Stiff Profile Front Ends Once Again

Just had to reiterate how important a solid front end in on a profile model. Just got back from flying and ran into it yet again.

Plane one- the Tricolor Interceptor design I posted a pic of awhile back. Nice looking, Ambroid Scrapper type plane, but 740 squares, LA 40, 11/4 Zinger, GRW 4.25 oz. tank. The thing runs like a top and has now for the last 25 flights. Easy to set, tracks the NV nicely, and holds a setting for the flight.

Plane Two- the sort Spitfire in a couple recent posts. Ran quite well for a dozen or so flights. Had a couple brief ground encounters due to wind and an over-eager pilot, but nothing broke. Today I started bubbles in the fuel line, and nothing works. The motor runs terrible. Tried removing the spinner, several different props, more foam under the tank, more Armorall in the fuel, nothing works and it runs bad. Exact same tank/engine/fuel/prop as the Interceptor.

So the bottom line is the nose on a profile has to be super stiff and strong. Apparently the Spit has developed some cracks or something internally and the nose is now softer. We've seen this on several planes. I'm convinced that 95% of the engine run problems on a profile plane are due to this soft nose/resonance phenom. I've seen many built up planes that showed bubbles in the fuel line and still ran just fine. The profile nose structure/length must just aggravate the problem where a built up nose just doesn't seem to generate the resonances.

I'll try adding some glass to the nose. If that doesn't do the trick, I'll just have to cut it off and graft a new one on!!!

Funny, I was just thinking about this today.

A lot of posts have talked about the Fox .35 not running well on profiles. My Ringmaster aka "The Pigmaster", has zero problems with it. I bet part of the reason is the rock hard front end. Unfortuanetly, the backend is just as rock hard

But, it makes sense to me that the solid nose helps tremendously on engine runs.

If the tail shakes around that is just as bad as if the front end does. The tail points the airplane where it is going by altering the angle of attack of the wing and if isn't soild and/or the controls aren't either, you are wasting your time. After all, you can wind fly without the motor running.

Jake

I'm rebuilding a Midwest Profile P51 for my neighbor, the motor mount was mushy and cracked. Was suprised to see that what you could see of the mount was all there was. My little Sig 15 ackro has mounts that go way back into the sandwich, so I rebuilt the P51 that way. Are long mounts the norm in profiles? I thought they should be, just for structural integrity, and now it seems there's another reason. Don

You're exactly right. As soon as I started building profile noses with full length hard maple engine mounts and SIG aircraft plywood, I found that profiles could be made to run and fly quite well.

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"I never liked my own species." -Gary Larsen

Great point. But how thick and what kind of ply is used to reinforce the nose? On a Twister I'm building, I replaced the stock ply doublers with thicker harder ply. The doublers actually weighed 4 ounces, 2 ounces heavier then the stock light ply that came with the kit. After talking this over with club members I went for the heavy ply despite the 2 ounce penalty. The plane is building up light enough as is, but 2 ounces seem like quite a bit to add. The motor mounts were also extended to the leading edge.

So, what is the secret formula (type of ply, thickness of ply, motor mount length, etc.) that firms up the front end of killer profiles?

Has anyone drilled out the ply doublers to save weight but retain strength? I thought lightening holes behind the motor mount cut out might keep the structural integrety while loosing an ounce or so.

I'm a newbie, but I would gladly sacrifice a bit of weight for strength, especially on a profile. Perhaps not wise, but workable.

Actually, using 1/16" five-ply aircraft ply instead of three-ply stuff is plenty of improvement. Long maple bearers are the only way to get decent longevity out of aprofile nose. Any plywood between the bearers and the engine lugs, however, will eventually crush, so I started cutting the outboard doublers with an outline where the lugs would go, or eliminating the inboard doubler entirely, and using pads between the bearers and the engine made of either aluminum or 1/16" circuit board material.

When Rat Racing was first getting a little popular, we often used old trainers with new noses on them, such as a Guillow's III with the old, origial pine upright mount removed, and a sideways mount substituted. One of the popular engines then was the Fox 35 "Series 3" (I presume that the two prior Fox Combat Specials were used some, also, but I didn't see any of those at the time I first experimented with Rats).

The old Fox was a vibrating monster, and the front ends of those Rats would shake apart rapidly. That was 1961, and I talked to some folks who were into speed, like George Aldrich, and was impressed by how rigidly they wanted their engines mounted. By creating a CRUTCH for a profile, same as folks later started making for builtup fuselage model planes, the vibration was averted, more power went to the prop, th eplanes lasted longer, and it was easier to get a needle on them.

I started with an "H" of long maple bearers with a cross piece of the same material behind the engine. The crutch was assembled on a piece of 3/8" glass (shelving) salvaged from an old bathroom medicine chest, with a slow-curing epoxy, weighted down totally flat. When Slow combat was born, I used the same type nose for those. The first Rats that I built that way dated back to 1962 or so.

.

Kiwi

I have successfully done a couple of things to get VERY rigid profile noses.

1. Fancherized twister kit. I used stock lite-ply doublers but put 5.0 oz carbon cloth/epoxy between doublers and fuse core. no other tripler etc and this nose is exceptionally stiff. THe carbon cloth--not veil or mat, gives all the structure needed, the light-ply just gives something smooth to finish.

2. "Impertinent" original design used 1/16" hard-ply doublers with carbon veil between them and the 4.5Lb balsa core. Again, very effective even with a SSW .36 Pro and 12-1/2 in prop. Plane has about 400 flights on it without a problem. again, no triplers just large fillets at the wing and epoxy lamination adhesive.

FWIW

On my lightweight Ringmaster project, I ran the doublers to the high pt of the airfoil, and ended the mounts as they ran into the wing. Also, I lightened the mounts with holes that are hidden behind the doublers.

The fuse is contest grade, and the inboard ply is of the lite variety. I may have gone to far for lightness here, but we will see. One thing, though...it won't have a shakin' Fox on it; it will be various OS 20s and 25s, both FPs and earlier. And I may take up Mr Buck's challenge, and put an FP 15 on it, although I may have made my mounts too far apart, and would have to add something. The landing gear is 3/32" and ultra lite wheels.

Projected wt is low 20 ounces. Right now, without landing gear, tank, motor, or covering for the wing, but with leads and bellcrank, and about 4 or 5 coats of very thin clear, thin filler, and sprayed dyed clear, it is 13 ounces.

My existing beater resto Ringmaster, without tank or motor, is 22 ounces right now.

Well, Lite Ply has its uses, but it is a poor choice for nose doublers on a profile. Lite Ply is soft and weak (and light!), try breaking a scrap of it sometime - you'll need TWO fingers. (Yes, you CAN help it with CF, you can also simply replace it with some appropriately stiff material.)

Also, shorter sections of basswood are a poor material to replace longer hard maple engine mounts.

That "crummy run" reputation of profile ships is rooted in use of such materials. Yes, that's what comes with many of the kits - that doesn't mean it is the best material.

As for the Twister, a stock one with an unmuffled Fox .35 is a rather skittish flyer, hardly a fine, pleasant stunt ship. Ted's tail moment elongation helps. A .40 FPS engine, being HEAVIER, actually tames the Twister a little, and improves line tension (in addition to running sweeter). I've built several Twisters, and the BEST one of all had 3/4" ADDED to the nose moment!

I've also built a profile ship with an elongated tail moment that came out TAIL HEAVY. It was not a pleasant ship, in fact it was a handful to even fly. (No doubt, it had a GREAT corner!)

Humor me. Get some 5-ply SIG birch aircraft plywood (use 1/8"). This stuff is incredibly tough and strong. Replace your Twister kit's doublers with this material. You'll be amazed at how much better life is in general, when the engine runs properly.

And before you assume that it will end up nose heavy and doggy, actually take it out and FLY it. You'll find it capable of doing a fine pattern, a smooth flier, (EASY to fly!), and the engine fins will not turn into a blurr when it starts. Notice how good the line tension is! It's a joy to fly, and it inspires confidence.

If you want a ship that behaves like a Flite Streak, of course, don't do this! (Remember, the Flite Streak is a "combat trainer", not a stunter.)

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"Anybody who'd build a $#%$ CLOWN.." -the late Ted Snow, at VSC several years ago.

There is another cause for this problem that isn't related to structural problems. I have had situations where after a minor smack the threaded portion of the crankshaft has been bent. I'm talking .015" which isn't a lot but can be seen if you pay attention and will cause vibration problems.

The effects of this are magnified in a profile as compared to a built-up fuselage.

So if you smack it in... check to see if the crank has been bent. It can be readily bent even in a minor crash if the angle is right.

Phil,
Perhaps you might try adding a plywood gusset between the inboard fuselage nose and the wing LE to steady it a bit.
You're a combat guy so maybe you've already tried that fix.

Thanks for the interesting replies. I plan to clean off the paint and add a couple of layers of 2 oz glass to the whole front end. I LIKE the carbon fiber idea, but I don't think it is worth the cost for this plane. It would take half my supply of heavy CF cloth. If glass doesn't do it, I'll just take the whole nose off and splice on a new one. There is no fuse cutout, so trying to gusset the nose won't do much. It's already braced by the leading edge.

Since the plane has hit the ground I'm pretty sure some of the wood in the front end has gotten mushed a bit. I'm pretty sure the crank isn't bent. There's no visible teeter when the prop is running. I've already pounded LA 40's into the ground much harder with no ill effects.

Maybe the gusset idea would work. I could rout slots in the fuse and lay in strips of 1/8 ply horizontally and run them back to the wing area to tie the motor mounts in. Glass over that and any balsa in the front end would be just filler. I'll have to think about it a bit. Too much work for today.

To prevent crushing, and loose screws on my profiles, I bush all engine, tank and landing gear mounting holes with either brass or aluminum tubing. Using a trick I learned from this forum, I make either a steel or aluminum large area plate to place between the engine and fuselage to increase the engine mount footprint. I don't use blind nuts on profiles, but instead, use screws, washers and lock nuts. If you insert the screws with washers from the inboard side, use washers and locknuts on the outboard side, then cut them to the perfect length, it makes a very neat installation, and they won't get loose!

On my profile the front end shook to much so I cut a groove in the bottom of the fuse from behind the leading edge of the wing forward about 5 inches and epoxied a carbon fiber tube in the groove. Didn't totally fix it but made it a whole lot better. Grins

Some followup for your edification and education.

I tried ground running the motor on the plane and started taking off rubber bands from the tank. With just one rubber band and the tank floating loosely on a half inch thick piece of soft foam I still got bubbles. So I tried a different metal tank. Exactly the same problem. Bubble when it was strapped on. Bubbles even with one rubber band. Bubbles mostly went away when I held the tank in my hand. I'll have to open up the tank and see what I can see. I think the fuel line was feeding enough vibration back into the tank to generate some bubbles.

Not wanting to rebuild the front end, I built up a Sullivan SS-4. Just a simple clunk to the back of the tank and a single vent in the corner(mounted it with the small dimension spanwise). Couple of rubber bands on the same rubber that was under the metal tanks. Ran flawlessly with absolutely no bubbles. Did manage one flight in a 20 mph wind. It was a bit rich and slow, but ran very steadily. I expected it to lean out towards the end, but muffler pressure kept it running smooth. It actually richened up a bit towards the end of the tank.

So for now I'll play with the Sullivan tank and save rebuilding the fuse for the next crash!!

Here's a tip - I found that you can get those flat "pancake" rubber sink stoppers for very little cost, usually in a nice white rubber that is about 1/8" thick. With scissors you can cut a piece to put between your tank and the profile fuselage nose, and it will often eliminate a bit of vibration problems.

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"Basic research is what I am doing when I don't know what I'm doing." -Wernher von Braun

They sell a real nice white foam at your LHS. It comes in different thickness. It's used by the RC guys for wrapping there receivers and batteries in. That's what I've been using between my tank and fuse.

Phil,

This is a secret, so don't tell anyone, please. But just between you and I it has recently been discovered that mounting these plastic tanks on profile fuselage models with the narrow side against the fuselage works pretty well.

No, probably not in all cases. But you might want to try it.

what I was interested in was a way to mount SOMETHING on the consarned plane that would feed fuel and not produce bubbles!! I was really surprised to see bubbles when I was holding the tank in my hand, totally unattached to the plane!! It was cool outside, but I wasn't shivering that bad.

What, pray tell O Wise One, would be the point of mounting the tank on the narrow face when it seems to work just fine mounted on the wide face?

If I remember the thread correctly, the recommendation was based on the user's idea that it reduced the amount of head pressure change from full to empty. THis is sound logic until the plane starts making three G's while flying..then the best head change geometry is rotated 120 degrees or so.

So, if I have it figured correctly, it is better to have it long dimension vertical.

LAST EDITED ON Apr-12-04 AT 06:15 PM (CDT)
 
>If I remember the thread correctly, the recommendation was
>based on the user's idea that it reduced the amount of head
>pressure change from full to empty. THis is sound logic
>until the plane starts making three G's while flying..then
>the best head change geometry is rotated 120 degrees or so.
>
>So, if I have it figured correctly, it is better to have it
>long dimension vertical.
>
>Curt

This neatly explains why all my full-bodied Stunt models exhibit such horrendous engine runs. Note to self: First chance, dig all those hard tanks out, rotate 90 degrees and reinstall.