Twin engine downthrust
11-13-2002, 08:06 PM
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Twin engine downthrust
I don't think downthrust is requirement of a twin, my Twinstar doesn't have any downthrust.
I think it's more related to wing position. Is the duellist a high wing plane?
I think it's more related to wing position. Is the duellist a high wing plane?
11-13-2002, 09:16 PM
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Downthrust....
JG...How's things in Miami?.....Downthrust and sometimes O/B offset is used to help keeping a Twin Engine airplane from Yawing or worse yet from rolling upside down.....When an engine fails or loses power...our models are more apt to do this because of inherant/parasitic drag ratios higher than the full scale counterparts...also keep in mind we're flying and have to react after a problem occurs rather than being at the controls and making changes as problems arise....As an example..my B-25 lost #1 eng. in a shallow r/h climbing turn and was upside down in a "flash" with 3Deg of downthrust in each engine...it rolled to the left so I just keep going and rolled it back to straight and level and reduced power to idle and bellied it in ....unfortunately it was on a rockpile and was heavily damaged....I had a duelist with 2 deg downthrust and it would fly all day on one engine....so it also matters how much wing loading ,type of airfoil,distance between engines (farther apart exaggerates adverse yaw)Fuselage length inherant/parasitic drag coefficients....so what this means is there are several variables involved and each twin behaves differently....I would guess the Warbirds are most succeptable due to high wing loadings.....Bill 
11-14-2002, 04:46 AM
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Twin engine downthrust
My twinstar has 3deg down and 3deg out on each engine. Really makes a great flying plane. I'm assuming the down thrust is there for the same reason as on a single. Some of my planes without down thrust change elevator trim at differing throttle settings. The down thrust helps keep it consistant for a give power arrangement. When I went from the .25fp's to the TT .36 pros's I was not able to keep the elevator trim the same at all throttle settings.
Edwin
Edwin
11-14-2002, 05:09 AM
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Twin engine downthrust
The amount of down thrust is really a function of nacelle location. My BE1900 has nacelles like a King Air or anything else with the P&W PT6. With this nacelle the crank shaft is slightly above the wing and the engines thrust will cause a downward moment about the CG. In this situation, no downthrust is needed. Now I understand that you are building a duelist. If I remember correctly, I believe that the crank is just above the leading edge, so I would think that no downthrust would be necessary.
11-14-2002, 10:46 PM
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Downthrust.....
JG....Downthrust is just not a function of the centerline of crankshaft in relation to wing chord centerline...there are many other variables involved....most....but not all prop engines are typically mounted close to chord centerline and a small offset up or down makes little or no difference on effectively making a plane climb or descend....wing/stab incidence..... inherent/parasitic drag coefficients....airfoil design....fuselage design..only to name a few can influence undesired movement along any of the three Axis....Roll, Pitch or Yaw.....I've experimented on several large warbirds...I.E a Seafury and my Corsair with downthrust( Or rt thrust ) and my conclusion is theres not enough difference to warrant its use ...on the other hand on a twin its most appropriate to help during single engine operation....If one happens to lose an engine and slows to VMC(Speed at which eng. thrust overrides full rudder deflection...) no amount of down thrust will help....so each twin has its own engine out characteristics some better than others....Duelist happens to behave quite nicely under single engine ops....However B-25 loses an engine...other engine flys you to the crash site....Bill....
11-14-2002, 11:52 PM
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Twin engine downthrust
Thats an interesting point that you bring up. Vmc. Although we experience it frequently and live to learn about it. Most people do believe that life ends at Vmc. It would be very interesting to pick a copy of Flight Safety and read an article intitled "Recovering from Vmc".
The worst behavior that I have experienced, with one engine out, is as follows : sticks in the corners, the plane is headed away from the feild and climbing. I can't turn in the direction I wish and I can not arrest the climb. I sence the plane is slowing and I am only a second or two from spinning in. I must do something, even if it is wrong! If "no action" will result in a crash, then I must do "something".
I have had this happen to me a dozen times in the eleven years that I owned a twin. One of the worst traits the plane had or that any twin can have is that tendency to climb with an engine out.
To that extent, I would stongly advise against the typical downthrust application to engine instulation. If the plane is rigged to fly straight and level with the engines running and then the engine dies and the downward moment of that engines thrust is removed , you will experience a subtle climb tendency.
If you look at some aircraft nacelles like the L188, you will notice that the cranks are not only above the leading edge, but there is an incredible amount of up-thrust in the engines. This will counter the apparrent downthrust from having mounted the engines so high above the wing.
Another factor to consider while building you Duelist is dihederal. Dihedral is a twins enemy!! That is why scale models typically don't fly as well as the sport twins.
The worst behavior that I have experienced, with one engine out, is as follows : sticks in the corners, the plane is headed away from the feild and climbing. I can't turn in the direction I wish and I can not arrest the climb. I sence the plane is slowing and I am only a second or two from spinning in. I must do something, even if it is wrong! If "no action" will result in a crash, then I must do "something".
I have had this happen to me a dozen times in the eleven years that I owned a twin. One of the worst traits the plane had or that any twin can have is that tendency to climb with an engine out.
To that extent, I would stongly advise against the typical downthrust application to engine instulation. If the plane is rigged to fly straight and level with the engines running and then the engine dies and the downward moment of that engines thrust is removed , you will experience a subtle climb tendency.
If you look at some aircraft nacelles like the L188, you will notice that the cranks are not only above the leading edge, but there is an incredible amount of up-thrust in the engines. This will counter the apparrent downthrust from having mounted the engines so high above the wing.
Another factor to consider while building you Duelist is dihederal. Dihedral is a twins enemy!! That is why scale models typically don't fly as well as the sport twins.
11-15-2002, 01:28 AM
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Twins......
KingWoodBarney..Et All....One other item thats not mentioned enough is wing loading.....a duelist has a "relatively "light wing loading making it a better candidate for single engine flying....it also has a pattern plane type airfoil which is quite effecient....engine thrust also makes a big difference if the plane is marginally powered or is powered accurately leaving a good margin for single engine flying....Its best to find out the flight characteristics of a twin in a single engine profile before an engine fails but this makes me alittle uneasy flying a twin and filling one tank halfway and waiting for the engine to quit....its a Catch 22....hoping an engine doesn't fail or "attempting " to find flt characteristics on one engine....I fly large Warbirds (singles,twins and soon 4 engs)and between the high wing loadings/High drag coefficients/fair engine performance....Its just prudent if an engine fails , to reduce power ,push nose down and land as soon as possible .I've just seen way too many warbird twins spiral to their deaths after engine failure...Only know of one Warbird twin that flys "GOOD" on one engine......so far....P-38 lightning...have seen 5 B-25's crater on one eng...Bill.....
11-15-2002, 07:54 AM
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Several advantages
I've just read NASA Technical Memorandum 4541, which is a discussion of the reduction of drag and increases in aircraft performance at "dangerous" speeds, such as landing with high-lift devices deployed, go-around situations, etc..
Referring to twins, with nacelle-mounted engines:
One of the items tested was to increase (downward) the inclination of the nacelle (thrust line) during these times, and it did indeed reduce drag and improve performance, especially at low airspeeds where the wing was pitched up, and the propeller was running at high speed. It was even suggested in the report that a moveable nacelle might be a worthwhile improvement to existing twins.
NASA has the best web site on the planet, for those of you who don't already know. There are TONS of free images, drawings, movies, technical reports, and other info, freely downloadable. One of the best sources is the Langley Technical Reports Server, at http://techreports.larc.nasa.gov/ltrs/
Referring to twins, with nacelle-mounted engines:
One of the items tested was to increase (downward) the inclination of the nacelle (thrust line) during these times, and it did indeed reduce drag and improve performance, especially at low airspeeds where the wing was pitched up, and the propeller was running at high speed. It was even suggested in the report that a moveable nacelle might be a worthwhile improvement to existing twins.
NASA has the best web site on the planet, for those of you who don't already know. There are TONS of free images, drawings, movies, technical reports, and other info, freely downloadable. One of the best sources is the Langley Technical Reports Server, at http://techreports.larc.nasa.gov/ltrs/
11-15-2002, 08:10 AM
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Twin engine downthrust
Sure! The prudent thing would be to throtle back the good engine and glide her in! But I love the forbidden fruit of buzzing around on the deck!! It just so much sweeter!! I figure that the plane is honking along with plenty of kinetic. Energy is energy! Well, that is what I tell myself. That kinetic energy bleeds off in a hurry!
Good find on the NASA site. I figure thats the answer we paid for! Funny that they waited so long to research this!
Good find on the NASA site. I figure thats the answer we paid for! Funny that they waited so long to research this!
11-15-2002, 02:05 PM
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Twin engine downthrust
thats the one thing i figured out when messing around in Realflight.
speed == good.
if you lose and engine and still are well above Vmc then keep it lit and line it up for landing, because once you chop the throttle you usually are committed to a landing were ever you are.
I am hopeing that the twin I'm going to design this winter will fly well with one engine. I'm taking all these little tidbits I read on this forum and fileing them away in my head to remember for when I acually start cutting wood.
I am hopeing that it will be a aerobatic twin that is capable of stupid things like mixing throttle to rudder to induce flat spins and high G pullouts.
I guess we will see next spring when i either post a set of plans or post in the crash forum
BTW if it does fly like I want it to I will make up short kits. my friend is a kit cutter and I am sure he will not be against letting me use his shop to knock out a few bits of wood.
speed == good.
if you lose and engine and still are well above Vmc then keep it lit and line it up for landing, because once you chop the throttle you usually are committed to a landing were ever you are.
I am hopeing that the twin I'm going to design this winter will fly well with one engine. I'm taking all these little tidbits I read on this forum and fileing them away in my head to remember for when I acually start cutting wood.
I am hopeing that it will be a aerobatic twin that is capable of stupid things like mixing throttle to rudder to induce flat spins and high G pullouts.
I guess we will see next spring when i either post a set of plans or post in the crash forum
BTW if it does fly like I want it to I will make up short kits. my friend is a kit cutter and I am sure he will not be against letting me use his shop to knock out a few bits of wood.
11-15-2002, 10:51 PM
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Thrust lines
Hello all, I'm new to this thread, but if you will check the thread titled "Berkley B-26 Invader Model" there is a lot of discussion on thrust lines. More in the thread "Pica Duellist." But neither really gets into the reason for downthrust.
A stable, easy flying airplane will normally have the wing set at a positive incidence relative to the stabilizer, usually just the wing positive with the stab set at zero. Sometimes you will find the wing positive, and negative incidence in the rear. Least common has the horizontal fin installed with negative incidence and the wing at zero.
The reason for these incidence angles is purely one of stability. At normal cruising speeds the airplane, slightly nose heavy, has lift from the wing, and a downforce from the stab keeps the plane stable in level flight. If the bird slows, the down force will decrease, the plane pitches down, the speed increases giving more down force, the airplane levels off. If the speed increases in level flight, the down force increases, the nose pitches up, and so forth. Combine this with the wing's dihedral angle and you get positive stability. The airplane will fly itself, and recover from maneuvers with no control input.
Now we get to the engine thrust line. With the wing/stab incidences set for our stable flight, when we add power the plane does not speed up, it climbs. decrease power and it merely descends, but does not slow down. If you read Wolfgang Langsweische's (Spellinng?) book "Stick and Rudder" this is where he gets the statement "The throttle controls altitude, and the elevator controls speed." Simplistically that is true. By adding the right amount of down thrust on the engine you counteract the tendency to climb with power. That is the ONLY reason for down thrust. It is not uncommon to have your trainer climb suddenly when you chop the throttle, now you know why.
All this positive stability is why you can take your fingers off the sticks, and the plane will recover itself, given enough altitude.
Side thrust is to counteract the "P-factor," which will not be discussed here, I've gone on long enough already except for one additional consideration. Aerobatics.
A high performance aerobatic plane will usually have the wing and stab at zero incidence, and the engine set at zero up-down, and zero side thrust. This makes the plane fly the same upright and inverted, slow and fast, and gives the airplane generally neutral stability, that is, it goes where you point it, you have to fly it all the time. Also, the lack of stab downforce requires the center of gravity to be moved rearward, making the airplane more responsive to control inputs.
So, stable or twitchy, make your choice and set your angles. If you want to experiment with the angles, get a copy of Great Planes' "Real Flight Simulator." It lets you alter all these parameters and fly the result. It's also a great tool for scratch building, you can put your numbers in, and alter them to get the flight characteristics you want before you start cutting wood. It really works. My first scratch twin is a dog, didn't use RF on that one. Scratch twin #2 is a super nice airplane. I used RF to check the design. If you are building from plans, that's not scratch building and you don't need RF to check it, the designer has done all the brain work.
Hope this answers some, if not most of the questions.
Build a twin, you'll find you're "IN."
WLR
A stable, easy flying airplane will normally have the wing set at a positive incidence relative to the stabilizer, usually just the wing positive with the stab set at zero. Sometimes you will find the wing positive, and negative incidence in the rear. Least common has the horizontal fin installed with negative incidence and the wing at zero.
The reason for these incidence angles is purely one of stability. At normal cruising speeds the airplane, slightly nose heavy, has lift from the wing, and a downforce from the stab keeps the plane stable in level flight. If the bird slows, the down force will decrease, the plane pitches down, the speed increases giving more down force, the airplane levels off. If the speed increases in level flight, the down force increases, the nose pitches up, and so forth. Combine this with the wing's dihedral angle and you get positive stability. The airplane will fly itself, and recover from maneuvers with no control input.
Now we get to the engine thrust line. With the wing/stab incidences set for our stable flight, when we add power the plane does not speed up, it climbs. decrease power and it merely descends, but does not slow down. If you read Wolfgang Langsweische's (Spellinng?) book "Stick and Rudder" this is where he gets the statement "The throttle controls altitude, and the elevator controls speed." Simplistically that is true. By adding the right amount of down thrust on the engine you counteract the tendency to climb with power. That is the ONLY reason for down thrust. It is not uncommon to have your trainer climb suddenly when you chop the throttle, now you know why.
All this positive stability is why you can take your fingers off the sticks, and the plane will recover itself, given enough altitude.
Side thrust is to counteract the "P-factor," which will not be discussed here, I've gone on long enough already except for one additional consideration. Aerobatics.
A high performance aerobatic plane will usually have the wing and stab at zero incidence, and the engine set at zero up-down, and zero side thrust. This makes the plane fly the same upright and inverted, slow and fast, and gives the airplane generally neutral stability, that is, it goes where you point it, you have to fly it all the time. Also, the lack of stab downforce requires the center of gravity to be moved rearward, making the airplane more responsive to control inputs.
So, stable or twitchy, make your choice and set your angles. If you want to experiment with the angles, get a copy of Great Planes' "Real Flight Simulator." It lets you alter all these parameters and fly the result. It's also a great tool for scratch building, you can put your numbers in, and alter them to get the flight characteristics you want before you start cutting wood. It really works. My first scratch twin is a dog, didn't use RF on that one. Scratch twin #2 is a super nice airplane. I used RF to check the design. If you are building from plans, that's not scratch building and you don't need RF to check it, the designer has done all the brain work.
Hope this answers some, if not most of the questions.
Build a twin, you'll find you're "IN."
WLR
11-16-2002, 12:04 AM
#15
twin lines
Thanks everybdy for the replays. W. Robison summarized the facts very good and things start to make sense in my little brain.
When i started the design, I copied everything from the full size plan, but then I realized the models are much harder to fly in abnormal situations, so I want to add more stability. I heard a long time ago that downthrust has a way of cancelling out some of the left turn tendencies, but nobody seemed to confirm that.
Here is my project...
JG
When i started the design, I copied everything from the full size plan, but then I realized the models are much harder to fly in abnormal situations, so I want to add more stability. I heard a long time ago that downthrust has a way of cancelling out some of the left turn tendencies, but nobody seemed to confirm that.
Here is my project...
JG
11-16-2002, 12:34 AM
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Twin engine downthrust
JGrc, Get a Pica Duellist 2/40 and bash it. Check it out, Duellist 3-view is very close to the DeHavilland wooden wonder, should be an easy alteration.
Single engine, not a pair? At least you're halfway there.
WLR
Single engine, not a pair? At least you're halfway there.
WLR
11-19-2002, 08:09 PM
#17
Wooden wonder
WLR,
I would get the Duellist except that is too small & I have the Hornet pretty drawn and ready to cut. Also I checked the dimentions and like you said is very close to the Duellist , I figured it might not be a visiuos airplane. To compare I measured distance from CG to vertical, angle and distance from props to CG then calculated percentage of the wingspan and is all within 4%. The Hornet has 0 downthrust, 0 sidethrust, 0 horinzontal and +1.5º wing incidence.
JG
I would get the Duellist except that is too small & I have the Hornet pretty drawn and ready to cut. Also I checked the dimentions and like you said is very close to the Duellist , I figured it might not be a visiuos airplane. To compare I measured distance from CG to vertical, angle and distance from props to CG then calculated percentage of the wingspan and is all within 4%. The Hornet has 0 downthrust, 0 sidethrust, 0 horinzontal and +1.5º wing incidence.
JG
11-19-2002, 08:34 PM
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Twin engine downthrust
JG, Thanks for saying "Hornet." I knew it wasn't a "'Skeeter," but I couldn't remember what the later plane was called. Re-reading your earlier posts I can see that you were looking at a larger model to start with, had I seen that before I would not have suggested the bash.
But, as I have said before, I have had the best flying planes using everything at zero-zero, both single and twin engined. The 1 1/5 degree on the wing will make it easier to fly upright, but inverted you might not like it. You could attach the wing in a manner to be adjustable, as most pattern planes are, but that would affect the thrust line also with a twin. Oh, well.
Have fun, that's what it's all about.
With a double fan, you've shown you're a man!
WLR
But, as I have said before, I have had the best flying planes using everything at zero-zero, both single and twin engined. The 1 1/5 degree on the wing will make it easier to fly upright, but inverted you might not like it. You could attach the wing in a manner to be adjustable, as most pattern planes are, but that would affect the thrust line also with a twin. Oh, well.
Have fun, that's what it's all about.
With a double fan, you've shown you're a man!
WLR
