DH88 Comet
12-12-2002, 02:26 AM
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DH88 Comet
I downloaded a set of plans for the DH88 Comet from the web. They were drawn by a man named Joe Colletti.
They are a nice set of plans, but I was just wondering about the following: all angles and incidences are set to zero on these plans. This can't be right I gather.
The plane is a 93" span twin, with high taper wings and a symmetrical airfoil.
Is there someone out there that can give me some advice on engine thrust lines, wing incidence and wash-out?
Anyone who knows the Comet will agree that this thing defenetly needs some wash-out, because stall properties without it would be terrible...
Some input would be helpfull
They are a nice set of plans, but I was just wondering about the following: all angles and incidences are set to zero on these plans. This can't be right I gather.
The plane is a 93" span twin, with high taper wings and a symmetrical airfoil.
Is there someone out there that can give me some advice on engine thrust lines, wing incidence and wash-out?
Anyone who knows the Comet will agree that this thing defenetly needs some wash-out, because stall properties without it would be terrible...
Some input would be helpfull
12-13-2002, 07:46 AM
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Rude One:
My best flying scratch twin has all incidences set to zero, and the engine thrust lines are 0-0-0. With no washout on the wings. This gives me maximum maneuverability, no change flying right side up or inverted.
The Duellist 2/40 (I have two) is also a very nice flying twin. On the Duellist the wing and stab are both two degrees positive, the engines have seven degrees down thrust (no side thrust), and the wing has two degrees washout at the tips.
Joe Bridi agrees with my set up, everything at zero, but Dave Platt designed the Duellist and his set up works very well also.
Many people swear by out thrust on twins as well. If this is your first twin I'll go along with out thrust: Three degrees out (right) on the right engine, and one degree left on the left engine. Difference is due to engine torque.
What I'm attempting (poorly) to say is that the plane will fly regardless of the angles if it's done well. Build it per the plans, except maybe add some washout if you wish. Or fly as I do, and avoid slowing to stall speed unless you want it to stall.
Build the giant deHavilland twin, no money left for food - you'll stay thin.
Bill.
My best flying scratch twin has all incidences set to zero, and the engine thrust lines are 0-0-0. With no washout on the wings. This gives me maximum maneuverability, no change flying right side up or inverted.
The Duellist 2/40 (I have two) is also a very nice flying twin. On the Duellist the wing and stab are both two degrees positive, the engines have seven degrees down thrust (no side thrust), and the wing has two degrees washout at the tips.
Joe Bridi agrees with my set up, everything at zero, but Dave Platt designed the Duellist and his set up works very well also.
Many people swear by out thrust on twins as well. If this is your first twin I'll go along with out thrust: Three degrees out (right) on the right engine, and one degree left on the left engine. Difference is due to engine torque.
What I'm attempting (poorly) to say is that the plane will fly regardless of the angles if it's done well. Build it per the plans, except maybe add some washout if you wish. Or fly as I do, and avoid slowing to stall speed unless you want it to stall.
Build the giant deHavilland twin, no money left for food - you'll stay thin.
Bill.
12-13-2002, 02:29 PM
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Engine thrustlines
Thanks for the reaction William.
This is the first twin design I'm looking at. However, I think I will be building a twin trainer of some sort first. Something simple to practice on single engine landings and stuff. Any designs out there you would recommend for this purpose?
Why would you use out thrust on the engines? Is this done just to get some airflow on the rudder at low speeds? Or does this help in an engine-out situation also?
I will be putting the wing at an incidence and use some wash out. I think it would have rather nasty snap characteristic without it.
This is the first twin design I'm looking at. However, I think I will be building a twin trainer of some sort first. Something simple to practice on single engine landings and stuff. Any designs out there you would recommend for this purpose?
Why would you use out thrust on the engines? Is this done just to get some airflow on the rudder at low speeds? Or does this help in an engine-out situation also?
I will be putting the wing at an incidence and use some wash out. I think it would have rather nasty snap characteristic without it.
12-13-2002, 03:07 PM
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Go to "Slowjoe"'s webpage and email him....he's a great guy and a good designer. I'm sure if the prototype didn't fly as well as he wanted, he will tell you what he would do differently. Just ask!
Go here: http://members.telocity.com/slowjoe/slowjoe170/sj.html
Go here: http://members.telocity.com/slowjoe/slowjoe170/sj.html
12-13-2002, 06:22 PM
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Rude One:
From the looks of it, that DH88 woulds not be a bad twin trainer. But it would be a real shame to build it and lose it on its first flight because you made a dumb mistake that you never would have with some twin stick time under your belt. Something REALLY dumb like using ailerons when you should have used rudder. That is absolutely the biggest/hardest thing to learn after getting lazy with a single. Until you unlearn the "Ailerons only" method of turning you're going to lose airplanes. Maybe (with luck) just spin in with minor damage. Or if REALLY lucky, just scare the pooh out of yourself, and recover from a departure without crashing.
For a twin trainer the Northeast Aerodynamics "Twin-Air 45" is hard to beat. Wood like you had picked it yourself, easy build, and a really good flyer.
www.ne-aero.com
John Marien (NE Aero) is one of the good guys.
The out thrust is indeed for easing engine out characteristics. Since the engine torque tends to turn the plane to the left, the torque and off-center thrust add when the right engine only is running, but with the left only the torque, still trying to roll the plane to the left, counteracts some of the right turn tendency. Therefore, more out thrust is used on the right engine.
Hope this helps.
Twins give your senses a push, until you land in the bush.
Bill.
From the looks of it, that DH88 woulds not be a bad twin trainer. But it would be a real shame to build it and lose it on its first flight because you made a dumb mistake that you never would have with some twin stick time under your belt. Something REALLY dumb like using ailerons when you should have used rudder. That is absolutely the biggest/hardest thing to learn after getting lazy with a single. Until you unlearn the "Ailerons only" method of turning you're going to lose airplanes. Maybe (with luck) just spin in with minor damage. Or if REALLY lucky, just scare the pooh out of yourself, and recover from a departure without crashing.
For a twin trainer the Northeast Aerodynamics "Twin-Air 45" is hard to beat. Wood like you had picked it yourself, easy build, and a really good flyer.
www.ne-aero.com
John Marien (NE Aero) is one of the good guys.
The out thrust is indeed for easing engine out characteristics. Since the engine torque tends to turn the plane to the left, the torque and off-center thrust add when the right engine only is running, but with the left only the torque, still trying to roll the plane to the left, counteracts some of the right turn tendency. Therefore, more out thrust is used on the right engine.
Hope this helps.
Twins give your senses a push, until you land in the bush.
Bill.
12-14-2002, 04:15 AM
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Dickeybird,
been there, done that... I sent Joe a mail a couple of days ago, but didn't get a reaction. That's why I came here looking for advice in the first place... I guess Joe is a pretty busy guy, judging by his website.
William,
Thanks for the input. You're right about one thing: single engine pilots get lazy on the rudder. I only use this thing when planing close to the ground at very low speeds (3D type flying), and for basic acro of course.
The offset engine thrustlines part in your reply really makes sense. I will be giving that a go.
I also have a very old (like 10 years or so) article somewhere explaining how you can setup your transmitter to simulate an engine-out. I will try that too. It is one of the few old magazines I didn't throw out over the years...I guess that twin must have been in the back of my head somewhere all the time, but I just didn't get around to it yet.
been there, done that... I sent Joe a mail a couple of days ago, but didn't get a reaction. That's why I came here looking for advice in the first place... I guess Joe is a pretty busy guy, judging by his website.
William,
Thanks for the input. You're right about one thing: single engine pilots get lazy on the rudder. I only use this thing when planing close to the ground at very low speeds (3D type flying), and for basic acro of course.
The offset engine thrustlines part in your reply really makes sense. I will be giving that a go.
I also have a very old (like 10 years or so) article somewhere explaining how you can setup your transmitter to simulate an engine-out. I will try that too. It is one of the few old magazines I didn't throw out over the years...I guess that twin must have been in the back of my head somewhere all the time, but I just didn't get around to it yet.
01-05-2003, 03:51 PM
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Originally posted by AmishWarlord
Hey where did you find those plans at rude? I'd like to get a coppy of them.
Hey where did you find those plans at rude? I'd like to get a coppy of them.
Joe Colletti's homepage
01-05-2003, 04:00 PM
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WOW!!!!
Anyone have any info on those plans??? How are they?
All wood construction?
The Comet in quarter scale would really be something...
I've just been browsing that site a little...and came across a 312" wingspan quarterscale PBY Catalina...you cannot call this modelling anymore. In a Cat that size there's room for an adult person...
Put a couple of Rotax engines on it and some instruments and you've got yourself a homebuilt!
All wood construction?
The Comet in quarter scale would really be something...
I've just been browsing that site a little...and came across a 312" wingspan quarterscale PBY Catalina...you cannot call this modelling anymore. In a Cat that size there's room for an adult person...
Put a couple of Rotax engines on it and some instruments and you've got yourself a homebuilt!
01-05-2003, 10:47 PM
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Yep, Steve Holland's Comet is a real beast. Did you know he built in those Zenoah's without possibility to take them out? He just made a few small service hatches...
I've got a nice pic at home of him lying under the wing of the plane to get some shadow on a hot day. I've got no scanner though, so I can't post it.
I'm planning on going to Great Brittain this summer to get a lot of pictures of the real one. Canopy and landing gear details and stuff.
What engines would you put on the 132" span Comet?
I've got a nice pic at home of him lying under the wing of the plane to get some shadow on a hot day. I've got no scanner though, so I can't post it.
I'm planning on going to Great Brittain this summer to get a lot of pictures of the real one. Canopy and landing gear details and stuff.
What engines would you put on the 132" span Comet?
01-05-2003, 11:31 PM
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OS BGX's converted to diesel, that way they would put out 6.15 HP each! 12.3 HP should move that plane. LOL my just go with diesel 108s The BGX's may be a bite much.
