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5 hours ago, 911hillclimber said:

Update:

2 deg right now confirmed, and zero degrees up or down, that is the motor shaft axis is parallel to the engine bearers.

 

On this design of frame, the wing is not mounted parallel to the engine bearer 'line', the leading edge is about 3 deg higher, the tail plane is parallel to the engine bearer 'line'.

There are no ailerons on this plane, essentially a 3 channel design.

Sounds like the down thrust is built into the design in that case.

Kim

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On 05/05/2022 at 11:33, 911hillclimber said:

Update:

2 deg right now confirmed, and zero degrees up or down, that is the motor shaft axis is parallel to the engine bearers.

 

On this design of frame, the wing is not mounted parallel to the engine bearer 'line', the leading edge is about 3 deg higher, the tail plane is parallel to the engine bearer 'line'.

There are no ailerons on this plane, essentially a 3 channel design.

As the tailplane is parallel to the engine bearers then there is no down thrust and 3 deg of wing incidence makes me think this model is a free flight design. If that is the case then they were designed to climb under power and the power would have been marginal requiring hand launch because there was not enough power for ground take off.

 

Putting this all together, a lot of assumptions on my part, tells me your model is doing what it was designed to do. Are we talking about your Buccaneer?

 

There are other topics on here discussing how vintage FF designs are flown when converted to RC. Leaving it as designed then only fly on calm days, climb under power using the rudder to control direction and when at height cut the power and gently circle back down to ground. Otherwise you can fly with more power and control which will require the design to be modified. At my club there are a number of these vintage models converted to RC that have down thrust added, Junior 60 for example, the other option is to reduce the wing incidence by packing up the trailing edge, or a combination of both.

 

I was given a Super 60 that had a lot of down thrust added even though some was already built in and more bizarrely the LE had been packed up, although it flew I could not see any logic in these changes so I removed the added down thrust and the packing from the LE. In my opinion it flew even better and certainly was not any worse. Why did they do it? Maybe taking a design that had been modified to RC and returning it back to FF with the increased wing incidence then adding down thrust to compensate. Crazy?

 

I have not measured the wing incidence and down thrust on the Super 60 but 2 deg down thrust and 1 deg of wing incidence would be fairly typical. Then there is CofG to play with, as it's 3 channel if the CofG is not far enough forward it will tend to wallow when coming out of a turn.

 

Plenty for you to experiment with which will require time to get it flying the way you want it to. 

 

Lastly when I was at your stage of learning I could not have done this and it was something like a year after passing my A that I started to notice how small changes in down thrust, wing incidence and CofG affected the way the model flew which enabled me to sort the Super 60 out. It's all good fun and keep us posted as to how you get on.

 

Steve

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On 05/05/2022 at 13:09, john stones 1 - Moderator said:

 

We are talking about a model aeroplane, it's not uncommon for many designers to give a starting point re side/downthrust on their plans, in fact it is quite common,  it is also common that deflections on your models have consequences through the speed range and differing maneuovres.

And the oft said "Well I can use my rudder" is another deflection.

Full size aircraft are designed by professionals who have access to wind tunnels & measuring equipment then have the prototypes proved by professional test pilots etc etc before any production aircraft are approved.

Model aircraft designers, except for a very few, don't really "design" their models. They generally copy previous model's features into their own using the TLAR &/or cut 'n try methods. They are also prone to accepting hangover features from free flight & restricted RC systems of the past without thinking that perhaps those features are no longer necessary when we have full proportional control available.  

 

Nothing wrong with using rudder deflection, it's easier to adjust than engine deflection, in fact many full size have the fin & rudder offset built in 

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14 hours ago, EarlyBird said:

As the tailplane is parallel to the engine bearers then there is no down thrust and 3 deg of wing incidence makes me think this model is a free flight design. If that is the case then they were designed to climb under power and the power would have been marginal requiring hand launch because there was not enough power for ground take off.

 

 

What makes you think that the thust-line is parallel to the tailplane ?

 

Free flight designs were not hand launched due to marginal power. In fact they were often launched from the ground (ROG). However there was (still is) sometimes other reasons to hand launch them, e.g. they had no rudder control to keep them on course, if it's difficult to keep them heading into wind or a bit too gusty hand launching is more practical. 

Bear in mind what are now vintage FF models were not designed by vintage modelers, many of them were not meant to trundle round in the manner we're used to seeing today. Back in the 1950's it was reckoned that most FF duration models could climb as fast as a Spitfire.  

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15 hours ago, EarlyBird said:

There are other topics on here discussing how vintage FF designs are flown when converted to RC. Leaving it as designed then only fly on calm days, climb under power using the rudder to control direction and when at height cut the power and gently circle back down to ground. Otherwise you can fly with more power and control which will require the design to be modified. At my club there are a number of these vintage models converted to RC that have down thrust added, Junior 60 for example, the other option is to reduce the wing incidence by packing up the trailing edge, or a combination of both.

 

 

The options you mention are exact opposites. The combination will just cancel each other out but the model will fly tail down for the model to achieve the same angle of attack at any given airspeed.

The thrust line and wing incidence are fixed with respect to each other. If extra down thrust is applied the angle between the two increases, if the thrust line isn't altered but the incidence is reduced obviously the angle between them also reduces.  

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8 hours ago, PatMc said:

What makes you think that the thust-line is parallel to the tailplane ?

The windowsill is parallel to the tailplane and the engine bearers are at an angle to that line.

 

image.thumb.png.25815e910820f098df6c7096688bb20f.png

 

on some models there is zero down thrust and then I have found the thrust line to be parallel to the tailplane.

 

Interesting that you seem to be saying that the down thrust is the angle between the wing and engine bearers. I will have to think about that. On some models there is zero down thrust and zero wing incidence, Phil Kraft Stik for example, but most of my builds are Peter Miller's designs that have zero down thrust but I never thought of comparing the thrust line with the wing angle, interesting, I will check.

 

Steve 

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To my understanding 'downthrust' is an attempt to reduce the effect of the difference between the thrust line and the centre of drag.   

The centre of drag is entirely dependent on the configurtion of the airframe.  
It follows that the airframe configuration  and the physical value of its thrust have to be considered to determine what effect a change in thrsut line may have.

 

For example the effect of down thrust will be completely different between a high and low wing design.

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3 hours ago, Simon Chaddock said:

To my understanding 'downthrust' is an attempt to reduce the effect of the difference between the thrust line and the centre of drag.   

The centre of drag is entirely dependent on the configurtion of the airframe.  
It follows that the airframe configuration  and the physical value of its thrust have to be considered to determine what effect a change in thrsut line may have.

 

For example the effect of down thrust will be completely different between a high and low wing design.

Simon, if that's the case then why isn't it a common feature in full size high wing aircraft ?

Also why do some scale models incorporate down thrust that isn't in the aircraft being portrayed ?

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4 hours ago, EarlyBird said:

The windowsill is parallel to the tailplane and the engine bearers are at an angle to that line.

 

image.thumb.png.25815e910820f098df6c7096688bb20f.png

 

on some models there is zero down thrust and then I have found the thrust line to be parallel to the tailplane.

 

Interesting that you seem to be saying that the down thrust is the angle between the wing and engine bearers. I will have to think about that. On some models there is zero down thrust and zero wing incidence, Phil Kraft Stik for example, but most of my builds are Peter Miller's designs that have zero down thrust but I never thought of comparing the thrust line with the wing angle, interesting, I will check.

 

Steve 

Actually what I'm saying is that the tailplane isn't necessarily parallel to the ground when the aircraft is in flight also the wing incidence isn't necessarily the angle that the wing presents to the air (Angle of Attack) it's penetrating.

Considering an aircraft in steady straight and level flight - since the thrust line is tied to the wing incidence any change in AofA during flight causes the same change to the thrust line - i.e. the thrust line angle alters depending on airspeed. 

All this boils down to remembering that the thrust line, wing & tailplane incidences and any datum line shown on a plan are no more than rigging references.

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I have said this many times but it seems that it needs saying again.

 

Go and buy Kemode's book "Flight Without Formulae"  First written in the 30s and so good that it is still in print today.

 

If you are into mathematics buy Kermode's "The Mechanics of Flight" which is basically the same book with all the maths you could wish for.

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4 hours ago, PatMc said:

Simon, if that's the case then why isn't it a common feature in full size high wing aircraft ?

PatMc,

 

With full size aircraft, the trim is adjusted for a fixed set of circumstances.

Change anything and the plane gets re-trimmed.

We as model flyers expect our planes not to change trim through the flight envelope, hence we modify our set up to limit the amount of change for variations in power / speed / attitude which would make full size aircraft inefficient due to extra drag produced.

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It is worth remembering that any thrust angle change  (or trim) can only create level flight at one specific air speed and thus one power setting.

This was a practical situation for un- throttled free flight planes but not so relevant for full house RC or full size.

Of course if you throw in a 6 axis gyro with GPS the effect of throttle changes can be almost completely mitigated.

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5 hours ago, Andy Gates said:

PatMc,

 

With full size aircraft, the trim is adjusted for a fixed set of circumstances.

Change anything and the plane gets re-trimmed.

We as model flyers expect our planes not to change trim through the flight envelope, hence we modify our set up to limit the amount of change for variations in power / speed / attitude which would make full size aircraft inefficient due to extra drag produced.

Sorry Andy but I don't think that addresses my response or Simon's reasoning.

The thrust line will remain through the same place wrt drag no matter what the power changes are made.

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I'm trying to follow some of the views above with many apparently valid but contradictory arguments.

 

Is it not the case that in most models with the possible exception of some specialist designs far removed from a Ben Buckle Buccaneer, a pitch change due to a different power setting from the moment between the thrust direction and the centre of pressure will cause the centre of pressure to move as the angle of attack changes?  This will necessitate a change of trim unless the pilot wishes to hold the stick pressure - in most real world flying, these changes are less frequent and there's only a small workload in re-trimming.  Contrast this with a scale or sports model flying circuits - instead of a couple of minutes or more between power changes, we're only spending a few seconds between climb, cruise and descent - hence some designs adding semi-automatic compensation for pitch and yaw effects in the form of down and side thrust.

 

Perhaps - and especially in regard to the OP's question - it's simply a case that there's no right, wrong or absolute answers here - just explanation of the reasoning behind individual designers' and pilots' preferences...

 

 

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5 hours ago, Martin Harris - Moderator said:

Perhaps - and especially in regard to the OP's question - it's simply a case that there's no right, wrong or absolute answers here - just explanation of the reasoning behind individual designers' and pilots' preferences...

I agree it's not a case of right or wrong and has more to do with the way the pilot wants the model to fly. If the model has been built to the design it will fly as intended by the designer. Take on old FF design then I would expect it to climb under power and descend to a gentle landing when the fuel runs out. The perception many make when converting to RC with increased power and control is that there is something wrong with the design when the pilot wants to fly the model in a way that was not intended. Converting to RC is not a problem but changes need to be made particularly with the extra power available. Increasing the power will increase the rate of ascent and in the extreme induce loops. What is the best approach for this model? On the Mini Super down thrust is increased as a more powerful engine is used. I have seen others increase the down thrust so I assume it is the accepted method. I have also adjusted the wing incidence on a couple of models which worked in that they flew better. I have also seen others simply apply a lot of elevator down trim which to my mind did not look correct but it also flew. My preference is to have the elevator in line with the tailplane and to minimise the down thrust adjusting the wing incidence as I feel fit. Another preference I have is to fly in winds over 10 mph which dictates that I have ample power to on hand. Another can of worms?

 

All personal preferences and the consequential adjustments I like to make.

 

Steve

 

 

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 On my Mercury Matador I have a small elevator which is more of a trim tab than elevator. Works on the stick in the normal way but also have a mix to a two position switch. Once up to a hight that I want switch puts in some down trim to keep it level [ish] in powered flight.  Once out of fuel on the glide switch back to normal position for best glide but also handy if some wind to keep it down so penetrating back to landing site and flicked out at the last moment for touch down.

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This is the most interesting design I came across for this topic

 

image.thumb.png.9257317de5f850e742ae51e63c33316f.png

 

No elevator, engine thrust line at window sill level and wing seat parallel to the thrust line. Most interesting is the tailplane has negative incidence and only rudder under RC control. I am still thinking how did this design fly so well, it was a classic old time FF model. I could build one and find out ? I could build a Buccaneer just to test my method for RC conversion.

 

Steve 

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 I think the engine mount can pivot about a point altering the thrust line but this is a method of protecting the prop in the event of a whoopsy, The match stick behind the pivot point acts as a sheer pin.

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