Questions after maiden flight today

[DavidKP]

The weather was glorious today in Northern Switzerland, and after getting through the shopping with the wife I was able to get to the field with my new Chapter One build by 4 in the afternoon, would have been a bit sooner if I hadnt left the TX behind and had to go back for it.

Anyway, the model was shown to the club expert Mode 2 flyer and he agreed to make the maiden flight. After 2 circuits he said he was having to hold in too much down elevator so brought it in to land. A few turns on the clevis and it was back in the air, flying nice and level on about half throttle, but every time he opened up the throttle it was climbing at an alarming rate. He landed it again and there was a discussion among the club pros, in Swiss German so I couldnt understand anything. At the end the recommendation was to add more downthrust, 2 degrees, by adding washers under the top mounting bolts, and changing to a 'slow flight' propeller, something with less pitch.

The motor is an AXI 2814/16 which is rated 1035 RPM/V, the LiPo is a 3S 3800 MaH. The recommended propellor on the AXI site for a trainer aircraft is 9.5x5. I remember when I bought the motor in the local shop that I had written down that propellor size, but they didnt have any in stock, so the sales guy assured me that a 9x6 would be just as good, up on one measurement but down on the other. So I bought and fitted an APC 9x6 Elektro.

The plane is built with the downthrust indicated on the plan, does it sound right considering the description that I have given that another washer added to the top bolts will help?

And what would be your recommedations about changing the propellor size and/or pitch?

[bert baker]

Is the c-g correct

[PatMc]

If it's "flying nice and level on about half throttle" why not just fly it at half throttle most of the time ? Use higher power settings for take off & to climb.

Adjusting downthrust is a throw back to free flight & single channel days when we didn't have proportional throttle & elevator control.

[Denis Watkins]

You have down thrust built in David as you say

This is not a racer, and more throttle makes a plane ascend.

Best advice was given, to fly at half throttle and enjoy the extra time in the air.

[john stones 1 - Moderator]

Your props not making much difference, the c.g could be aft, but your test pilot never said it was twitchy, go with a bit more downthrust, if you trust him to maiden it, trust his advice.

[DavidKP]

Should have said that we moved the LiPo about 5mm forward before the first flight so that 'Nico' was happy that the CofG was towards the front end of the range on the plan which is where he prefers it. But no it was never 'twitchy', it flew smoothly but just nose up. Anyway, I will add the washers as advised, and buy a couple of different propellors on Saturday morning and let's just hope that the weather is as good as today, 20 degrees and standing around in a short sleeved shirt in mid-October ..... just great

[Martin Harris - Moderator]

Posted by PatMc on 14/10/2017 19:45:04:

If it's "flying nice and level on about half throttle" why not just fly it at half throttle most of the time ? Use higher power settings for take off & to climb.

Adjusting downthrust is a throw back to free flight & single channel days when we didn't have proportional throttle & elevator control.

I can't agree with that. Having flown a few models with insufficient downthrust - or even upthrust - over the years when test flying new models, a significant pitch change with power as described in the OP is most unpleasant.

There are two basic cures - adjust the thrust line or use a throttle to elevator mix to compensate. With a pattern ship or aerobat, zero pitch change is desirable, trainers and scale models benefit from slight pitch up with power and vice versa. Getting the thrust line right is the responsibility of the designer but trimming is for pilot comfort so I would recommend a bit of fine tuning.

[john stones 1 - Moderator]

At field today myself David T shirt n sunshine.

[PatMc]

Posted by Martin Harris on 14/10/2017 20:19:48:

Posted by PatMc on 14/10/2017 19:45:04:

If it's "flying nice and level on about half throttle" why not just fly it at half throttle most of the time ? Use higher power settings for take off & to climb.

Adjusting downthrust is a throw back to free flight & single channel days when we didn't have proportional throttle & elevator control.

I can't agree with that. Having flown a few models with insufficient downthrust - or even upthrust - over the years when test flying new models, a significant pitch change with power as described in the OP is most unpleasant.

Just reduce the power in those situations or add some elevator down trim if you want to increase the speed without climbing.

Actually here's no such thing as "down thrust", the thrust line changes dynamically with the wing's angle of attack.

[PatMc]

Posted by DavidKP on 14/10/2017 20:18:16:

Should have said that we moved the LiPo about 5mm forward before the first flight so that 'Nico' was happy that the CofG was towards the front end of the range on the plan which is where he prefers it. But no it was never 'twitchy', it flew smoothly but just nose up. Anyway, I will add the washers as advised, and buy a couple of different propellors on Saturday morning and let's just hope that the weather is as good as today, 20 degrees and standing around in a short sleeved shirt in mid-October ..... just great

Moving the cg forward will have made the model more pitch sensitive. A lower pitch prop should make it less so.

[Martin Harris - Moderator]

Posted by PatMc on 14/10/2017 20:39:45:

Moving the cg forward will have made the model more pitch sensitive.

Surely a forward C of G makes an aircraft less sensitive in pitch?

The thrust line is relative to the fuselage datum - as is the tail and wing incidence so to say there's no such thing doesn't make sense to me. That thrust line has a direct influence on a model's pitch behaviour with power changes.

As stated above, there is a fixed and physical relationship between three main rigging angles so changing any one of them can affect the overall behaviour but getting them right is a major factor in producing an aircraft which is pleasant to fly.

[PatMc]

Posted by Martin Harris on 14/10/2017 20:51:04:

Posted by PatMc on 14/10/2017 20:39:45:

 

Moving the cg forward will have made the model more pitch sensitive.

Surely a forward C of G makes an aircraft less sensitive in pitch?

The thrust line is relative to the fuselage datum - as is the tail and wing incidence so to say there's no such thing doesn't make sense to me. That thrust line has a direct influence on a model's pitch behaviour with power changes.

As stated above, there is a fixed and physical relationship between three main rigging angles so changing any one of them can affect the overall behaviour but getting them right is a major factor in producing an aircraft which is pleasant to fly.

If a model is trimmed for a forward cg it will be flying with the wings at a higher AoA therefore the thrustline will be at a higher angle relative to the models flight attitude. It's similar to the principle of the well known "dive test".

I assume by the three main rigging angles you mean thrust line, wing incidence & tailplane incidence.
Obviously the tail incidence isn't fixed. The AoA is determined by the CG & tail incidence {within the possible flight envelope]. Since the thrust line is fixed in relation to the wing incidence any elevator changes which alter the AoA will alter the thrust line by the same degree.

PS the fuselage datum is an arbitary line, aproximately level with the flight attitude. It's a drawing board & rigging reference line not the line that the model flies along. 

Edited By PatMc on 14/10/2017 21:17:27

[Martin Harris - Moderator]

Yes, there are dynamic changes in tail incidence due to elevator movement - I shouldn't have added that as the designed tail incidence is an aid to ending up with a neutral elevator at cruising AoA.

However, although the datum is arbitrary, it does help define the relationship between the wing rigging angle and the alleged non-existent thrust line. Although angle of attack changes occur throughout a flight, the thrust line remains at the same angle in relation to the rigging angle of the wing at any speed or attitude short of structural failure or wing band stretch where a whole new can of worms is opened.

None of these semantics affect the fact that adjusting the allegedly non existent thrust line can make a significant improvement to the "flyability" of an aircraft...

...and I've not yet found a model that has become more pitch sensitive by moving the C of G forward.  I have flown a full size glider (with permission from a BGA inspector) with a large pupil who put the C of G forward of the normal limit and I can assure you that it was anything but unstable in pitch!

Edited By Martin Harris on 14/10/2017 22:06:57

[PatMc]

I have never said that the thrust line is non existent, I have stated that the thrust line is fixed in relation to the wing incidence.

As you stated "Although angle of attack changes occur throughout a flight, the thrust line remains at the same angle in relation to the rigging angle of the wing at any speed or attitude..." changes to the AoA are relative to the rigged incidence therefore if the thrust line remains at the same relative angle to the incidence it is changing to the same degree as the AoA. i.e. the thrust line changes dynamically with the wing's angle of attack.

The ability to manage throttle & elevator trim makes clumsy alterations to thrust line unnecessary in most models including trainers. Very few models have any "flyability" improvements due to alterations to thrust line.

[PatMc]

Posted by Martin Harris on 14/10/2017 21:56:19:

...and I've not yet found a model that has become more pitch sensitive by moving the C of G forward. I have flown a full size glider (with permission from a BGA inspector) with a large pupil who put the C of G forward of the normal limit and I can assure you that it was anything but unstable in pitch!

The pitch sensitivity refered to was in relation to changes in throttle settings. With a rearward cg the model won't tend to pitch up to the same degree from S&L when the throttle is opened as it would with a forward cg.

[Martin Harris - Moderator]

We'll need to agree to disagree on the aerodynamic explanations and I still can't understand your assertion that moving a C of G forward makes a model less stable in pitch but could you explain how I might have gone about using elevator trimming on a Flair SE5a that I maidened for a friend which was not ridiculously overpowered but would loop with full power and dive violently at idle with no elevator inputs when trimmed straight and level at a little over mid throttle. Application of a some down thrust transformed the model into a pleasant flyer.

Perhaps your build standards are such that you've never experienced such a model but I can assure you that the thrust line change on this model (and quite a few others with similar if not quite as extreme behaviour) was more than worth the price of a couple of washers.

Edited By Martin Harris on 14/10/2017 22:59:36

[PatMc]

Posted by Martin Harris on 14/10/2017 22:52:04:

We'll need to agree to disagree on this but could you explain how trimming could have helped on a Flair SE5a that I maidened for a friend which was not ridiculously overpowered but would loop with full power and dive violently at idle with no elevator inputs when trimmed straight and level at a little over mid throttle. Application of a some down thrust transformed the model into a pleasant flyer.

Perhaps your build standards are such that you've never experienced such a model but I can assure you that the thrust line change on this model (and quite a few others with similar if not quite as extreme behaviour) was more than worth the price of a couple of washers.

It's ludicrous to ask me to diagnose the described situation when I have had no hands on experience of the model.

I've never yet changed the thrust line of any model after it's been built to improve it's general behaviour. I have reduced the downthrust on several designs [usually to zero] during build then adjusted the cg rearwards during test flights [DB Mannock springs to mind as one example] & I have tweeked the thrust lines of a couple of "pattern ships" when I had ambitions to be competetive in that field.

Perhaps you can explain why upthrust is more common than downthrust in full size aircraft but having the thrust line on the datum is most common.

[Martin Harris - Moderator]

Posted by PatMc on 14/10/2017 23:27:22:

Posted by Martin Harris on 14/10/2017 22:52:04:

We'll need to agree to disagree on this but could you explain how trimming could have helped on a Flair SE5a that I maidened for a friend which was not ridiculously overpowered but would loop with full power and dive violently at idle with no elevator inputs when trimmed straight and level at a little over mid throttle. Application of a some down thrust transformed the model into a pleasant flyer.

Perhaps your build standards are such that you've never experienced such a model but I can assure you that the thrust line change on this model (and quite a few others with similar if not quite as extreme behaviour) was more than worth the price of a couple of washers.

It's ludicrous to ask me to diagnose the described situation when I have had no hands on experience of the model.

I've never yet changed the thrust line of any model after it's been built to improve it's general behaviour. I have reduced the downthrust on several designs [usually to zero] during build then adjusted the cg rearwards during test flights [DB Mannock springs to mind as one example] & I have tweeked the thrust lines of a couple of "pattern ships" when I had ambitions to be competetive in that field.

Perhaps you can explain why upthrust is more common than downthrust in full size aircraft but having the thrust line on the datum is most common.

I didn't consider it ludicrous as you've been more than happy to claim that adjusting a thrust line won't help on models in general and that you can adjust out the described tendencies with elevator trim and C of G changes. I do consider it ludicrous to claim that there is any way to do so on a model which had its extreme pitching problems cured by the addition of some down thrust and no other changes - unless you think the weight of the washers moved the C of G forward enough to make a difference? To damp out such violent pitch changes with a sufficiently rearward C of G would surely reduce the model's pitch stability beyond reasonable limits.

Upthrust common on full size? Not something I've ever been aware of but if that's so, a possible explanation is where full size designers choose to draw their datum lines? I'd be grateful if you could quote some examples as it might increase my understanding if I could research them - I have seen several full size references to downthrust e.g. Van's RV6, Piper Cub...both in the region of 4 degrees.

It's just occurred to me that most datum lines I've seen have been drawn parallel with the tailplane and that is probably responsible for my clumsy reference to the tailplane earlier in the thread.

Edited By Martin Harris on 15/10/2017 00:44:18