Phenomenal P61 Black Widow project! - Page 3 - RC Groups
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Nov 02, 2005, 06:05 PM

Other aircraft of interest

Not sure this is the right place to post this type of thing but thought some of you might be interested in the "normal" types of aircraft I build - typically sailplanes and typically made mostly of composites and foam cores - this is why this P61 project is very "alien"" to me and why some of the techniques I use cross the "boundaries" of my traditional sailplane building skills and the ones needed for the P61. Both of these are scale replicas of flying aircraft and the primary reason I like scale in general.

There are two sailplanes here - a 10.6m SB-10 (first/one pic) and a 10.3m Eta (three pics)

Anyway, you can ignore if you need to.
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Nov 02, 2005, 06:08 PM

Other aircraft of interest

Forgot to mention, both of these are scratch builds from three-views - concept thru design and then to flying.
Nov 02, 2005, 06:35 PM
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Wow. Impressive. Do you make molds for a big fuse like that or do a one off?
Nov 02, 2005, 06:56 PM

Other aircraft of interest

I still have the mold for the SB-10 - it started as a plug, then mold and I, in total, pulled four parts from it experimenting with layups for strength and weight. The last part was the one I used for this one

For the Eta, I lied a little on that one - it was not fully scratch built - but almost. The fuse for this aricraft actually came from Germany for a different aircraft altogether. I chopped it up, removed several parts of the fiberglass fuse and re-built it with glass, kevlar and carbon to match the three view. It was fortunate that some parts of the existing fuse I got from Germany were somewhat similar otherwise it would have too been a plug and mold. The lost foam technique was used on the parts that had to be rebuilt/built. Other than that, all the rest was scratch built, and no, I don't have a mold for this one - this was a one-off.
Nov 02, 2005, 07:04 PM

Other aircraft of interest

I could post a whole article about the eta too but I don't think this is the right forum for that - no engine although the term fuel could be interpreted in many ways!!! Batteries after all, are reseviors for fuel - the only difference is this fuel is electrons!
Nov 02, 2005, 07:07 PM
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Last edited by Rcflyer9; Sep 01, 2007 at 09:51 PM.
Nov 02, 2005, 08:03 PM
You win again, gravity!
Muxje's Avatar
Building up the wings in a foam cradle seems an interesting technique. Most people (like me) build wings on a flat surface, either in a jig or by using tabbed ribs (with the tabs cut off after construction). Is this method significantly easier or more accurate?

Very nice model and build! Nice and tidy equipment installation... mine always looks rather messy even if I try and make it look neat.
Nov 02, 2005, 09:55 PM
Originally Posted by SoarScale2
The data I have in the documentation says different to your statements.
Again, I was not and am not attempting to disparage you or your model.

With respect to the aileron travel and fighter brakes in P-61C and subsequent variants, I would refer you to AN 01-15FB-2 and AN 01-15FC-1. The "C" revision is dated July 1945 and the final "-1" revision is dated 14 October 1949. These of course are the last major and minor revisions to the P-61 type operations manual.

I rather doubt the USAF would have been told it's pilots that the aircraft had up aileron travel of 65 degrees and down aileron travel of 28 degrees, or that the aircraft had operational fighter brakes, if such was not the case, even if the aircraft were at the end of their active service life.

I've always thought that a good source of pretty aircraft pictures was commercially produced "aircraft documentation", e.g. the P-61 books by Thompson, Johnsen, Davis and Menard, Pape, and McLaren, but that accurate aircraft facts were best found in the USAF publications issued to the folks who put their feet on the pedals.

Once again, it appears you're crafting a very good rendition of a P-61C, and we will all be looking forward to seeing it in future scale event reports.
Nov 03, 2005, 12:14 PM
Mike Denest's Avatar
Go to to see a P-61 under restoration by the Mid-Atlantic Air Museum. Look for the "Widow's Web" page. The aircraft is being restored to flying status and will be the only one in the world to do so.
Nov 03, 2005, 12:19 PM

Foam cores/surface

Muxje, Since this is the first time I have actually built a stick/rib model of this type, I couldn't answer the question regarding ease or accuracy since I have nothing to reference it to.

What I can say is that I have watched others do the traditional way as you describe with ramps, wedges, jigs, pins, tabs etc and I personally thought there must be an easier way to do this using some of the experiences I had with the sailplanes.

Using the foam beds created a complete top and bottom "mold" or "jig" so to speak, of the top and bottom surfaces. Most folks I have seen that build built-up wings apply sheeting in pieces and the results are not always as they desire.

When I took the model to the scale masters qualifier for viewing, a significant number of folks asked - how did you get your sheeting to look so clean?. I told them - and there were a few people that just looked puzzled (probably never cut cores of their own), some indicated that this was not the correct way to do it (die-hards, I assume, of traditional techniques - not the way they have done it in the past and wary of change). Others showed clear interest in learning how to do it this way (who, by the way, have been shown this method since).

The key however, is accurate foam cores/beds with equal joint heights and accutrate washouts cut in. Once you have this you end up with a wing shaped building surface that when glued (temporarily) to a flat surface, provides a very accurate (depening on the accuracy of your foam cores/beds) SOLID surface to build on.

I personally found this an easy method due to my experience with core cutting. The results were very accurate (as measure by incidence meter) and the joining of wing sections (center to tip panels) was a breeze and were also accurate. TE's were very straight because the core TE's were very straight!

Bottom line, the foam beds are simply another type of jig, probably similar to the ones you already build except in the foam case, you do not have any parts of the wing "floating" in the air - it is completely captured/supported at every point on every surface.

Final sheeting is also easy as you can use, again, a single, pre-prepared sheet for the remaining unsheeted surface. Use polyurethane glue and the remaining foam bed to "encase" the whole wing structure, weight this evenly and the sheeting has even pressure over the whole surface as it is pressed into the "mold". Again, as long as the foam beds are accurate, the sheeting glue cures and the whole wing is locked into the exact shape, with washout and straight TE's. It acts just like a two-part mold.

It will be my method of choice in the future for sure - for me the results were exactly as I had desired.

I also used the same technique for the stab and elevator but the pictures didn't show this.
Nov 03, 2005, 12:33 PM

Foam cores/surface

Forgot to mention the "downsides" for this technique

I am not sure I would actually call it a "downside". It should probably be considered a process change to cater for the use of this type of jig.

The change from the traditional methods was having to transfer the rib spacing and component placement (spars etc) to the sheeting. Since you no longer have a plan visible below the wing structure because you're building the wing on the bottom or top sheeting, accurate spacing and component locating/marking is required.

Believe it or not, the level of "difficulty" with this is dependant on whether you build the wing right side up or up side down. The curvature of the sheeting in the foam bed varies this difficulty factor. Right side up is easy because, generally, the sheeting has minimal curvature. Upside down is a little more difficult due to the increased curvature of the top surface sheeting.

Also, the sheeting was always cut a little oversize where possible and trimmed/sanded at the appropriate stage.
Nov 03, 2005, 12:39 PM
You win again, gravity!
Muxje's Avatar
What I can say is that I have watched others do the traditional way as you describe with ramps, wedges, jigs, pins, tabs etc and I personally thought there must be an easier way to do this using some of the experiences I had with the sailplanes.
Easier.... if you know how to cut foam cores Maybe I should learn this skill.

I can see how this helps with sheeting. I do sheeting with the wing spars set firmly in a jig, which gives accurate results but the balsa sheets do not always line up as neatly as I wanted, since weighing down has to be done using stacks of magazines or similarly flexible stuff. A bit hit and miss, but nothing a little glue and sanding won't solve.

A question though: do you form the bottom sheeting into the foam mold first, and then glue the ribs and spars in place on the sheeting? It looks that way from the pictures.
Nov 03, 2005, 12:45 PM
You win again, gravity!
Muxje's Avatar
Ah, it seems your 2nd post answered my question

As for positioning the ribs, perhaps it is easier to transfer the location of the ribs to the leading and training edges, then (depending on the construction of your particular wing) glue these in place first or temporarily fix them to the foam bed. This should ensure your ribs go in at the right places and at a right angle.
Last edited by Muxje; Nov 03, 2005 at 12:53 PM.
Nov 03, 2005, 02:11 PM

Foam cores/surface

Yes, the sheeting is pre-shaped in the beds beforehand to minimize stresses. Water/alcohol mix misted and paper either side of the sheeting in the beds/cores to draw moisture and aid drying.

As far as your comments on the equipment installation, I have added some pics of the final layout and some notes if you are intersted - different from the one posted before.

The electronics and air service panel are all mounted to the removable structure and the structure itself is bolted to brackets on the wing.

From the first picture, from left to right, brake servo, air pressure guages, air connections from valves in nose and tanks in fuse, brake valve below air fill valves, three gyro's, 12 CH Rx, 2 x 8 Ch opto-isolators, 4 matchboxes and one Equalizer II for spoilerons.

Matchboxes are there to match servos on:

1). Rudders/Stearing
2). Elevator
3). Flaps
4). Ailerons/spoilerons (via one Rx channel)

The two, dual battery input powerbox sensor switches will mount to the top of the electronics structure (pic#2) and the switches will be accessed through the top panel in the fuse (where the gun turret was originally mounted). No external switches, no further hatches needed.

The structure took a little planning but with a little time spent, I am happy with the result. Air lines travel from the nose/fuse, to the service panel through the wing center section, T-ed here where necessary and along to the middle of the electronics package and back into the wing. Then along the service tubes to the nacelles for connection to gear and door cylinders.

Power to the electronics systems will be via 4 LiPo's. 2 for the RX, 2 for the servo's. About 720maHr each for the receiver through the regulated Powerbox Sensor switch to the RX, the other two about 2100mAhr each for the servos to the output side of the opto-isolators, again through the regulated switches.

Just a note on the spoilerons and the Equalizer II. Since I decided to mount the servo for each spoileron in each wing panel, another deviation from the Ziroli plans, a problem then existed on how to control the servo so that it moved ONLY in one direction - center to up and no center to down movement.

End-point adjustment was required. Now, this can be done several ways - via the Tx, in the servo itself since they are Hitec digitals, or via some form of EPA from a matchbox type device.

The Matchboxes are not designed to provide a zero movement on one side for EPA so this wouldn't work. Given all the channels I needed for the control surfaces/functions AND in consideration of the function I wanted from the outer flaps (flaperons), I was out of channels with my 12 channel TX/RX combo to control the spoilerons separately. Since the Matchbox wouldn't do it, I reviewed the feasibility of direct servo programming of the Hitec Digital - relatively easy to do with the HFP-10 programmer but not readily, and I might add, easily field adjustable if it was required.

I eventually called the folks at Smartfly and asked them if the Equalizer II would do this. They tested it. They found a "wrap-around" feature built into the firmware, resolved it and sent me a sample - Viola! This device now limits the travel in one direction to zero, and provides addjustable EPA for the other direction. It is readily accessible under the main, detachable fuse center and can be adjusted in unison with the Tx - ideal.
Nov 03, 2005, 02:24 PM

P61C Black Widow series (#21)

Nacelles/Electronic ignitions and battery mounting

No big deal here I thought - well, since I wanted to build this aircraft as light as possible I chose to go with the battery technology that provided the highest energy density at the lightest weight - LiPo's. With all the cautions and warnings about charging procedures for LiPo's it occured to me that charging the LiPo while it was still in the aircraft may not be the safest thing to do. So, for the iginition units, I deceided they had to be removable - but how.

I eventually decided to build removable firewall to wing LE fairings from a light layup of fiberglass. The pictures show them.

A liPo will be mounted beneath each one and the fairing will be removed so the battery can be removed for charging. The 5V regulators and electronic switches will be mounted below the wing, the ignition modules ahead of the firewall. We'll see how this goes!

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