut weighed 10 grams and the 23” WS Fiat G-55, 19 grams. Despite their light weight, the three models were built rugged enough to withstand years of mass launch flying often in windy weather. No special skill was required to build these models to the noted weights. Micro-X strip wood (circa 1975) and 6 lb. sheet balsa was used exclusively. Tiny portions of thin CyA was used on all tight fitting wood to wood joints and Duco for filling gaps. The tissue was affixed to the frame with Titebond thinned with 50% water. After the tissue was shrunk with water misted from a perfume dispenser, a few dust coats of nitrate dope mixed with about 60% thinner was applied. Had these models been equipped with plastic props, they would have become nose heavy and ballast would have to have been added to the tails increasing the final weight and defeating attempts to build them light. Besides, plastic props do not fit into my equation for success with 15% motors. Unless scraped down, they are very heavy and their pitch is too low. In addition, the Peck type props will cause excessive drag when freewheeling because their blades are very wide towards the tip where the pitch angle flattens. The advertised pitch of the Gizmo props is too high for my purposes. Sleek Streak type props may be OK for smaller models but, since I have never tied them, cannot say for sure.

Thanks to the generous tail volume of the models noted in the foregoing, the CGs could be safely set about 35-36% chord. The effect of the cambered stabs contributed to an increase in the tail volume. It is a shame that cambered stabs have been bad mouthed on some of the internet forums and a major national newsletter in recent years. I have used them on all my scale models built during the 70s and everyone built after returning to the hobby with excellent results. Cambered stabilizers are stronger and more warp resistant. The lift they create does not cause any instability but does allow the CG to be moved slightly rearward. This reduces drag (less decalage required), loop tendencies and helps to reduce or eliminate the need for ballast. . The critics of cambered stabs claim that their lifting effect causes the nose to drop and the model to “dive in”. Seems to make sense but sound theory and practical application prove it wrong. The scientific explanation as to why this theory is incorrect has been posted on various internet forums but is rather lengthy and beyond the scope of this article. Models can exhibit symptoms of pitch instability like failure to recover from stalls if the CG is set to far to the rear. However, in my opinion, the most common cause for a model diving in when the thrust drops off, is improper surface alignment. Thrust keeps the nose up during the power mode but surface alignment takes over during the glide mode. Some modelers still insist on test gliding models with the free wheeling props attached and the motor installed. Their reasoning is that this simulates how the model will glide when power runs down and it goes into the glide mode. Seems logical but the problem is that, when hand gliding a model on level ground, the sink rate will be so rapid because of the drag of the prop and the weight of the motor that is very difficult to tell what it is doing in the few seconds of flight. Hand gliding a model from a hill or rooftop would extend the time the model is in the air and, with such a test, it might be possible to make fairly accurate decalage adjustments. However it is doubtful if many modelers follow such a procedure. It has been my experience that the best way to test glide a scale model for the purpose of setting surface alignment is to test glide it balanced, but with the prop and motor removed. This is a tried and true method that I have been using for decades without a single problem. After proceeding to test the models under power, they rarely need anything more than thrust adjustments. I have included detailed explanations of trimming procedures in every construction article I have ever written in magazines and newsletters if anyone is interested.

The prop/motor combination is the most important factor when flying with 15% motors regardless of the weight of the model. I use props carved from light balsa with the same pitch distribution on models ranging from P-Nuts to my twins which have won Jumbo and Giant Scale at Geneseo numerous times. These non helical props do not have any under camber and are strengthened by a rubbed down coat of CYA. A prop block diagram and a pitch distribution chart is included with this article. (See attachments) As with popular F1B props, the pitch diminishes towards the tip. Since these props work as well with heavy motors, they do not have to be changed when switching events, e.g., the Shell Speed Dash (unlimited motor) to the Greve (15% motor).

During years of contest flying, I can only remember one incident of prop breakage.

I have devised a very simple and effective means to determine the optimum size of the rubber motor . Since the weight of the motor has been already determined by the weight of the airframe, all that has to be done is to calculate the length and cross section of a motor that will SAFELY take about 1100 turns. Another way of saying this is that 1100 turns are about 90% of maximum turns. It took me years of trial and error to finally get to this magic number which works well with all the different sized models I use for the mass launch events. I don’t know how to calculate the size of such a motor using mathematics but, through years of experience, have learned to make close estimates. In some cases is it not possible to make a 15% motor which will take 1100 turns with standard width rubber strip and the use of a rubber stripper may be required.

One thing that I have noticed during the heat of competition is that the other models will often climb higher and have longer motor runs than mine. Invariably, they come done much faster. The low wing loading of my models allows them to glide much longer. Surprisingly, these light models do very well in strong winds. They may flip flop and tumble but recover rapidly and rise above the ground turbulence.

Another curious phenomenon that I have noticed is that that these small models seem to fly better than larger versions of the same subject. My Chambermaid P-Nut will fly much better than my 23” WS version with both equipped with 15% motors. The big Chambermaid is no slouch. With a heavy motor, it won the Shell Speed Dash with 3 maxes (flyoff) and the Greve at Geneseo, 2006. I built 26” WS versions of both the Fiat G-55 and another Smith design, 19 gram, Reggiane RE-2005. Both of these models fly well with heavy motors but do not do as well as the smaller versions with 15% motors. The smaller RE2005 has not seen much competition but flys as well as the G-55.

Since radial engined subjects flown with 15% motors have won the WWII event at a few major contests recently, theories are abounding as to their superiority. I believe their success to be an anomaly and that the inline engine subjects that have dominated the Greve and WWII for years will again gravitate to the top once competitors get the hang of flying with the light motors. Radial engine subjects have obvious disadvantages as compared to long nosed subjects with inline engines. They have a larger frontal area and tend to be heavier. Although I don’t fly the Thompson, I have been watching the event for years and it is obvious that the draggy radials with their short noses don’t fly nearly as good as subjects with inline engines, cleaner configuration and longer noses. Even with short motors and light tails, it is likely that they will still require ballast which will result in a higher wing loading. Heavier models must fly faster to stay in the air and drag increases exponentially with the speed. I believe it would be very difficult for even a highly skilled modeler to build a 23” WS, full scale (not a slab sided ghosty) radial engine subject, rugged enough to withstand years of windy weather mass launch flying to the same weight as my G-55 and RE2005. Anyone who has ever seen a Dave Smith plan should realize that his designs are true to scale with plenty of stringers and nice oval fuselage cross sections. One can argue that the radials with their short noses are more stable than subjects with longer noses. While this may be true, experience with my models has indicated that this is not an important factor. My light models with relatively long noses have never had stability problems even with the rearward CGs I have been using.

While my experiences with lightly built models lead me to believe that they are capable of greater duration with 15% motors, heavy models will also benefit when using the propeller/motor combination noted above. My grossly overweight, battered, much repaired Mig 9, flown for years in local Modern Military events (mass launch/15% motor), has a good contest record. While the 1110 turns figure noted in the foregoing is not an absolute, my experience has indicated that it may be within 50-100 turns of optimum. Certainly others who have been flying under the 15% rule for years have developed their own secret of success which may differ from mine. It would be nice to hear from them. All I can say is that the techniques described in the foregoing have worked for me for almost a decade.

----------------------------------------
It should be noted that the chart on the right indicates the block dimensions and pitch for the 13.5 D prop In used on my
Giant Scale TA152H. It is a modified Coupe prop with pitch distribution similar to the Andruikov. John Barker, Hepcat, used his prop picker program to design this prop. This is a different prop than used on my smaller models.

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Winning with 15% Motors
Winning with 15% Motors
« Last Edit: March 30, 2018, 03:57:41 PM by Ratz » Logged
PeeTee
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« Reply #1 on: February 10, 2009, 04:16:07 PM »

Bill

Thanks for the insight into FAC mass launches and your proven technique for success. Although I only fly duration models I found the write-up most interesting. Keep 'em coming please.

Peter T
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thymekiller
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« Reply #2 on: February 10, 2009, 08:24:57 PM »

Yes, Please!
thymekiller
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slipstick
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« Reply #3 on: February 11, 2009, 05:03:37 AM »

I'm probably going to feel really stupid when I see the answer (nothing new there) but...if the rule is motor may not weigh more than bare airframe where does the 15% come into it ?

Steve
(still confused after all these years Wink)
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schnellwilli
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« Reply #4 on: February 11, 2009, 08:48:18 AM »

The motor may not weigh more than 15% of the weight of the empty airframe. When I first got back into the hobby after a 20 year layoff, about 10 years ago, the CD of our low key local contests used to weigh the models and motors of the models that placed 1-2-3. Then he would do the math on a calculator. It was a big nuisance for the CD so we abandoned the weighing process and now fly honor system. We are a close group and I don't think anyone cheats.
Some CDs use a simple balance, the motor on one side and the model on the other with the fulcrum set accordingly.
I have not been to Geneseo since 15% was used ther and do not know how it is done. Maybe some one will tell me.
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« Reply #5 on: February 11, 2009, 12:02:30 PM »

Hi Bill,
I was helping Richar Zapf on the 15% weigh ins @ Geneseo this past summer. We weighed the model and the motor prior to the event and then the winners and those that placed promptly after the event.

Both a scale and a balance were available. It went smoothly and quickly. Not a complaint from a single flyer.

FWIW, those the flew in the event were in the air for long fiights. Most landed just short of the dreaded potatoes. I think a couple still managed
to find their way in.

I think it's a good idea for the reasons you stated earlier. Furthermore, a lot of fantastic models get lost in the crops. I understand that is a part of competitive FF, but this seemed to cut that back a bit.

Besides, it stinks to see some guy out in the summer heat beating through the crops trying to find a model when he could be on the field with his buddies flying etc.  Additionally, some guys could get hurt in there. That's not good.

I tried to help find a model in there and could barely pass. It's incredibly dense. Not worth getting hurt or losing a good model (IMHO)

Thanks for the insight on 15% flying. The prop stuff is great too.
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« Reply #6 on: February 11, 2009, 02:56:26 PM »

Thanks for such an informative article.
Lots of stuff in there; I read it twice and still need to print it off to get the best from it. Just starting scale after years of duration flying and competition. This sort of article takes a lot of time, first to acquire the knowledge then to disseminate it in a clear and easily understood way.
Respect. Smiley
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schnellwilli
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« Reply #7 on: February 11, 2009, 03:58:11 PM »

I just described what worked for me. There must be other guys on this forum who have also had success using 15% motors, perhaps using entirely different approaches. It would be helpful if they chimed in too.
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Ratz
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« Reply #8 on: February 11, 2009, 04:11:00 PM »

A link to a PDF has been added to the first post for those that are interested.

Ratz
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« Reply #9 on: October 05, 2010, 08:12:59 PM »

Hi all,

Would somebody please refresh me/us as to the math - how to figure the 15% percent of total airframe+motor weight?

thanks In Advance,

Dunce Gilbert
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« Reply #10 on: October 06, 2010, 03:09:27 AM »

Dunce, it is not 15% percent of total airframe+motor weight. In the earlier post of Schnellwilli he mentioned:

Quote
The motor may not weigh more than 15% of the weight of the empty airframe

so the math is: weigh your empty model, multiply by 1.5, divide by 10 and the outcome is the maximum motor weight.

Wout
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« Reply #11 on: October 06, 2010, 08:24:37 AM »

Hi all,

Would somebody please refresh me/us as to the math - how to figure the 15% percent of total airframe+motor weight?

thanks In Advance,

Dunce Gilbert

Weigh the motor and the plane. Multiply that total by 15%. Smiley
If the plane doesn't fly good, make another, LARGER motor and weigh it with the plane. Then multiply THAT by 15%. Grin

Is that what yer needin' to know?
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schnellwilli
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« Reply #12 on: October 06, 2010, 09:09:10 AM »

Wout got it right. That's what we have been doing at contests here in the NE for more than ten years. I have not flown at Durham lately but believe they have a simple balance stick to check the empty models on one side and the motor on the other.

BTW, how come this thread has been resurrected after it died about 1.5 yrs ago?
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« Reply #13 on: October 06, 2010, 10:21:57 AM »

The question I have actually pertains to all weight limit motors. I understand that the weight is to include whatever lube is used, but are extra do-dads like bobbins/Tim Gray hooks, rear peg counted as motor or airframe weight?

Right now, I'm just curious, but G'town is on the "next place to go" list.

Old threads will likely resurface as newer people come on board. Some will actually go thru old threads to familiarize themselves with what's gone on, posting when they come to a stumbling block (or "point of interest") rather than starting a whole new thread.
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« Reply #14 on: October 06, 2010, 10:43:26 AM »

Hmmm,
Well, for FAI models such as F1G (Coupe) and F1B (Wake) the extra Doodads do indeed count as airframe weight. Rubber weight is just the weight of the rubber motor itself with lube. As for the other classes I have complete, blissful ignorance.... Grin

Tony
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« Reply #15 on: October 06, 2010, 11:46:32 AM »

Thanks Tony. That's pretty much what I logicized (new word?).  Rulemakers and interpreters being what they are (oft logically illogical) I, and I'm sure others, am curious as I've seen mention where the "extras" WERE counted as "motor weight", or it might have been a misinterpretation by the contestant.
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schnellwilli
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« Reply #16 on: October 06, 2010, 12:01:36 PM »

Whatever is attached to the motor is counted..like lube and "0" rings that are built into the motors but not metal hooks that lock into the end loop of a motor (crokett hooks) but are removable. Few scale guys use such things anyway especially in mass launch events. The rear pegs are not attached to the motors and stay attached to the model as it is weighed.
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« Reply #17 on: October 06, 2010, 01:07:22 PM »

So an "anti-bunch" sleeve at the back would then count as motor weight, since it is essentially "captured"?
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Re: Winning with 15% Motors
« Last Edit: October 06, 2010, 01:24:43 PM by Pit » Logged

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« Reply #18 on: October 06, 2010, 01:46:39 PM »

I'd argue no as it's part of the rear peg system and doesn't contribute to rubber energy storage. Then again neither does the rubber bands used to hold the motors together but.....

Tony
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« Reply #19 on: October 06, 2010, 03:52:32 PM »

It counts.
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« Reply #20 on: October 06, 2010, 04:23:14 PM »

I don't see where that counts. It can be manually removed. I had my 8 strand Corsair motor braided onto a sleeve like that and removed it for weighing. No complaints where heard.
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« Reply #21 on: October 06, 2010, 04:28:20 PM »

OHBOY! Wormsville...
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