Engine alignment

[Sam Longley]

I am building a new model. The plan says 1.5 degrees down thrust. Now this question can apply to either glow or electric power units. I have not decide which yet so I may not just follow the engin bearer layout if going electric.

Can someone tell me first

How do i know that I have the unit aligned with the centre of the plane?

Then

How do I measure 1.5 degree down thrust accurately?

 

Obviously some might say - look at the drawing. But I recently bought a new Cougar & it turned out that when I first flew it the engine was pointing at the sky & I only just managed to avoid a crash. I have since aimed it down but it is still not right. Somehow the model has been supplied with the bulkhead missaligned. I have had it in store for 2 years so can hardly return it now having already tried to fly it

So my questions above could equally apply to this plane as it does to the one that I am building

Thanks for any help

Sam L

[Engine Doctor]

Google engine offset calculator and from that you should be able to work out the measurment for 1.5 deg of side or down thrust.

[Nigel R]

What are you building?

Picture paints a thousand words. Can you post a shot of the nose of the model?

 

 

 

[Martin McIntosh]

Setting the firewall at the desired angles of down and side thrust is usually the first job to do on any model, and they all differ in construction. One way would be to use tangent tables but a simple protractor is all you need. Mark the prop driver centre on the plan then place the protractor centre over it, aligning with the model centre line. Mark at 1.5 deg. and draw a line through the two points. Where this touches the firewall will be the new mount centre so draw a line at 90 deg. to this to get the new angle. Side thrust can then be done using the same method, so the mount will now be offset upwards and to the left. Much easier to do than explain. In practice a little inaccuracy with the angles should not be noticeable in flight but you must end up with the prop driver exactly central. The nose ring, if fitted, should follow the same angles as the firewall.

Always use a commercial mount, not old fashioned wooden bearers.

[Martin Harris - Moderator]

The ideal thrust line depends on the layout of the model and the relationship between the thrust line and centre of aerodynamic pressure.

 

In general, a low wing will need little or no down thrust and a high wing will need some downthrust to resist the pitch up tendency.  In the case of a high mounted engine, such as seen in some seaplanes, for example, upthrust will be required. 
 

In the case of your Cougar (assuming of the fun fly variety?) there’s very little downthrust required and probably none provided - easy visual check is to I align the propeller vertically and look the model from the side.  I assume you had sufficient wing bands - if the leasing edge could lift it would cause it to behave rather nastily.  

 

P.S. I just checked with the designer (a clubmate) and the original Cougar had some downthrust but the Cougar 2000 had none due to a change of wing section.

Edited December 9, 2023 by Martin Harris - Moderator

[Martin McIntosh]

Some models, such as the Curare, have no firewall angle, compensated for by an increase in wing and tail incidence. Much simpler.

[kc]

There is a school of thought that says that side and downthrust are remnants of free flight and are not needed with RC.  Much easier if the motor has no down or sidethrust - cowl fits nicely etc.

[Martin Harris - Moderator]

…especially as most radios these days allow throttle to elevator/rudder mixes which can be used to fine tune behaviour if your thumbs can’t cope. 
 

And it’s not really cheating as the majority of full sized aircraft use adjustable trims for different power settings/flight phases. 

[Christopher Wolfe]

1 hour ago, kc said:

There is a school of thought that says that side and downthrust are remnants of free flight and are not needed with RC.  Much easier if the motor has no down or sidethrust - cowl fits nicely etc.

 

Believe it or not, a model trimmed with the correct thrust line adjustments might fly better when the engine is running or stopped than a pretty cowling can achieve. Whether Free Flight or r/c the same rules apply for engine torque and spiral prop wash and the model does not even know how it is being controlled.

 

Try to attend a better school of thought and do not rely on fiddling with your transmitter to compensate for design flaws in the layout and setup of your model 🙃

[Martin McIntosh]

No doubt that mixers apply now, but when I was flying F3A in the 70`s we were lucky to get even rate switches let alone anything else. Many models are being built these days from the `golden era` and the plans show engine offset, which I still do.

Interestingly, Spits and Hurries have none as per full size and do not show any adverse effects but then, they were not designed for precision aerobatics.

[Sam Longley]

5 hours ago, Nigel R said:

What are you building?

Picture paints a thousand words. Can you post a shot of the nose of the model?

 

 

 

Avanti F3 successful pattern plane from the dark ages

[PatMc]

40 minutes ago, Martin McIntosh said:

 

Interestingly, Spits and Hurries have none as per full size and do not show any adverse effects but then, they were not designed for precision aerobatics.

Very few models are designed for precision aerobatics & even fewer model pilots are capable of executing anything like precision aerobatics.

[Geoff S]

I was gifted a DB 58" ws Tiger Moth by the adult son of a friend a few years ago.  He was a total novice builder but mechanically skilled (he built a kit car later, for instance) and there were a few faults built in.  When I eventually got it flying electrically powered, I found it wouldn't stall turn to the right because I'd used up too much rudder on the vertical climb.  I managed to get some right thrust without fouling the cowl, which improved the situation.

 

I've just set up the motor on my DB 60" Sopwith Pup to have 2 deg right and down thrust (it's easy to do with 4mm cap-head bolts used as motor mounts).  I use my Robart incidence meter to make the measurements - one of the brackets has a fitting to allow attachment to the prop driver.

[john stones 1 - Moderator]

It's a dubious claim that only precision aerobatic models benefit from engine offsets, laws of physics applys to all types with a prop on. Landing with trim in the rudder/elevator causes a reaction would we class that as a precision manouvre ?

A nice line up in the cowl, o.k if it's a static, if it's for flying another story.

Physics is physics and you canna change the laws of physics...Montgomery Scott.

[PatMc]

4 minutes ago, john stones 1 - Moderator said:

It's a dubious claim that only precision aerobatic models benefit from engine offsets, laws of physics applys to all types with a prop on. Landing with trim in the rudder/elevator causes a reaction would we class that as a precision manouvre ?

A nice line up in the cowl, o.k if it's a static, if it's for flying another story.

Physics is physics and you canna change the laws of physics...Montgomery Scott.

When an r/c model is being flown the applied physics are constantly being changed. The effective thust line is also changing with every change to the wing's AofA.    

[john stones 1 - Moderator]

Just now, PatMc said:

When an r/c model is being flown the applied physics are constantly being changed. The effective thust line is also changing with every change to the wing's AofA.    

 

Yep, but it's still the best method we have other than fly by wire, and that's cheating. 😉

[Sam Longley]

4 hours ago, Martin Harris - Moderator said:

The ideal thrust line depends on the layout of the model and the relationship between the thrust line and centre of aerodynamic pressure.

 

In general, a low wing will need little or no down thrust and a high wing will need some downthrust to resist the pitch up tendency.  In the case of a high mounted engine, such as seen in some seaplanes, for example, upthrust will be required. 
 

In the case of your Cougar (assuming of the fun fly variety?) there’s very little downthrust required and probably none provided - easy visual check is to I align the propeller vertically and look the model from the side.  I assume you had sufficient wing bands - if the leasing edge could lift it would cause it to behave rather nastily.  

 

P.S. I just checked with the designer (a clubmate) and the original Cougar had some downthrust but the Cougar 2000 had none due to a change of wing section.

My cougar has been manufactured & supplied with a massive up thrust which needs attention. I have placed about 5 washers in the top of the engine mount & am still not sure it is right. I have checked the mount against others & it is not (as I have seen) upside down.

I always use 6 wing bands

[Sam Longley]

50 minutes ago, PatMc said:

Very few models are designed for precision aerobatics & even fewer model pilots are capable of executing anything like precision aerobatics.

The Avanti was a championship winning model. I take the point about pilots not being good enough. However, Youtube vids suggest this to be an excellent plane to fly. There is no point - as I have already found out a number of times- trying to fly a badly set up plane --or one that has crashed & rebuilt as an old dog.

I am currently recovering from a Broken femour ( October) & building this is a great pastime during recovery. I have had similar models & expect to be able to fly this reasonably oK provided I do not start doing what I tend to do - pushing my luck!!. I have a 75 inch MX, a 60 inch Pitts & the cougar for that

Edited December 9, 2023 by Sam Longley

[Nigel R]

1 hour ago, Sam Longley said:

Avanti F3 successful pattern plane from the dark ages

 

If you're sticking with hardwood bearers, just fit them exactly as per plan.

 

If you're converting to a plastic mount, you'll want to align the firewall at right angles to where the hardwood bearers would have gone.

[Sam Longley]

1 hour ago, Nigel R said:

 

If you're sticking with hardwood bearers, just fit them exactly as per plan.

 

If you're converting to a plastic mount, you'll want to align the firewall at right angles to where the hardwood bearers would have gone.

Yes but it then says- Adjust motor to give 1.5 degrees down thrust

I want to know how to acurately measure & set up down thrust

It seems that nobody knows.

Sam L

[Peter Jenkins]

The reason that many full size aircraft do not have any up or side thrust to the engine is because there is a pilot sitting in them with a set of instruments that tells them if the aircraft is slipping, skidding, climbing or diving.  All aircraft have an elevator trimmer that is used regularly during different phases of flight e.g. take off, climb, cruise etc.  All aircraft will have small trim tabs on the rudder and aileron and these are set so that at cruising speed the aircraft flies with wings level and without any need for rudder.  The rudder is used most of the time when not in cruise to balance the aircraft.  The fixed tabs are set by the flight test guys at the manufacturer or your licensed engineer who can adjust them to sort out these issues.

 

Larger aircraft, especially multi engined ones, also have aileron and rudder trimmers to easy crew work load.

 

As soon as you apply or reduce power in a full size aircraft you will see the nose rise/fall and swing from side to side.  As we don't sit in our aircraft we don't generally see these changes in attitude immediately but it soon becomes apparent that your aircraft is climbing or diving with the application of power.  When I watch the average club pilot flying, they rarely fly straight and level for very long and often say they have trimmed out their aircraft.  On many occasions when I am asked to fly an aircraft it is rare to find that it is trimmed to fly straight and level hands off.  I did that recently to a club members model and found that even then he was constantly moving the stick (mode 2 so RH stick) even when he didn't need to.  I told him to get it flying S&L and let go of the stick.  It flew in a straighter line than he had ever managed while I had been watching.  He was just so used to constantly having to correct for an out of trim model it had become standard flying practice.  He also was not used to flying the aircraft in a straight line.  Take off was invariably curved and the climb out always curved despite there being no reason for either of these issues.  Trying to get him to fly a rectangular circuit was very difficult as he tended to fly in a meandering flight path and seemed oblivious to this. 

 

Getting the aircraft's motor thrust line set up so that even quite large power changes do  not have an immediate effect of aircraft attitude is well worth doing.  It's not a case of just building in the stated down and side thrust but also flying the model and observing what the application and removal of power does and then fine tuning the engine mounting so that you get to the position that all specialist aerobatic aircraft end up in. 

 

Why bother?  It makes the aircraft much easier to fly so that you can consider other things such as what you want to fly instead of spending all your time just flying your aircraft.  It really is worth the bother.  I let the club mate I am referring to fly my 2 m F3A aircraft and it was if he was flying on rails - once I'd told him he didn't need to move the stick constantly.  Aircraft don't fly accurately straight out of the box without being set up and trimmed even 2 m F3A airframes!

[Nigel R]

15 minutes ago, Sam Longley said:

Yes but it then says- Adjust motor to give 1.5 degrees down thrust

I want to know how to acurately measure & set up down thrust

It seems that nobody knows.

 

I've just checked the plan on outerzone. It shows the zero degree datum line along the fuselage side, which the bearers are parallel to, and what it then says is simply a one word instruction - "shim" - by the engine. The angle is drawn on the plan. Personally, if sticking with hardwood bearers, I would notch the formers F2 and F3 in a slightly different place, to account for this angle. It would be easy to extend the thrust line already shown into these formers, and cut appropriately. That said, I wouldn't use hardwood bearers. I'd just angle F2 at 1.5 deg (or whatever it is) and fit a glass/nylon mount to it.

 

 

[Peter Jenkins]

6 minutes ago, Sam Longley said:

Yes but it then says- Adjust motor to give 1.5 degrees down thrust

I want to know how to acurately measure & set up down thrust

It seems that nobody knows.

Sam L

On the contrary, someone has already told you how to do this.  So, can I ask if you own and know how to use a protractor?  As has already been said, you can draw lines on the plan but if you don't have a plan then you can draw lines on a piece of paper.  Mark a point along the line that represents the front of the fire wall and, using the protractor, measure a point that is 1.5 deg down from the datum line and one at 90 deg to that new point.  Draw lines to connect your new 90 deg point passing through the origin and then measure, at bolt height, the distance between the fire wall and the new 90 deg line.  That will give you the amount you need to pack up the top bolts to achieve your 1.5 deg downthrust.

 

Of course, your firewall might be mounted at a different angle than at 90 deg to the aircraft's datum line.  To find out what it is mounted at you will need to draw a datum line on your model.  Set your datum line along the length of the fuselage aligned with the tail plane.  Most, but not all, tail planes tend to be mounted at 0 deg to datum.  Now measure from the top and bottom of the firewall and transfer these positions to the side of the fuselage and draw a line through them crossing your horizontal datum line.  Now use your protractor to measure the angle between these two lines.  The could be 90 deg in which case you have now established that there is no up/down thrust.  In your case, it may well have some up thrust built in. So measure the distance between your the line at 90 deg to the datum line and the fire wall line.  This amount of built in up thrust needs to be added to your downthrust figure on your piece of paper and gives the total packing needed between the motor mount and the firewall.

 

That's how it's done when the model has been built.  Alternatively, you can just keep packing the top motor mounting bolts until when you apply power the aircraft does not change its flying attitude.  

 

If you have to get the motor shaft exiting from the nose at a set point in the cowling then simply use the paper drawing you have to mark the position of the cowling hole.  Then adjust the vertical position of the motor until the line representing the major axis of the motor crosses the position of the centre of the cowling hole.  Move the engine mounting bolt holes to this new position and, hey presto, the motor shaft will exit the cowling in the right position.

[Martin McIntosh]

Assuming that the motor is either upright or inverted and you use wooden bearers you are stuck with only being able to change the downthrust. Those things were notorious for the bolts/screws working loose due to compression of the wood. They also added considerable weight. It is a very bad idea to pack the lugs on an i/c engine with washers because the crankcase can easily distort. As Peter says, attending to these simple details from the word go can make a huge difference to a model, not to mention that the pilot can then concentrate on improving their flying rather than fighting the thing all the time.

[Sam Longley]

21 hours ago, Nigel R said:

 

I've just checked the plan on outerzone. It shows the zero degree datum line along the fuselage side, which the bearers are parallel to, and what it then says is simply a one word instruction - "shim" - by the engine. The angle is drawn on the plan. Personally, if sticking with hardwood bearers, I would notch the formers F2 and F3 in a slightly different place, to account for this angle. It would be easy to extend the thrust line already shown into these formers, and cut appropriately. That said, I wouldn't use hardwood bearers. I'd just angle F2 at 1.5 deg (or whatever it is) and fit a glass/nylon mount to it.

 

 

 

I think that I would rather stick to the design. Especially as I have already cut the slots for the engine bearers. The fus sides & bulkheads are already done.

Thinking about it-I can machine top shims to 1.5 degrees in brass on my mill & set them in the wood. That would stop the engine mounts wearing into the wood.

The design does not make it easy to put nuts on the underside of a nylon mount so it has a brass plate set on the underside of the wood bearers taped 3mm for the bolts.

I have been considering bolting an aluminium angle on the prop shaft. Then a digital angle guage placed on the top flat of this would measure any tilt in the engine.. Even if a bit out --ie not dead in line with the shaft, it would show relative changes. Set the digital level to zero then (without rotating the shaft of course) just measure the change in angle. A reference mark placed at a convenient position to ensure that the shaft was not rotated.

I could machine a piece of aluminium in my mill to get all the faces nice & square. That would guarantee that the flat was in line ( albeit offset by a determined amount) with the shaft.