Martin 167 Maryland design and build. - Page 4 - RC Groups
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Nov 30, 2008, 08:14 PM
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RyanPSU21's Avatar
Depending on the tab and self jigging structure you might be able to do it with wood glue instead of CA. I think on my Cub build (Sig 1/5 scale that is in barebones state right now for Funbuild2) very little of the fuselage could be, especially the top wing center section since there would be no way to get the glue on all the 10 or so parts and get everything twisted and forced into place before the glue set. You can't really just put glue on the already in place joint since it won't really wick into the joint the way CA will.

I'm not old enough to know Ambroid. I'm only 32. I built my first 10 to 15 RC planes with CA. Not counting several rubber powered planes done when I was really young that were all Elmers. It's only more recently I've started using Titebond instead. I think my P47 was the first I did that way. My Albatross and B24 were about half and half titebond and CA. My Cub was entirely CA just because it didn't lend itself well to Titebond and it was supposed to be a fast summer build. This thing will be mostly Titebond unless I get to a step where I just can't figure out how to pin or tape something in place to dry.
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Nov 30, 2008, 09:12 PM
Two left thumbs
Ambroid was - and still is, if you can find it - a cellulose-based glue that works really well for sheeting, planking, and/or for very light structures that require more flex than CA provides. Similar products are Sigment and Duco household cement.

Nov 30, 2008, 11:43 PM
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lake flyer's Avatar
This one looks like it might be a touchy flyer , I hope I am wrong about that .

Good luck.
Dec 01, 2008, 06:44 PM
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RyanPSU21's Avatar
Originally Posted by lake flyer
This one looks like it might be a touchy flyer , I hope I am wrong about that .

Good luck.
There really is no reason that it should be. The proportions are fairly generic. The wing area is generous for it's span and the tailmoment is long with decent sized fin and stab. Hopefully it will come in well south of 20oz/ft^2 wing loading so it should be fairly well behaved.
Dec 02, 2008, 08:39 PM
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RyanPSU21's Avatar
More work done the past couple of days. Both fuselage sides were finished. The second built on top of the first to get 2 sides that are the same. They were pinned down to the building board and jigged up to get eveything aligned. Then the center section cross pieces were added. Tonight I fixtured up the rear section and added the rear cross pieces.
Dec 02, 2008, 10:23 PM
Two left thumbs
You're giving Maltone a good run for his money WRT getting things build in a hurry. Looks very nice!

Dec 02, 2008, 10:34 PM
a.k.a Maltone
Pat Lynch's Avatar
Looking good - it has an almost British look to it. Blenheim-ish I guess.

The glue discussion is a hardy perennial and will never be resolved I think. Provided the joint doesn't fail due to the glue used and it suits your building style, then why not? I've used almost totally medium CA in recent times and haven't had anything fall apart yet

It's great that quite a few folk are still going down the hand cut path - keeping old skills alive. (or is that some of us (read - me) haven't conquered laser files and 3D yet - )

Last edited by Pat Lynch; Dec 02, 2008 at 11:39 PM.
Dec 02, 2008, 10:50 PM
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RyanPSU21's Avatar
I can draw 3D CAD just fine. I just can't afford software that can do 3D decently. I drew this on Turbocad V12 which I bought of Ebay for $15. I upgraded a day or so ago to V14 for $24 since it is supposed to run better on Vista. The deluxe versions which are cheap have very limited 3D functionality. As much as I'd like Rhino I just can't justify $1000 for hobby purposes. Way back when for a Coop job during college I used to work at a software company doing customer support for a 3D modelling software.

The other real advantage for this construction style is that is can be mostly built from the scrap balsa bin that I've accumulated over the years. So far I think I've spent $10 on balsa for this to get a few 48 inch sticks. If I did lasering I'd have to spend money on all new wood for it plus lasering costs.
Dec 03, 2008, 07:21 PM
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RyanPSU21's Avatar
I got the cross pieces glued into the front of the fuselage tonight. Tomorrow the pins can come out and the basic fuselage box can come off the building board. Next will be figuring out some way for mounting the tail wheel and making the top and bottom formers. I'm going to sheet with 1/16" just the top and bottom surfaces of the fuselage. I thought of just doing stringers except in a few spots around the cockpit but I think the weight save to do that verses sheeting won't be enough to make up for the hurt to the appearance it will make.
Dec 04, 2008, 07:49 PM
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RyanPSU21's Avatar
Tonight the fuselage structure is unpinned and off the plans. The side plans went back onto the building board to built the 2 side structures where the fuselage rear increases in height near the tail. This plane has a rather odd fuselage profile reduces in height just aft of the wing and tapers back until just before the tail where it tapers larger again.
Dec 04, 2008, 09:08 PM
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RyanPSU21's Avatar
I am trying to sort out the power system for this thing tonight. It's going to end up with something similar to what was in my shortlived Albatross DVa. Motors and Controllers from Balsa Products. Looking for around 100watts/lb which will be more then decent overkill. Scale for this would be 3 bladers around 14.5 inches. After mucking with motocalc for the last while I can't seem to come up with any good combination involving 3 bladers that doesn't leave me with a really low pitch speed. I'm going to drop down to 13inch 2 bladers since I seem to get the best numbers there with the various motor combinations I tried.

Below for record keeping is my motocalc output for this thing with a fairly pessimistic close to 8 lb flying weight calculated in.

MotOpinion - Martin167 Maryland
Sea Level, 29.92inHg, 59F

Motor: BP3520-6; 800rpm/V; 2.1A no-load; 0.05 Ohms.
Battery: A123 ANR26650 M1 (30C); 4 series x 2 parallel cells; 2300mAh @ 3.3V; 0.016 Ohms/cell.
Speed Control: BP 60amp; 2 controls (separate); 0.008 Ohms; High rate.
Drive System: apc eprop; 2 motors (parallel); 13x8 (Pconst=1.17; Tconst=1) direct drive.
Airframe: Martin167 Maryland;; 122.5oz RTF; 18oz/sq.ft; Cd=0.055; Cl=0.56; Clopt=0.67; Clmax=1.2.
Stats: 99 W/lb in; 70 W/lb out; 20mph stall; 27mph opt @ 49% (36:59, 84F); 29mph level @ 53% (31:51, 87F); 1484ft/min @ 39.5; -219ft/min @ -5.4.

Power System Notes:

The full-throttle motor current at the best lift-to-drag ratio airspeed (34.2A) falls approximately between the motor's maximum efficiency current (21.2A) and its current at theoretical maximum output (108.4A), thus making effective use of the motor.

Aerodynamic Notes:

The static pitch speed (48mph) is within the range of approximately 2.5 to 3 times the model's stall speed (20mph), which is considered ideal for good performance.
With a wing loading of 18oz/sq.ft, a model of this size will have very sedate flying characteristics. It will be suitable for relaxed flying, in calm or very light wind conditions.
The static thrust (122.9oz) to weight (122.5oz) ratio is 1:1, which will result in very short take-off runs, no difficulty taking off from grass surfaces (assuming sufficiently large wheels), and steep climb-outs.
At the best lift-to-drag ratio airspeed, the excess-thrust (75.6oz) to weight (122.5oz) ratio is 0.62:1, which will give steep climbs and excellent acceleration. This model should be able to do consecutive loops, and has sufficient in-flight thrust for almost any aerobatic maneuver.

General Notes:

This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
Dec 07, 2008, 03:01 PM
eye4wings's Avatar
Is that the TowerPro motor TP3520-670? If so it's my current favourite. Very economic.
Shame if you have to back off from scale prop size though. One of the benefits of outrunners is that you can fly with scale props - unlike the IC boys who have to change theirs for flying! We can look good on the ground AND in the air without any effort!
For what its worth I never do the calculations (which is probably why some of mine are REALLY economic and some not) but I run as few cells series as possible. I'd rather start from the scale sized props and save a cell. There's little cost difference between ECSs for 30A and 40A anyway. And three cells mean I can use the ESC's BECs, with a set of four nicads in parallel with the BECs as a capacitor to supply any over demand from the retracts.
From what you just said it looks as if you've got it well sorted. She'll look really scale in flight. Looking forward to the video!
Dec 07, 2008, 04:33 PM
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RyanPSU21's Avatar
I think it's probably not the same motor you mentioned. They don't look the same and have different kv ratings.

I could do 3 blades and get a decent pitch speed but it required going up to the 4120 sized motor. It would turn a Master Airscrew 13x9 3 blade okay but to drop back to a 2 blade needed atleast a 15 inch prop which may not be possible ground clearance wise. That motor size also weighs 3 ounces more or 6 ounces more for 2 of them and they are considerably more expensive.

There is a possibility I could use the APC 13.4x13.5 3 blade prop that may work on the motors I chose but I'd need to test run to see what the current does.

If I had an unlimited budget for this I'd buy another Mega22/20/2 with MEC gearbox and CC Phoenix 45 to have a matching pair with the one I already have and then get a set of vario 3 blade props.

This thing is getting a set of Robart Air Retracts that I already have. I got them on the cheap by getting a Top Flite P51 kit on Ebay that was partially started and inclued the retacts and air system for the normal price that the Robart retracts normally cost. I bought it, kept the retracts and re-ebayed the P51 kit so got the retracts for a net total of about $20. I don't feel like messing around with another set of servo driven retracts for a while. On my B24 I did them with 3 servos for the retracts and made up a wiring harness so that the retracts ran on a totally seperate power supply from the receiver. This plane will get a UBEC that will be wired in at the point where the power splits to the 2 ESC's. The 4S A123 packs are right in the grey area of okay to not okay to use the BEC on the speed control. BalsaPR ESC's don't have BEC's built into them anyways so something external is needed.
Dec 07, 2008, 08:45 PM
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RyanPSU21's Avatar
The top rear of the fuselage structure is on and completed now. I cut out the fuselage top formers and attached and put the stringers on. I am still debating whether or not to sheet the fuselage top and bottom or to just leave it as stringers.
Dec 08, 2008, 08:16 PM
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RyanPSU21's Avatar
Tonight I decided to suck it up and start the top sheeting on the fuselage. I got one side done. I always dread this part but it didn't go so bad. Trying to find wood to use I stuck several sheets of 1/16 that I had in the bath tub and let them soak a while and then pulled out the softest one. The bad news is there was only one sheet in the bunch that seemed to soften up enough to really be useable. So it will be off to the LHS tomorrow to get some more. This was the last 3 or 4 1/16sheets from Lonestar that I think were shipped right before the fire. Quality on these was really lousy. 2 of the 4 are split down the center due to some weird grain and a rigid zone near it.

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