William Robison

My Feedback: (3)

So buy another one, Rich, this is your big chance.

Pardon the jab, it's meant in fun, but if I can get enough business for John mebbe he'll give me a second kit. For free. I did pay for the first.

AKMac. Why three fuselages? What advantages do you see accruing from this planform? Looks sleek as can be. Please share your design goals. I'm curious, and while everybody else is curious as hell I'm the couthless one with sufficient scrotum content to ask.

A twin that sleek, surely must be unique.

Bill

AKMac

My Feedback: (1)

Well, really the only big advantage I can think of is that the servo, and linkage for the elevator would be in a easily accessible spot (in the middle fuselage.) Other then that the design is really just something I drew on a napkin on the way home from a vacation. I just wanted something different, sleek looking, and moderately fast. Honestly, I don't think the middle fuse is really necessary, but to me it just looks right on this plane.

Forgot to add thanks for the encouraging words DocYates

William Robison

My Feedback: (3)

Thanks for the reply, AKMac, maybe it wasn't curiosity, but just plain nosiness.

I'll add my encouragement to that given by Tommy Yates.

With twin engines, Dick,
you've twice as many paddles when you're up the "Crick."

Bill

Mike James

AkMac,

I agree with the others. You obviously like the looks of your design, and a lot of us do too. (I followed your original threads, and the comments from others, in the "Scratch-building/CAD" forum earlier this year.

Dave Platt has said, "You may as well just start in and do the job.", meaning that for every step you take, you're that much closer to the end. Don't let your initial fears keep you from following through. The worst-case scenario is that you'll learn a lot from doing a project like this... Some will be "things NOT to do", and some will be immensely rewarding.

Personally, I'd like to see you build and fly it. If any parts of it really spook you, then simply sub-contract those parts. There are individuals here who can cut foam wing cores for you, do some calculations about weight and loading, and even airfoils, and there are probably people who would even build plugs and molds for you. Your "modernized P-38" is a good-looking aircraft that would give twin fans something different to build, so I say, "Just go for it!"

MormonMike

My Feedback: (2)

Your the neatest bunch of guys! i wish the heck i could meet all of you, You always have something interesting to talk about and it sure would be a lot more fun than being at this miserable hospital job working for a thankless boss. Seeya guys, MjS

LaCerne

My Feedback: (16)

W.R.
Are you trying to take credit for bringing NE aero to this board's attention? If you look back you sounded as though you'd never heard of them until I posted my reply to this thread.

HMMM????

William Robison

My Feedback: (3)

LaCerne:

I refer you to your post, #12 in the series and my post #13, responding.

I truly had not heard of NE Aero before that. As is plainly stated in that post. And I thank you for bringing them to my attention.

Also, if you check later posts referring to NE Aero, ALL entries thought NE Aero was long gone. So if there is any "Credit" due I think you and I would have to share it.

Regards a free kit, am I not allowed an improbable dream once in a while?

I see you've posted your AMA number; if you want to play "Who did it first" I am AMA #25139, I was a callow youth when I joined more than 40 years ago. Dues were $5. Ah, the good old days! Or ws it $4.oo ?

A Northeast Twin-Air, the way to get there.

Bill

Unstable

william,

whats with the rhymes at the end,
before you click send

William Robison

My Feedback: (3)

Unstable:

A Cockney
told me:

Writers of doggerel,
wont be sent "Ter 'El."

Nothing worse
than blank verse.

Your "End, Send." line.
You do pretty fine.

It's just something I started,
to cheer the down hearted.

I like twins,
so I keep them in.

Bill

LaCerne

My Feedback: (16)

W.R.

I just wanted to see if I could "stir the Pot" a little like you do.

I just received an email from John at NE aero, telling me he was mailing some sheeting that he said was missing from my kit. I had no idea anything ws missing, but according to his email through elimination mine was missing those parts. Hard to beat that kind o service huh?

William Robison

My Feedback: (3)

LaCerne;

All is calm, all is bright (If the sun is out). Pot stirring can be fun.

John (Northeast Aero) struck me as one of the good guys, glad to see he is proving that he is.

Opinions of the Twin-Air? Let us know, I'll know my opinions soon.

See the pot-waters boil, set your twin to "Boil."

Bill

William Robison

My Feedback: (3)

Today, Saturday the 23rd of November, 2002, is the day my Northeast Aerostar "Twin-Air 40" arrived in the hands of my mailman. Except she's a mailwoman.

All angles are zero-zero. That, I wanted to see.

After examination I have decided it is indeed a simplified Duellist. Box fuselage instead of the planked oval, and constant chord wings instead of the Duellist's tapered wing. And, not least, longer landng gear.

Retract taildragger wont be as simple a conversion as I had hoped, might leave it as a fixed gear tricycle. Not enough length in the nose for trike retracts uless it sits very low in the nose. Still a lot of time to ponder, a real Duellist and a J-3 are in line ahead of it.

Weather's fine, fly a twin of mine.

Bill

LaCerne

My Feedback: (16)

I just received the updates and can't see much differece except for the 3/8" triangle stock in the nacelles. I'm just finishing up a goldberg cub to put on floats. I can't decide if I should start the twin-air or my house of balsa PT-19.

As for the retracts in the nose, I believe the nosegear may have to slide rewards, then retract forwards. Might be doable with a mechanical type retract, or might be a pipe-dream. I have also decided to split the ailerons and make the inboard portion flaps. the servos will fit in the engine nacelles.

Anyhow, it's fun to day-dream about.

William Robison

My Feedback: (3)

LaCerne:

The HOB PT-19 would probably be the faster to complete, there's a LOT of waste balsa on the Twin-Air that has to be carved/sanded off. PT-19 cheaper to build also.

Do you have the engines for the T-A on hand? What are you planning to use? Fellow local flier talked me out of my long-pipe Rossi engine pair, so I don't yet know what mine will get. I have a beautiful K&B Q40, Perry carb and pump, if I could find its mate there'd be no question. The Duellist is getting #4011 K&B engines.

As drawn there is no room to put retracts for tail dragger configuration within the nacelles, but a slight deepening of the nacelles will allow it.

You almost had a good idea with the moving mount of the front strut. I think a sliding mount would be way too heavy. Change it to have the gear leg hinged about 2/3 of the way down, the upper part would swing up rearwards while the lower part folds up forward putting the nose wheel beside the upper part as the gear completes retraction. If you are familiar with the DC-2 and DC-3 mains, something like that except a single leg instead of the double strut. If it weren't for steering you could almost do a direct copy.

This is off the twins line, but I need to share my pain.

I destroyed an airplane this afternoon. I strongly suspect I will find nothing but crash damage when I check, but I could have had the elevator servo fail. I was doing a low fast inverted pass, the plane started to lose altitude, too low to roll it out, and it did not respond to down elevator. Hit amost flat, no more than a 15 degree down angle, but the air speed was up, running full throttle. On our asphalt paved runway. D**n it, it should have climbed out! The runway survived. Not the plane.

The airplane was only four months old.

At least it was a single, not one of my twins.

I can fix it easily if I spend the $140 to buy another ARF - it was the RCM Trainer 40. With which I had no business flying a low fast pass, much less a low inverted fast pass.

Oh well, as has been said, it's not whether, it's when you will crash. But other than two heli crashes, one of which was an obvious part failure - one of the tail rotor grips broke with no notice, shook the bird apart in midair - and I still don't know the cause of the other - I've not had a bad crash in years. I am not used to seeing my planes stacked in. Maybe I was just due for a bad one.

Got caught by the crash curse,
with a twin it would've been worse.

Bill

LaCerne

My Feedback: (16)

William

Sorry to hear about your crash. I once lost an airplane like that. The elevator pushrod flexed when it moved out too far and at high speed the airflow kept it from moving the right way. Had I reacted a little quiker and rolled it over I could have saved it. Oh well I know how you feel!

I am planning Saito .30 fourstrokes for the twin-air. They're light and fairly powerful. I'll need to keep it light which is why I'm going back and forth about retracts. There's just no way to do a light-weight installation. I may keep it fixed and see how the power is, then build the next with RG. I'm also planning on the small light-weight servos.

The DC-3 concept is very interesting and I think would be simpler. As you said the steering will be the issue.

Maybe we should start a twin-air thread?

William Robison

My Feedback: (3)

LaCerne:

Push rod flex is a possibility, the horn is on the top and I was going fast. If it did flex that could have removed all the down I didn't get.

Did you get the Twin-Air 20? I got the T-A 40, and I don't think the Saito 30 engines would fly it for poo. They'd be nice on the 20.

Regards a T-A thread, I had already pondered one for the constructuion, sort of an ongoing review of the airplane, but I'd rather have Model Airplane News pay me for it. Or even Gordon Banks might take a buck or two out of RC Report's pizza fund if it were sent to him.

Main problem as I see it is that the Twin-Air is an unknown airplane, people might browse but as no one is building one we'd have no participation other than cheers and teasing. I wouldn't mind either one. Do you think there might be interest anyway?

Another possibility would be to have John Marien sponsor the construction series, I suppose we could ask.

Share your thoughts.

As we each build our Twin-Air,
others should watch, it's only fair.

Bill

LaCerne

My Feedback: (16)

I got the twin-air .45. This is the same wing and fuse (with the exception of the obvious differences) as the Sport-air .60. I believe the saito's will fly it well enough, just not balistic. With 800 + sq inches of semi-symetrical wing, it shouldn't need too much. The Saito .30 I have now turns a graupner 10-6 at about 9000 to 9500 rpm. It's got to have four-strokes for me, but the next size up is the OS 40, or the Saito .56. The OS is over 3oz heavier each, and the Saito 6oz heavier each. The .40 doesn't appear to put out a significant amount of Hp over the .30 Saito. The Saito .56 is just too much. I left out the Saito .50 because it is only 1/4 oz lighter than the .56.

The whole twin-air came about because I have a DC-3 kit that I want to put the Saitos on, but I'd like to know beforehand if the .30s are enough.

Convoluted, I know, but...

William Robison

My Feedback: (3)

LaCerne:

If we didn't involve ourselves in twisty (convoluted) thinking we'd take up basket weaving. No, that requires a lot of warpage also.

Saito 30, 10x6 at 9000? Are you sure nobody slipped a 0.56 crank and piston/cylinder set in there? That's a "Honkin" 30! Should drag it respectably.

Concerning Twin-Air 40 or T-A 45 - my error, this one is also a T-A 45, don't know why I was saying 40. John confirmed it is a bash on the Sport-AIR 60, in fact the T-A 45 IS the S-A 60 with the nacelles stuck on. all the structure for the 60 is still in the nose. You can save weight by replacing the nose mounting plate with 1/8" ply, and eliminating some of the other bracing there. But leave the bracing if you like to land with a high rate of sink.

I am an appearance junkie as much as a builder and flier, and this severely limits my engine choices on twins. For me to be happy the right engine must have its crankcase reversed to allow both cylinders on the fuselage side of the nacelles, this leads to muffler fit problems. See my note about the mufflers on the 4011 K&B engines. I could, of course, use rear exhausts, but they are usually found only on racing engines and I don't want to go quite that fast. Not to mention starting prices in the $300 plus range.

Regards the T-A as a test bed for the DC-3 engines, that's not a good idea. You're going to like the T-A so much you'll be forced to buy two more for the Douglas bird. HAW!

Of course you might also get a pair of hot two strokes for the T-A.

We'll see.

K&Bs or Saitos in her, she gotta have Tru-Turn spinners.

Bill

LaCerne

My Feedback: (16)

William,

I am in total agreement about the cylinder heads. I think I can put a cooling scoop on top of the nacelles and run the right side muffler over and down behind the engine. The mufflers on that Saito are so small it might just work??

William Robison

My Feedback: (3)

LaCerne:

Using 2-strokes, an extension on one engine only almost certainly would not work (unless the same length was added to both) it would affect the power curve of the engine - like having both engines piped, but different pipes on the two engines.

Four strokes will react in a similar way, but they seem to be less critical. I wouldn't have any hesitation about using an exhaust extension, but to avoid possible problems I'd be sure to use an equal length extension on both.

With hot cams the effect will be more pronounced, simplistically speaking the longer the pipe the lower the power peak rpm. The peak can be widened, over a greater rpm range, by having the pipe diameter increase with increasing distance from the exhaust valve. This is the purpose of a "Megaphone" exhaust. But they are loud. To quiet the noise the diameter can be brought back down to a smaller size for the outlet. If done properly this can cause an even greater increase in power as well as quiteing the exhaust. You then have what is properly called a "Reverse cone megaphone," it is commonly called a "Tuned pipe."

In reality, tuning the exhaust is a black art, there are computer models that will get you close, but peaking it is still cut and try. When you are truly peaked you might find enough atmospheric variation from late morning to early afternoon that you've lost peak. There are even variations in the same run of props - you might do nothing other than pull an APC 9x8 and put another APC 9x8 on - lose power.

A manufacturer can say "This pipe works fine on that engine at such-and-such rpm" and be quite correct. But then you have to pick and choose props that are both efficient and will maintain that particular rpm at the speed you want. All we have available are "Good enough" pipes, but not truly tuned. Too many variables. Just a small dent from careless handling can turn a sweet set up into a dog.

Sorry I ran on so long, I have a tendency to switch into "Schoolteacher" mode.

Fit your twin with a pipe, get a sound that's ripe.

Bill

LaCerne

My Feedback: (16)

William

My understanding is that a 2-strokes responds to a tuned pipe due to the fuel air mixture actually being pulled into the pipe then shoved back in to the cylinder due to resonance and giving the engine a much denser fuel-air charge for ignition. This doesn't work for a four-stroke unless there is a large amount of over-lap in the intake and exhaust valves, which few model engines have. I think the OS Supercharged 1.20 is one of the few.

Once again this is my understanding, but the only real gain in power from the exhaust side of the 4-stroke is in relieving back-pressure. When all is said and done you can always throttle back the stronger engine. As you well know full scale twins don't match engines for that reason.

LaCerne

My Feedback: (16)

Now that I re-read your post you are saying almost the same thing(I think?) in a different way.

AmishWarlord

My Feedback: (5)

Cloud Dancer 120 bashed into a twin. Satio 91's for power

William Robison

My Feedback: (3)

LaCerne:

Two stroke engines are generally more responsive to exhaust tuning than 4-strokes. But both can be made more powerful with careful pipe work. Inlet tuning can also be done.

A primer on exhaust tuning.

When a pressure wave comes to the open (outlet) end of a straight (Constant diameter, it can curve) pipe it reflects an opposite wave. When it comes to a closed end (exhaust port on 2s or exh valve on 4s) it reflects at the same "Polarity."

What this means is the ehaust pressure wave goes into the pipe and travels to the exit, somewhat leading the gas flow. When it gets to the end it reflects as a negative pressure wave. Then, when this wave gets to the exhaust port (or valve) there is an effective vacuum induced. If the exhaust opens as the negative wave arrives, we have a tuned pipe. A straight tuned pipe. Good for one rpm only. Raise or lower the rpm and the wave, which travels approx the speed of sound, will not be there at the right time. With enough rpm change, either up or down, a harmonic can arise in the pipe that is positive at port opening and will actually decrease the engine's power. And the tuned rpm is very narrow, could be as small as 100rpm up or down. Variable: pipe length. Shorter - pulse will get there faster, tuned for higher rpm. And of course, longer pipes come in at lower rpm. That is variable #1. And we aren't going to get anywhere near the efect of pipe diameter, we're going to have enough variables without that. Just pick a size that allows good flow, section area about the same as the port.

So what do we do to widen the power band? This is where the megaphones come in. Each point along the length of a cone acts as an open end to the pressure wave, so with the megaphone instead of one sharp pulse we get a much longer pulse, and this gives a wider range of time useable between port opening - now our power band could be as great as 2000rpm up and down. Longer cone lengths give wider expansion of the band, again the reverse is true. More variables are the taper rate, and the maximum diameter reached.

But now we are making a godawful racket. This is where the reverse cone comes in. Just as the increasing diameter of the first cone acts as an open end at each point in its length, the reverse cone with its decreasing diameter acts as a closed end at each point along its length. And reflects a broad opposite pulse. This has to be timed such that it does not cancel the good pulse from the increasing diameter section. Fun, eh? This is adjusted by varying the distance apart, usually with a straight section of pipe between the two cones. Since the decreasing diameter is more effective in reflection than the increasing diameter section, the reverse cone is seldom as much as 1/2 the length of the forward cone. Also by this time the exhaust gases have cooled a bit, and that affects the propagation speed of our pulses.

Do we have enough variables now? No? OK. Last two. Or at least the last two we're going to discuss in this session. The actual outlet pipe itself. For maximum quieting it should be as small as possible, but we do have to let the gases out. By now the actual gas flow has smoothed out from the hard pulses at the other end, so let's use a pipe 1/2 the diameter of the pipe from the engine to the first cone. Actually, this ratio is just about the best. But. There is another open pipe end. That one also reflects the pulses, so again we have to set the length of it as well.

See why it's a black art? Many cuts and many tries.

Any engine with pulsing exhaust can have its exhaust tuned. It is not uncommon to see tuned pipes on two stroke motorbikes, setting up a four stroke is such a b***h that it is seldom done. What you generally see on a 4s is a "Header," where each pipe is cut to the same length and then twisted as required for them to come together in a collector. With the pipes the same length you might not get any gain from tuning, you also have no loss from one cylinder getting a pressure pulse while another gets a vacuum pulse. Burned piston time.

Now that you're ready to design your own pipe set up, you only need one other thing. Starting number for pulse propagation speed in the exhaust is 3300 feet/second. This is affected by exhaust temperature, the amount of unburned oil, how fast it is cooled by contact with the pipe walls...

You can aslo tune the intake, as I said, but that is WAY beyond the length I want to go now. Your inlet stack starting number is 1700 f/s.

You think twins are a black art?
Exhaust tuning really makes you faint of heart!

Bill

91zulu

My Feedback: (2)

Hopefully Grumman Skyrocket..with Mag. 80s