How best to power the Umland Cobra?

Now don't anybody jump on me... there is still probably a fairly significant period of time before I'm actually ready to begin building this model, but having some down-time, I've decided I could do worse than to carefully consider what kind of power plant I will want for this plane. With a project like this, there are several ways one could go...

1. I could be a purist, and put a Fox 35 on the front. I have to admit, that is my natural inclination. The plane was originally designed around this engine, and I do have experience with it, and understand it. I'd need to use a muffler, but a tongue muffler would work well. If I wanted to go a bit more "high-zoot", I could get an L&J Fox 35 for it, or other similar...

2. I've seen at least one person planning to put a Brodak 40 on this plane. I'm sure power-wise that engine would handle the plane quite well, but how does it compare with the Fox 35 for weight? Would I expect any balance problems going this route?

3. I bet an OS LA 25 would work here as well. What about the 25 FP? I'm thinking the LA 40 would be too heavy...

4. No, I'm not going to put any sort of pipe setup on this plane...

5. If I wanted to be outlandish, I guess I could probably find a 4-stroke that would work. Naw, that's about as "out there" for me as the pipe setup...

So I think the real (read "practical") choices for me are either the Fox 35 or the Brodak 40, or maybe the OS 25 in some flavor...

There are a bunch of folks here who are no doubt building this kit for the coming season... what are y'all doing to spin the prop?

Tim

I was thinking OS MAX 35 S.

LAST EDITED ON Dec-12-04 AT 01:20 PM (CST)
 
>Now don't anybody jump on me... there is still probably a
>fairly significant period of time before I'm actually ready
>to begin building this model, but having some down-time,
>I've decided I could do worse than to carefully consider
>what kind of power plant I will want for this plane. With a
>project like this, there are several ways one could go...
>
>1. I could be a purist, and put a Fox 35 on the front. I
>have to admit, that is my natural inclination. The plane
>was originally designed around this engine, and I do have
>experience with it, and understand it. I'd need to use a
>muffler, but a tongue muffler would work well. If I wanted
>to go a bit more "high-zoot", I could get an L&J Fox 35 for
>it, or other similar...
>
>2. I've seen at least one person planning to put a Brodak
>40 on this plane. I'm sure power-wise that engine would
>handle the plane quite well, but how does it compare with
>the Fox 35 for weight? Would I expect any balance problems
>going this route?
>
>3. I bet an OS LA 25 would work here as well. What about
>the 25 FP? I'm thinking the LA 40 would be too heavy...
>
>4. No, I'm not going to put any sort of pipe setup on this
>plane...
>
>5. If I wanted to be outlandish, I guess I could probably
>find a 4-stroke that would work. Naw, that's about as "out
>there" for me as the pipe setup...
>
>So I think the real (read "practical") choices for me are
>either the Fox 35 or the Brodak 40, or maybe the OS 25 in
>some flavor...
>
>There are a bunch of folks here who are no doubt building
>this kit for the coming season... what are y'all doing to
>spin the prop?

I'm now ready to paint my Corbra. I'm putting in the Brodak 40. I also have a L&J Fox 35 ready.
Here's my take. The plane is going to come out at 43oz and that's light using the kit wood. (Kit has GREAT WOOD) I seen this plane fly well at 48oz......SO with that
ENGINES
Silver Fox 40
Brodak 40
FP-35 from Neuman
LA-40, 46
ST-46
Areo 36
McCoy 40 Tom Lay
an good OS-35s...maybe
DON'T EVEN look at an FP-25 or LA-25

Scott

LAST EDITED ON Dec-12-04 AT 02:56 PM (CST)
 
Hi Tim,
I have flown my Argus (Steve Wooley predecessor to the Cobra) on a "stock" Fox 35, L&J Fox 35, Webra 32, and a DS 40. All flew the ship quite nicely. It weighted ~37 oz. RTF with the Foxes and DS.
Having said that, the Cobra is quite larger!
I would go with a "stage 6" L&J Fox 35 or a Brodak 40. Those will be in the Fox "weight class", and try to keep it as light as safely possible. These old I-Beamers do not have the "thick" airfoils of today's ships to generate all that lift.
Also, build in an adjustable elevator horn, it definitely helps this category of plane. (about all others, too )
I put a *little* "scab" over the rear of the fuselage (usually 1/8th balsa) on both sides to allow for the clearance "holes" for the adjustable set up. Sanded in, it isn't very noticeable.
The Cobra is an absolutely great flier!
Bill <><

EDIT: Tim, I suggest these engines because of your desire to keep it in the "same ballpark" as the original. All these engines are currently available if you don't already have one. It's true, Steve, Billy, et.al., used to tip the nitro can for those Foxes more than people realized!
I wouldn't recommend a 25.

LAST EDITED ON Dec-12-04 AT 02:14 PM (CST)
 
While scanning the PAMPA Classic Plans book, it appears the Cobra and Argus had better airfoils than most I-Beams. I haven't built one yet, but I like the Wooley leading edge method instead of the Ares. The Ares also has that slight taper in the trailing edge - just another aggravation to deal with compared to the Cobra/Argus.

Bill:
Another thing I like in the Wooley designs - the tendency of tapering the bottom of the fuselage to a minimum depth at the tail. It looks good, but leaves little room for the control horn and pushrod connection. I've noticed the Umland kit and Johnston drawings seem to alter this. Did you build yours to the original design, and did you have trouble fitting the stuff in?

For a lot of stunt plane drawings, I've always wondered if the draftsman ever thought about the inside clearances at the tail.

EDIT: Oh, BTW - GREAT Question, GREAT Answers, very useful information.

>While scanning the PAMPA Classic Plans book, it appears the
>Cobra and Argus had better airfoils than most I-Beams. I
>haven't built one yet, but I like the Wooley leading edge
>method instead of the Ares. The Ares also has that slight
>taper in the trailing edge - just another aggravation to
>deal with compared to the Cobra/Argus.
>
>Bill:
>Another thing I like in the Wooley designs - the tendency of
>tapering the bottom of the fuselage to a minimum depth at
>the tail. It looks good, but leaves little room for the
>control horn and pushrod connection. I've noticed the
>Umland kit and Johnston drawings seem to alter this. Did
>you build yours to the original design, and did you have
>trouble fitting the stuff in?

Hi Tom,

My Argus was built directly to the plans. (Some drawings that Tom Dixon got from Doc Brown)
The hook up wasn't any tougher than usual on the old timers.
I did relieve some of the inside of the fuselage sides to prevent drag of the pushrod and keeper washer.

On the Werwage Vulcan, I cut out the sides of the fuselage everywhere the rod/screw hit (adj. elev. horn from Tom M.) and put some 1/8th inch balsa scabs over the resulting holes when everything was together. Sanded in they are barely noticeable, and gives the requisite room necessary.

When all that was used was a bent rod and washer soldered on as a keeper, not a lot of room is necessary, but I always have to relieve the inside of the fuselage sides, it seems.

I-Beamers forever! LOL!!

Bill <><
"It is better to remain silent and be thought a fool than to open your mouth and remove all doubt"
Abraham Lincoln

>When all that was used was a bent rod and washer soldered on
>as a keeper, not a lot of room is necessary, but I always
>have to relieve the inside of the fuselage sides, it seems.
>

Thanks for the reply, Bill. That's what I've always noticed, too - but nobody ever tells you that.

>While scanning the PAMPA Classic Plans book, it appears the
>Cobra and Argus had better airfoils than most I-Beams. I
>haven't built one yet, but I like the Wooley leading edge
>method instead of the Ares. The Ares also has that slight
>taper in the trailing edge - just another aggravation to
>deal with compared to the Cobra/Argus.

Hi Tom,
The originally published plans for the Ares were not of the plane Billy used in 1959. A different airfoil was used on his actual '59 Ares. The swept forward TE is pretty simple to do as you know.
The two different methods for the LE are interesting, and I haven't decided which one I really like best. Maybe the Wooley style.
Although better airfoils, they do not approach the SV or Patternmaster airfoils that seem to be the choice for so many today.

>Bill:
>Another thing I like in the Wooley designs - the tendency of
>tapering the bottom of the fuselage to a minimum depth at
>the tail. It looks good, but leaves little room for the
>control horn and pushrod connection. I've noticed the
>Umland kit and Johnston drawings seem to alter this. Did
>you build yours to the original design, and did you have
>trouble fitting the stuff in?

Have you seen the Airon?? Geez, there ain't NO fuselage back there!!

Bill <><

>
>Have you seen the Airon?? Geez, there ain't NO fuselage
>back there!!
>

Yeah, but only in the PAMPA book, and it's one of those MAN 3-views, so it's hard to tell how small the tail might really be.

By the way, I was watching an old war movie on Pearl Harbor Day, and the planes they were using in the movie made me realize I'd seen the look somewhere else before:

Tim,

I haven't built mine yet either, but will get around to it sooner or later.

Here are some numbers I have gleaned from my digital gram scale:
Double Star .40 = 6.00 ounces
Brodak .40 = 6.20 ounces
Fox .35 hemi = 6.30 ounces.

These are my engines and are basically ready to run, less mufflers. Any of the above three engines will work great. Your choice should depend on what you have and what the final airframe weighs. My Brodak .40 just loafs through the pattern in a steady four stroke pulling a 42 ounce Oriental. The DS and Fox interchange easily; the Brodak uses the O.S. drill pattern and is longer.

I still have a couple of O.S. 35S engines stuffed in a storage box, but that's where they will stay. The piston and liner never lasted long enough for me.

One thing to remember about Steve and his Cobra: he was not afraid to pour large amounts of nitro into the tank in order to get his Fox 35 to work - like 25 percent at some contests. If you try that, be prepared to replace engines more often.

My two cents.

Ron King

The Cobra is a pretty good sized airplane, something like 585 sq. in. I would use the Brodak .40, Aero Tiger .36, RO-Magnum .36 or a stock Magnum XLS .36, not being too keen on Fox .35's.

By the way, there was a Wooley Cobra kit available at this last Saturday's club swap meet, for $60 asking price. This was a fairly old kit, and seemed kinda heavy. I didn't see any info as to who kitted it. Who done it? Steve

I would sure tend to agree with Steve, especially if you were going to fly the Cobra competitively....even the Brodak .40 might be taxed.....I sure favor the Aero Tiger or ROJett .36 Magnum, but would imagine that Len's TT .36 would do well, too....

The best flying Cobra that I have seen had a ST .46 in it....horsepower helps a bunch.

See Ya

I have built four Cobras and three look a likes that had small changes.They have weighted from 44 to 52 ounces. One was powered with a ST/35 the rest have had ST/40-46's in them.I would recommend a TT/36, Rustler/40,or a ST 46 if you have one. A Fox will fly it but it would be a wimp with out a lot of Nitro and short lines.Even Paul Walker used a lot of Nitro and his weighted on 42 ounces. Steve used 10-20% Nitro and 9.5 inch prop.The Cobra with the bubble canopy that was in FM mag about 10+ years ago had a 46 in it.
Ed

4 Cobras! Wow! What was in the unusual colored one that you have?

If an ST 46 is not too big, what about the TT 42? I've been hearing some good things about that engine, and it's in the 8-9 oz. range w/o the muffler. How much did an ST 46 weigh? I had one when I had the Scorpio, but I can't remember how much it weighed.

Thanks guys. Great answers. Sure made me think. Also, I'm really happy to learn the REAL story behind just HOW someone could fly a plane like this competitively with a Fox 35. I have to admit it had me mystified.

I'll be very interested to hear from people who fly this plane with a Brodak 40, I have to think this would, under most circumstances, be a quite reasonable combination. I'll have to see if I can lay my hands on an Aero Tiger 36, but that may be out of my league. OTOH, it'll be a while before I start this kit. This one will have NO mistakes, NO shortcuts...

Tim

Fox engines can make you a better flyer. The fox on 60-foot lines doing a 5.0 lap with a 40oz plane will make you a better flyer. You have to wait for the plane with lack of power. You have to fly the tops of your maneuvers. You have to be light on the controls. You have to see the plane fly. YOU to Have confidence "it will make it"
Sure made me a better flyer

Tim, go with the Aero-Tiger 36, you won't be disappointed. I have one I'm breaking in now and the engine is terrific. By the time you spend 150 bucks trying to make a Fox run, your getting close to the price of the Aero-Tiger. The Rustler 40 seems to be a nice engine also, but from what I've seen the Aero-Tiger is the better engine.

Hi Tim,
If you CAN get an Aero Tiger 36, then by all means do so,
Run it exactly like Randy says and you will not be disappointed.
Very user friendly.
The Magnum 36 XLS II, is an extremely good, inexpensive engine that would have absolutely no problem pulling the Cobra. (Ask PTG, and Keith V.) Just don't try, or expect it, to run anywhere near like a Fox 35.
BTW: The Aero and Magnums like those Carbon props.
Bill <><