Effect of two identical propellers having the same direction of rotation

[Matt Piatkowski]

Hello,
This is obvious that this effect is as twice as big then for one propeller. The torque reacted on fuselage is 2x larger and the gyroscopic moment is 2x larger.

There exist, though, well flying stunt models having the motors with propellers having the same direction of rotation and mounted in the wings nacelles ( like P-38 Lighting).
I wonder what trimming is used to mitigate the torque and the gyroscopic moment in such models for them to fly a decent pattern. The methods used fall into two categories:
Aerodynamic deflections of steering surfaces ( differential flaps, elevator’s halves or differences is their sizes) and the thrusts deviations from the model’s rolling axis. Perhaps also
Larger wing tip weight is needed. I stress that I not aiming for perfection in quality of the stunt pattern but for the methods of limiting the effects of larger torques and gyro moments.
Thank you,M.

[Brett Buck]

Hello,
This is obvious that this effect is as twice as big then for one propeller. The torque reacted on fuselage is 2x larger and the gyroscopic moment is 2x larger.

There exist, though, well flying stunt models having the motors with propellers having the same direction of rotation and mounted in the wings nacelles ( like P-38 Lighting).

    I note that on  the real thing, the props went in different directions, making what could have been a very hard-to-handle machine very gentle where precession and P-fctor matter.

I wonder what trimming is used to mitigate the torque and the gyroscopic moment in such models for them to fly a decent pattern. The methods used fall into two categories:
Aerodynamic deflections of steering surfaces ( differential flaps, elevator’s halves or differences is their sizes) and the thrusts deviations from the model’s rolling axis. Perhaps also
Larger wing tip weight is needed. I stress that I not aiming for perfection in quality of the stunt pattern but for the methods of limiting the effects of larger torques and gyro moments.

     It's no different from anything else, the precession and P are not huge effects either way. even on high powered heavy prop conventional airplanes, people generally ignore it or make token adjustments for it,  with good results. From a practical standpoint, you adjust the various factors (leadouts, rudder, etc) to minimize it, it might have been different that before, but the residual uncompensated effects are not likely to be a lot different.  The same passive stability approaches should work, although, it might start looking pretty dorky with the giant fins.

      And I can't see any reason at all it would need more tip weight.

   The one big difference I see is that you might get a great deal of yaw torque if the engines don't respond equally, all the, that is, differential thrust. That would be true no matter which way the props spun. And what little I have seen of twin (or more) engine stunt planes suggest this is probably the biggest difficult-to-solve problem. Electric should not have this problem to any great degree, so I think that is a lot more fruitful way to go.

    Brett

[Air Ministry .]

Somewhere , Sometime , Suddenly , When you least expect it ,  .



Hand Lauching ! If the launchee DOSNT throw it opposite & Banked OUT , it can drop a wing , turn left , accelerate , rolling , aiming directly for the pilot .

More Side Area - seems to slow this down . Negate the effect . Lots more . little side area , ( Like the Hornet & Mosquito ) Iit can snap  & Spin . Get the lines in the props .
Mr Rabe was intending COUNTER ROTATEING .

A big gust in a sharp turn , Where it gets the motors full volume , the ' P ' effect AND Torrque reaction due to the RELITIVE acceleration 9 Your in a gust ) can push & hold the wing down,
or over into a spin , if its ' cut loose ' .

the 262 profile was way less trouble than the Mosq. , due to being a flying wing fence - very consistant line tension .
THIS was a bittoffa ballsup . 2.25 kilo ( 2 'd be o k - goal was 1.75 ) Deep Airfoil - worked good - But held the speed steady . The thinner Mew Gull'd glide 2 laps unassisted .
So a intermediate'd be the nexrt trick ( theyre the same wing 0 . this'll glide two , whipped . the initial pushrod had a bit of slack . ANNOYING .

Nevertheless on the olde OS 35 s 's Itd loop & wingover on the outer !

Needed 10 x 4 props ( 3 Bl ) .
21 inch centers ( nacelles ! ) ( Id also leave the tailplane tips fixed - a bit of ' counter ' to steady / firm up - responce - feedback )
THUS , the long ways bettween pieces seem to disuade misbehavior !


Flying the wrong way , dehidrated , after a long trip , isnt the best idea yet . V stable & steady , though . Even when the pilot isnt .



ALSO YOU VERY MUCH KNOW THE wheels ARE THERE ! A DECIDED WAKE ! . On Profiles the tend to inhibet performance - or throw the drag curve way up , and bias is offset from aerodynamic neutral point .
So disc wheels & even faired / streamline section LEGS , ARE AS IMPORTANT AS PROPS .

Oriental based profiles the thrustlines 14 apart = ea. side ( 7 ) .

You can see the ' long ways ' to the fins , large dimensional deflection for small angular movement = directional stability .

In a WHUMP / Drop off , where its all under duress - > is where your aware of the TOURQUE REATION from the dual R H props . Or progressively - . if the lines are loose .

So the KEY REQUIREMENT is to ensure (  design ) so the lines DONT Get Loose .

[Peter in Fairfax, VA]

Brett,

The fellow that owned the hobby shop where I worked in 1972 had been a WW II pilot, primarily PBY Catalina.  However, he related that he test flew the very early P-38 before they had rigged counter rotation.  According to him, it had a tendency to ground loop.  I do not have independent verification of that story.  Perhaps this actually relates to the Mosquito, which was well known to ground loop.

Peter

[Air Ministry .]

Mosquito was for aces , you used the throttle , on landing , to keep it straight . Fences became an issue .

this is fine for wingovers & rounds On the Outer . IN CALM AIR . too .

Big wing fences & lots of it . Least irksome of the wee ones .
67 span ( gota 2 mitre one ) FOUR INCH PITCH is a LOT SAFER . 6 in on ' cut loose gettaway ' things happen QUICKER !

These two are V similar at the handle , theflight loadings (  handle ) are HALF on the Red One . Roll tendancy in gusts is worse / snappier .
Despite the O A flap span being similar . as in about four feet - with a foot off taper outside it . more or less - Consider for aerodynamics .

Se the P-38 . FLAPS arnt ' way out there '

Twiin 8 in pitch props , and you wear them . hold Down on Take off run . Do NOT attempt lift off before airspeed obtained . Torque roll is a result of LOAD & ACCELERATION .

recover DOWNWARD - engine out - from inverted . do the fly fishing lead the tip , on light flying swine . The De H 88 did multiple houglass clover & Sq eight on the maiden flight . !
But ive flown ONCE in the last two years , so not apt to try getting to carried away , straight off the bat . Just Now . 

a load / aileron connection , or RABE RUDDER , might alleviate things , before they get nasty . You dont take things for granted with twins .

[pmackenzie]

Hello,
This is obvious that this effect is as twice as big then for one propeller. The torque reacted on fuselage is 2x larger and the gyroscopic moment is 2x larger.

There exist, though, well flying stunt models having the motors with propellers having the same direction of rotation and mounted in the wings nacelles ( like P-38 Lighting).
I wonder what trimming is used to mitigate the torque and the gyroscopic moment in such models for them to fly a decent pattern. The methods used fall into two categories:
Aerodynamic deflections of steering surfaces ( differential flaps, elevator’s halves or differences is their sizes) and the thrusts deviations from the model’s rolling axis. Perhaps also
Larger wing tip weight is needed. I stress that I not aiming for perfection in quality of the stunt pattern but for the methods of limiting the effects of larger torques and gyro moments.
Thank you,M.

The twin will also have smaller motors and props than the single, so I suspect if you work the numbers the total torque and gyro forces will be similar to the single prop setup.

[Brett Buck]

Brett,

The fellow that owned the hobby shop where I worked in 1972 had been a WW II pilot, primarily PBY Catalina.  However, he related that he test flew the very early P-38 before they had rigged counter rotation.  According to him, it had a tendency to ground loop.  I do not have independent verification of that story.  Perhaps this actually relates to the Mosquito, which was well known to ground loop.

Peter

   The P-38 had contra-rotating props from the beginning. Only for temporary tests was it every fitted with two of the same engine, and, they also swapped engines side-to-side on at least one occasion for test purposes.

     Brett

[Perry Rose]

The P-38's engines did rotate in opposite directions. Exactly opposite the best way. The left engine rotated clockwise and the right counter clockwise.. Something to do with canopy buffeting or something like that. The best way is the opposite of that. So you don't have two critical engines if one quits. Or at least both rotating in the same direction clockwise or counter.

[frank mccune]

      To Matt Spencer:

       Is that you you holding the P 38 in your left hand while wearing a brown jacket?

       Stay well,

       Frank McCune

[Air Ministry .]

No . thats Fred Astair .

P 38 got  a nose down trim at high speed , ' J ' s or something had ' dive brakes ' to stop them overspeeding . so they could pull out .
so tthe change was somrthing to do with propwash over the tailplane . Also - and this is vauguely relevant - and coincident to the rant 1

The P-38 , with a engine out - Go around - If the live engines put to FULL THROTTLE - Itll go in . Theres no controling it . SO THEY SAY - pilots notes or
the instructional film ( is on u tube ).
the Proceedure is to bring ( The live engine ) up to say 3/4 - get the thing flying straight , then milk a bit more power up .

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I guess this is what I is trying to get across . ' Inside the Envelope ' a good twin is like a good any other . a bad ones bad .
Airflow over ( strong ) flaps , with good controls , maybe the response is a little firmer - more deliberate . positive .

But ' outside the flight envelope ' or at the edges , with things ' loaded up ' is when the action can get exciteing .
Id found 10 % rather than 5 % ensured useable settings . saving wasted fuel & flights when it wouldnt get a decent somewhat matched setting .

So PREP is important ( I. C . , lectrics cheating ! ) Clear spraybars if its castor & unused a while . 20 5 Nitro and your laughing , rich . Tourque .

I guess its just theres the usual engine gremlins X 2 , or multiplied . You make your luck with preparidness . FLYING in NORMAL conditions  ,
its all pretty straightforward . Adverse Conditions the ' Oh Sheet ' factor may make itself known . For these - DURABILITY in all systems is more a prerquisette .
Still , with a light & lively one , the initiate flick at the elbow to hint the thing into the bend , Like the rally driver , or a dirt bike . Not a couch potatoe .

If its for Aerobatics , the 4 in pitch gives a lot less swing on takeoff . You Need To stay Ahead of IT , a more sedate jobs more sedate .

Had me thinking last night - a 10 in. x 60. in wing Mosquito profile ( as opposed to the 8 x 48 ) we timed that with the Veco 19s on 20 % at something like 90 flat out on 8 x 6 's
Appx line length was over - So a pretty definate 90 mph . Of course its backed off into a 4 - 2 run ish , foe F2B pattern , buts really more ' combatish ' to fly .

SO , its gunna depend on the AEROPLANE ! .   



nuver pitcher , was absolutely useless till I got the 10 x 4 three blades on it , then they ' hooked up ' .

Large side profile , oor area aft , fin etc , damp & calm the adverse effects considerably , so minamilist side areas will get more instant yaw and all its mechanations & carry ons .( unless its My P-38 ! )

[Air Ministry .]

A picture of the Aeromodelor MOSQUITO - to illustrate a minimilist fuselage and suchlike . Re YAW . If you look at the AIRFOIL youll see why anyone'd use a Oriental ( De Rice ) one .



Of course , the diesels are less prone to snapping and howling the crankey old glows . The OS max 20s ran pretty good . And on No Nitro - back in the 1970s .

Cant crop it , but the Side Area made it the Cadilac of the trio . the steadiest .

[Steve Helmick]

Two Bobs have twins flying, that I know of...both electron burners. Bob Hunt's lovely twin got tested with CW & CCW props on each/opposite sides, and he reported there was definitely a good way and a bad way.

Last weekend, I saw Bob Welch fly his big profile OD Grumman Tigercat, and it flew quite well, tho Bob flew (IMO) too many different models on one weekend and his profile flights suffered some. But his placings for all the events (Profile, Classic and Expert) won him the famous "Crock Pot" with $255.29 (?) enclosed. Not a bad weekend! I don't know which direction his motors rotate, but he's a smart guy, and I'd expect that he's experimented with it. Assuming you're using electrons, there's no reason not to use a combination of CW & CCW rotations.  Steve