Break away motor plate on Cobra

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Lou Melancon
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Break away motor plate on Cobra

Post by Lou Melancon »

My new Cobras will have breakaway motor plates and a stronger center section and spar.

The standard Cobra holds up very well in my collisions and dirt naps but I have been having problems breaking the spar at the center behind the motor plate in a hard tap.

Here is the center rib I will be using on the next batch:
Image

It is balsa, 1-1/2" wide and extends from LE to TE. The thinking is that the impact shocks will be absorbed by this piece of wood instead of being transferred to the spars. The notches are where the 1/8" glass rods will sit.

The breakaway motor plate is shown below:
Image

Three 10-24 nylon bolts are used to secure the HDPE motor plate to the balsa rib. Brass inserts are epoxied into the root rib. The nylon bolts and brass inserts were purchased at Ace Hardware. The thinking is that an impact will cause the 10-24 nylon bolts to sheer off. They can then be removed by cutting a slot across them with a dremel tool and backing them out with a flat blade screw driver. The motor plates will be tethered to the center rib. Also the motor plate goes over the notch for the 1/8" glass rod to further secure it to the root rib. As shown this assembly weighs 5-1/2 ounces.

Here is a view showing the top of the wing and how it will be assembled. I believe that the glass rod glued into its slot and secured with 5 tie wraps cinching both rods down to the foam will be very strong and damage resistant.

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Lou Melancon
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Post by boiler »

Good move Lou. If you can get 1/4 inch polypropolene instead of polyethethene, it would be even better. That stuff bends instead of breaking. You might want to use nylon nuts on the top so if you have a chance to re-lauch after breaking the bolts the operation would be faster than hunting for the dremel tool.......... I have another idea that may not be as good. I'll let you know when I start to develope it.

Bob Loescher
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Post by NAES »

At the field I have had the bolts shear off and just used a lighter to heat the tip of a screwdriver and jam it into the sheared nylon bolts. Works well in a pinch.

NAES

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Pat_Willcox
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Post by Pat_Willcox »

Excellent idea. I can see good results from that technique.
I will try that also.

Pat
Lou Melancon
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Post by Lou Melancon »

Based on Bob's comments I may add a fourth bolt hole and threaded insert to the design.

I think two bolts will hold it on just fine in the air, so I can put two in it and if I shear them off then use the other two holes for two more bolts.
Would be faster than trying to get bolts out and would allow the plane to be flown again in the same match.

Lou Melancon
Alpharetta, Georgia
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Post by thojo »

I've been toying with this idea too, but instead of a horizontally mounted engine, I am gonna use 1/4" balsa sandwitched with thin aircraft ply on both sides, ala Raptor and have a plate mounted vertically. Hopefully shop time will be available soon to test...

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Pat_Willcox
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Post by Pat_Willcox »

Lou,

Ref Sean's comment about heating a screw driver tip. I use that procedure a lot on installation of airvalves. I do a #1 (small) phillips screwdriver red hot from a propane torch and just stick it into the plastic bolt. I have never tried just using a lighter but that might work OK. You can try putting a scrap nylon bolt in a vise and experiment a little. It really does work pretty slick. When installing an airvalve, on the opposite side of the fuel dump brass screw I use a 10-32 nylon screw. After I have turned it in a couple turns, I cut it off flush with the case and then do the heated screwdriver "branding". This may be a bit confusing....I'll demo at hotel willcox in Nov. (grin) pw
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Post by Cajun »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><font color="red">I believe that the glass rod glued into its slot and secured with 5 tie wraps cinching both rods down to the foam will be very strong and damage resistant</font id="red"><hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Cut your grooves for the glass rods a quarter inch deep in the foam and polyU glue them in and the zip ties will not be necessary. I recess mine 5/16s deep and have never had a glass rod pull out in a crash.

The 1/8" grooves cut with a hot wire are not adequate, IMO, to hold the rod. A router groove is strait walled and much tighter.

CAJUN [8D]
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Lou Melancon
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Post by Lou Melancon »

Bill,
The tie wraps are really more of a building aid than an armor technique.

I cut the rod slots, then drill holes on each side of the slot for the tie wraps. After gluing them in I cinch up the tie wraps and shove the large end up into the foam.

Next wipe off any ooze, spray a little 3M 77 over the slot then apply tape tip to tip on top and bottom. This keeps the glue from oozing out and allows me to continue to work on the wing as the PolyU glue sets up.

Lou Melancon
Alpharetta, Georgia
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Post by PROPPY JON »

lou i didn't like the spar from the git go so i use 2 carbon fiber rods top and bottom then i use1/8th quality ply and put g-10 on each side then i poly glue it right to bottom of wing put two blind nuts in for motor plate which i build the same way with g-10 and bolt on with nylon screws it works very well i could see with the plate screwing into spare it was lines for broken spar.. your idea looks like it will work very well

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Lou Melancon
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Post by Lou Melancon »

Hi John,
I glued up four wings this evening and they are looking pretty good. I will be able to test at the Jacksonville contest in early October.

I took apart a damaged Cobra tonight and found that it like several others had bad spar breaks at the center and bad damage behind the spar.

On Lee's original Cobras he made a plate that extended from the motor nose block back to the TE to transfer some of the impact forces.

I think the combination of 10-24 nylon bolts and the wooden rib will work well. The rib will hold impact forces without damaging adjacent structures. If the strike is hard enough the bolts will shear.

Hope that tests prove it out.

Lou Melancon
Alpharetta, Georgia
sgilkey
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Post by sgilkey »

Lou, fyi, our Piranhas use three 10x24 nylon bolts to hold the HDPE motor mount plate to the ply fuse. In only one crash (of many) have the bolts sheared- straight in nose-first onto an ashpalt heli pad at the Havoc (yes it took incredible talent for me to find that 10x10 pad in the middle of a grass field). It surpises me how difficult it can be for nylon bolts to shear. You might want to use 8-32, or cut a slot in your 10-24 to create a shear point. Or don't apply much torque to the bolts. My guess is that the clamp loads of firmly-torqued 10-24 bolts are high enough that there is too much friction between the plate and fuse, and it will not shear off. Less torque, less friction and it might then shear as designed.

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Post by Which_way_is_up »

Or a little "car wax" between the HDPE and the rib to get the friction down. Or small bolts, I'm going to try using (3) 1/4-20 bolts on the existing design to hold the engine pod to the wooden tongue.

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Lou Melancon
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Post by Lou Melancon »

Scott & Roy,
Thanks guys for the ideas. I can see how tightening the bolts will cause a real increase in surface friction. I think I'll just use two and make sure the plates slide against each other by waxing or some other low friction treatment.

Thanks again for your insights. I think you are both right.

Roy, I think the 1/4-20s are too large, mine have never sheared off in impact on the standard design. When I've 1/4-20 to hold on landing gear on sport planes they shear easily, but the length of the LG creates a lot of leverage.

Lou Melancon
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Post by sgilkey »

Lou, I hear the Texans all are waxing their legs, in preparation for the thrashing they are going to get at the Scale TOC where they will be shown to be the girlie-men of combat compared to the manly northern hordes. You might be able to get some good wax from them for your motor plate experiment! While you're at it you might as well wax your legs as well, as you too will be given a whoopin' by the awesome Northern host.

Scott Gilkey
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