Unequal flap deflection good or bad?

[frank mccune]

     Hello:

      A friend of mine was gifted two Noblers and on one, the inside flap has more deflection than the outside.  Was this a common idea back in the day?  I can only guess at the reason and wether or not it was a good idea.

     Suggestions/comments

     Tia,

      Frank McCune

[Brett Buck]

A friend of mine was gifted two Noblers and on one, the inside flap has more deflection than the outside.  Was this a common idea back in the day?  I can only guess at the reason and wether or not it was a good idea.

    Bad. It is already prone to rolling back and forth, it doesn't need differential flap movement to make it worse.

    But let's be sure what we are talking about here, because it is not easy to make that happen - requires a special control setup that cannot be achieved with conventional parts. What you are suggesting that the inboard flap moves more in either direction that the outboard, and that the neutrals are even at neutral elevator, and that move the control, say,  5 degrees one direction and the inboard flap goes  8 degrees, and the outboard flap goes 5 degrees, and in the other direction, it also moves 8 on the inboard and 5 on the outboard in the other direction?  In other words, the rate of the motion of the inboard is faster than the inboard at all deflections?  Or is it only at full deflection (i.e. the inboard moves freely and the outboard binds up at the end of travel)?  Is there slop between the two flaps?   Are they both neutral at the same time?

    Brett

[Dan McEntee]

I'm not saying it's desirable. But didnt Bob Palmer have that onaversionof theThunderbird? Didnt Veco produce the hardware required for this? Makes me want toaskifit's a really old Nobler.
 Type at you later,
 Dan McEntee

[Trostle]

In the December 1959  issue of Model Airplane News, Bob Palmer wrote an article on Differential Flaps.  This was installed in a Thunderbird.  Each flap had its on control horn, the inboard horn was 3/4" while the outboard horn was 25/32" (that is 1/32" longer).  This provided for the inboard flap to move more than the outboard flap with the claim that the system provided "slight roll and greater line tension at points where it's most needed".

There were several problems with this.  The differential indeed would help on some turns, but it was detrimental in other others  The spit flap horn in itself is not a good idea unless there is a system employed  to not allow the flaps horns to flex differently under different flight loads unless the horns used employ a system which minimizes the differential twist (as practiced by Paul Walker and others who use Paul's system).

I was witness to conversations with Bob Palmer in Tucson after 2000 where he explained that the system did not work that well, in fact, he stated that it was more effective to have the outboard flap with increased travel but was still not a satisfactory approach.  Part of the effect for the different approach was that the increased travel of the outboard flap increased the drag on the outboard side but it still created trim problems that could not be cured.

Shortly after the MAN article, Aeromodeller magazine also published information on the Palmer concept.

I tried the approach from the MAN article in the early 60's.  I was not an accomplished pilot by any stretch, but I remember that I did not like it.  I think the main problem came from flexing of the horns by the pushrod set up that caused flexing of the control system that resulted in problems in the maneuvers, particularly unwanted characteristics in the transition of all of the eights.

(It is interesting to note that the Thunderbird Bob experimented with had the nose lengthened 1 1/2", 1" increase in stabilizer span and more area at the wing tips.)

Keith

[frank mccune]

     Hello:

     I just spoke to the owner of the Nobler and he told me that each flap is controlled individually.  He is favorably impressed by the craftsmanship that went into the construction of this airplane.  It looks like it has flown many flights.

     The plane is being stripped of the ancient tissue and will be covered in Monocote.

      I suggested that he fly the plane as is when completed.  If he is not satisfied, he can return it to being a wall hanger.

     Thanks for the replies,

     Frank McCune

     

[Istvan Travnik]

My last small model (built in 1987 for HP40) made with pretty effective flaps (concave airfoil, 30% of chord in root, 20% at tip) , made with adjustable, individual horns. The whole wing was totally symmetric. I used during 13years, up till the end of 1999. Flights: 2000 minimum, 3000 believable.
In recent 5 years came off the wall and was "hired" by two comrades. Me and all of others loved it, really.
The best adjustment was a little bit faster OUTSIDE flap, and more wingtip weight than conventional.
Other parameters were: flap movement: linear, + - 20°;  elevator: + - 45°; strongly exponential.
Istvan

[Ken Culbertson]

     Hello:

     I just spoke to the owner of the Nobler and he told me that each flap is controlled individually.  He is favorably impressed by the craftsmanship that went into the construction of this airplane.  It looks like it has flown many flights.

     The plane is being stripped of the ancient tissue and will be covered in Monocote.

      I suggested that he fly the plane as is when completed.  If he is not satisfied, he can return it to being a wall hanger.

     Thanks for the replies,

     Frank McCune

   

I was just a kid when I had my first Thunderbird and it had those flaps.  At 14 you don't question a legend like Bob Palmer so you use them.  Didn't like them, never used them again.  Occasionally one pops up and I get a kick out of that noticeable wiggle in the overhead 8.  In theory, if they were made to work in reverse coupled to a logarithmic system they might work..or not.  Keith's CAM rudder can be set to accomplish the same thing with no side effects.

DON"T DO IT - Ken

[Larry Renger]

Late in his life I had the privilege of speaking with Bob Palmer and asked him about the differential flap system. He said it was not good for competition but useful for inexperienced sport flyers.

On my Smoothie, I actually reduced the inboard flap area to cure square corner hingeing. The larger inboard wing was too much.

We have come a long way in understanding CL aerodynamics.

[Brett Buck]

Late in his life I had the privilege of speaking with Bob Palmer and asked him about the differential flap system. He said it was not good for competition but useful for inexperienced sport flyers.

   Not surprisingly, Bob Palmer was right. It is useful only if maintaining additional/extraneous line tension is more important than competitive maneuvering. The intent is to roll the airplane in the direction to add line tension (by rotating the lift vector to the outside of the circle). It's the same theory behind adding tip weight.

    The problem comes when you have to start maneuvering in either direction, quickly enough that it can't go from rolled one direction to the other fast enough. It just starts flopping around and while it is, you have no idea what a given amount of control pressure might do. The same idea as expressed here:

https://stunthanger.com/smf/at-the-handle/to-45-90-or-not-to-45-90-that-is-the-question/msg600662/#msg600662

    Differential flap movement is an extreme case of "out of trim" condtion that takes extraordinary effort to correct. I would add that many people figure "in trim" is the equivalent to "has enough line tension to get through the pattern most of the time", after which you are done and you should go out and practice until you win. That theory may have worked in the past but I don't think you are going to get too far with that plan today.

     Brett

[Brett Buck]

My last small model (built in 1987 for HP40) made with pretty effective flaps (concave airfoil, 30% of chord in root, 20% at tip) , made with adjustable, individual horns. The whole wing was totally symmetric. I used during 13years, up till the end of 1999. Flights: 2000 minimum, 3000 believable.
In recent 5 years came off the wall and was "hired" by two comrades. Me and all of others loved it, really.
The best adjustment was a little bit faster OUTSIDE flap, and more wingtip weight than conventional.
Other parameters were: flap movement: linear, + - 20°;  elevator: + - 45°; strongly exponential.
Istvan

   That is a reasonable way to use it  - an alternative to adding tabs to the flaps to change the area. Most airplanes end up with either slightly larger outboard flaps, or tabs to accomplish the same purpose - which is to keep the roll angle near zero at different load factors (corner tightness). If you don't do it, if you get it perfectly straight in level flight, it will roll a little bit in the round loops and a lot in the squares - like it needed different amounts of tip weight in different kinds of corners. Adding tab or outboard flap area  (or movement), it has progressively more effect as the load factor goes up.

   My airplanes have had about 1/8" extra flap chord to get this to come out about right without adding extra. It is not the same for every airplane, even of the same nominal design, it is a classic trimming adjustment. I claim that I invented the "adjustable tab" that held it with screws on the outboard flap and had slots to allow you to vary the area without gluing or cutting. Given that it seems pretty obvious, probably other people also "invented" it over the years, since it's a common problem.

   So far I have been doing pretty good with the Infinity series, my built-in differential flap area seems to have been pretty close to right and I have never had to add flaps or cut one down. I have changed flaps A LOT, but to change the overall area, not the differential. I am making some right now, smaller, for the latest Infinity, and have in the past made them as much as 3/4" larger to get more flap for a given pitch rate.

     Brett

[Ken Culbertson]

the "adjustable tab" that held it with screws on the outboard flap and had slots to allow you to vary the area without gluing or cutting.

Brett - is there a picture of this anywhere?  I have been using the 1/8" on the outboard since the 70's and didn't really know why until reading up on it here - thanks.  It is nice to *know* what you know. 

Ken

[John Park]

If anybody's got the original Finnish-language plan for Juhani Kari's Nakke, they'll see that the inboard flap horn's length is shown as 19mm., while the outboard one's length is 17.5mm - a full sixteenth of an inch shorter!  Build it like that, and the outboard flap has significantly MORE travel than the inboard.  Since Kari was a fan of the MK. 2 Thunderbird (he'd placed 2nd. behind Sirotkin in a major contest flying one built from a kit, solid leadouts and all), I'm assuming he simply copied the T'bird's control arrangement - so I've always taken the flap-horn lengths shown on the plan for a drafting error.  Could he possibly have built and flown the model as drawn?

[Brett Buck]

If anybody's got the original Finnish-language plan for Juhani Kari's Nakke, they'll see that the inboard flap horn's length is shown as 19mm., while the outboard one's length is 17.5mm - a full sixteenth of an inch shorter!  Build it like that, and the outboard flap has significantly MORE travel than the inboard.  Since Kari was a fan of the MK. 2 Thunderbird (he'd placed 2nd. behind Sirotkin in a major contest flying one built from a kit, solid leadouts and all), I'm assuming he simply copied the T'bird's control arrangement - so I've always taken the flap-horn lengths shown on the plan for a drafting error.  Could he possibly have built and flown the model as drawn?

   Probably not, but put enough tip weight in it, and it has a chance.

     Brett

[Lauri Malila]

That’s a mistake in original Nakke drawing. In the original model it was other way round, like the fashion was in those days. I did check that from the original model (picture) that is at the aviation museum in Vantaa. L

[John Park]

That’s a mistake in original Nakke drawing. In the original model it was other way round, like the fashion was in those days. I did check that from the original model (picture) that is at the aviation museum in Vantaa. L

Thanks.  It's what I always assumed - I wonder if anybody ever built one like that!

[rob biddle]

 A few years ago I built a Brodak original Nobler as per plan. Slightly porky at 49oz but with ample power from a stock La .46.

 It has around 2.5" asymmetry if memory serves correct. The inboard flap is absolutely huge in comparison to the outboard. In hard corners it would roll like a well trained Labrador begging for treats and the air brake effect of the extra inboard flap area would swing the nose in noticeably making the model fly diagonally through a mushy corner and requiring considerable intervention to retain line tension.

 I often wondered if an adjustable differential flap arrangement working in reverse to the original configuration would be worth pursuing? Probably far too much work to justify for an antique airframe design where an adjustable wart on the outboard flap would achieve the same result.

[Brett Buck]

A few years ago I built a Brodak original Nobler as per plan. Slightly porky at 49oz but with ample power from a stock La .46.

 It has around 2.5" asymmetry if memory serves correct. The inboard flap is absolutely huge in comparison to the outboard. In hard corners it would roll like a well trained Labrador begging for treats and the air brake effect of the extra inboard flap area would swing the nose in noticeably making the model fly diagonally through a mushy corner and requiring considerable intervention to retain line tension.

 I often wondered if an adjustable differential flap arrangement working in reverse to the original configuration would be worth pursuing? Probably far too much work to justify for an antique airframe design where an adjustable wart on the outboard flap would achieve the same result.

   49 ounces with an 46LA is not bad at all. the turn balance seems to be the best, assuming absolutely stock arrangement, in the mid-40's, with a Fox. It will be optimal at even higher weight with a 46LA sinc the speed will get held better.

   But yes, that was the problem they had - getting enough line tension to be able to fly under a reasonable range of conditions, and as slow as they could manage it. It may well have been the best solution for their set of conditions.

  My opinion is that you just live with the quirks, and if you don't want to, you can do a lot better than this with a different airplane.

    Brett

[Dwayne Donnelly]

     

     The plane is being stripped of the ancient tissue and will be covered in Monocote.

Thanks for the replies,

     Frank McCune

   

Monocote? NOOOO!!!   

[Dave_Trible]

The Palmer Hurricane also had the differential flaps.  I built one as a teen and was afraid to use them so put in a solid horn (so yes that can be said to be 'legal' to have one like it in classic).  I built another just a few years ago from the Brodak kit and again skipped the dual horns.  I think it's a novel idea that should be kept in the 'novel' box.  I DO use an outboard flap that is about 3/32" wider in chord than the inboard with equal span.  As I think about it the Hurricane is also supposed to have a little dihedral.  I never did that either although it is OK as long as you can still get the airplane aligned properly and don't have trouble with binding up the control horn.  Split horns and/or lucky boxes would help in some sense but then can create some other issues too.

Dave

[Brett Buck]

The Palmer Hurricane also had the differential flaps.  I built one as a teen and was afraid to use them so put in a solid horn (so yes that can be said to be 'legal' to have one like it in classic).  I built another just a few years ago from the Brodak kit and again skipped the dual horns.  I think it's a novel idea that should be kept in the 'novel' box.  I DO use an outboard flap that is about 3/32" wider in chord than the inboard with equal span.

Dave

   It always ends up like that anyway, so might as well build it in!

     Brett

[Dave Hull]

Piotr Zawada started off with differential flap horns on his Lucky 14 model. These are shown on the plans. (Model Aviation, May 1980.)  During repairs, he removed the differential flaps and also changed the control ratios. It is nice to have a guy tell you what they tried, what worked, and what they had to change.

[Lauri Malila]

Piotr Zawada started off with differential flap horns on his Lucky 14 model. These are shown on the plans. (Model Aviation, May 1980.)  During repairs, he removed the differential flaps and also changed the control ratios. It is nice to have a guy tell you what they tried, what worked, and what they had to change.

Also, Juhani Kari wrote in his 1964 Nakke-article that he likes the flap differential, when inside flap moves about 5mm more at maximum deflection.
It would have been interesting to see how it would have evolved from Nakke, but unfortunately he left us too early. L