Trimming an aircraft

[Ben B]

I was reading the article in this months magazine by Dave Burton on trimming ( btw) and it got the grey cells stirring.

I know for a trainer type plane (and certainly for a vintage) the idle = down , cruise = ISQ, WOT = climb but is that what we really do with sports planes or pylon racers? They tend to have more power and therefore height can be gained by direction of travel not just through speed related lift. The article shows a pylon racer and says "once nicely trimmed a pylon racer is a joy to fly" (or words to that effect). Now perhaps I'm doing it all wrong but I don't do it this way and wonder what other people do...

Certainly on super fast planes and pylon racers I trim it so that there isn't any net lift off the wing at full throttle and any climbiing is done by virtue of direction of travel IE use the elevator to get it pointing upwards and watch it climb. Otherwise a fast pass would require lots of down elevator to stop it climbing. On my sports models I tend to have it set up similarly so that I can cruise at 3/4 without any throttle and tend to adjust the down-thrust so that full throttle doesn't cause any significant climb. If I need to gain height I can adjust the angle of travel rather than the power. That way if I want to do a full bore fast pass I can without elevator input and giving it full throttle before (say a loop or knife edge) does not cause a significant change in altitude.

What does anyone else do? I guess what I'm saying is- with "point and shoot" type planes does the suggested trimming technique have any advantages or do we just rag round as quickly as possible on full power and use direction of travel to gain height?

Really love articles like that one. Gets me thinking and questioning what I do and whether I should be doing it

Answers on a postcard please :D

[Stephen Grigg]

Yes I also enjoyed BEBs article and it made so much sense to me and clarified some areas also.Tyhe problem I have is I cannot trim in the air.I wouls need a third hand,so tend to get the club expert to do the initial flights for me.

[Lee Damms]

I'm a bit like Steve I usually fly with my ex instructor and must admit he usually sets up the initial trim on the maiden ie into wind level flight at cruising speed 1/2 -3/4 throttle been an old so and so I don't fly fast I'm more a relaxing type. Having said that I've found myself doing more trim adjustments depending on weather conditions at the time. Due to finding myself a lone flyer at the field 10 mph wind and unable to take of with my Silhouette due to a badly cut field (council left it way too long) I was forced to hand launch my Mini Super which I usually prefer to fly on less windy days but I found 4 clicks of down elevator at 1/2- 3/4 throttle into wind suited my flying. Just had to remember put 4 clicks back in when launching. I now feel much more confident in trimming my own models during flight.

Regards Lee

[Biggles' Elder Brother - Moderator]

Very interesting comments Ben. When I write the articles there are always two problems - the first is I want to make it as applicable as I can to the "average flier" (like me!) so I tend to stick to the mainstream but I'm always aware that there are many exceptions to what I say. The second problem is of course the word limit. David will tell you that my 1st draft is always at least 50% too long! So the finer points about exceptions etc. tends to be the stuff that gets the chop!

One of the points I didn't make - and you deal with very well - is that there is no "right" trim. We need to trim the model with one eye on its design, another on what we want to do with it and a third eye (!) on how we like to fly. I selected the 1/2 to 3/4 throttle for level flight because that's a happy compromise for most cases. But you are of course dead right, if we take what might be called an extreme example like a pylon racer, then the rules of the game change. Here its all about speed. The model will spend all its working life at WOT, so obviously we should trim for level flight at WOT. The "how to do it" is of course exactly as I describe - just the throttle setting is different.

Now about will it climb then simply by pulling up the nose? Well this depends on the model and just how much power you have! Let's go back to the basic aerodynamics. Leaving aside air properties and wing size etc. two factors effect the lift a wing will produce; the airspeed and the AoA. The elevator controls the AoA, so if we pull back we increase the AoA and lift increases and the model climbs. BUT,.....the drag also increases so the speed will decrease and hence the proportion of lift we get from speed will drop.

Now its all about the balance between the drag characteristics of the aerofoil and just how much power you have! If the AoA-induced gain in lift outweighs the loss of lift from the reduced speed you will continue to climb.

But if the drag effects exceed the power unit's capability to provide enough speed generated lift, then you will climb for while but ultimately simply return to level flight at a new AoA and lower speed.

Obviously we have some models with sufficient power for unlimited vertical "flight" - becuase the engine can generate more thrust than the combined model weight plus the drag forces - so yes in these cases the application of "brute power" will make the model climb and of course it's perfectly reasonable to trim appropriate for those condition if that is what you want. Its all about what you are confortable with.

Personally even if the model is very powerful for its weight I still trim for level flight at 1/2 to 3/4 throttle - why? Well it means that when I go into a high G maneuoure and open the throttle wide on entry I'm getting some of my upward trajectory from the increased airspeed over the wing and some from the increased AoA from the elevator - as a consequence I'm flying with a slightly lower AoA than I might be doing otherwise - so I have some leeway before I need think about stall behaviour in very tight moves.

Just to summarise - it really is down to the individual flier how they trim their model - as long as you can achieve straight and level at some throttle setting, and you know what that setting is, it entirely your choice and yes where that point is will vary depending on what the model is designed to do.

BEB

[Bob Cotsford]

Even with fast symetrical wing models I like to trim the model to maintain height hands-off, which means the model is being trimmed with a hint of positive incidence on the wing in order to maintain altittude at say, 3/4 throttle. Due to the nature of the beasts and the fact that the airspeed is already high at 3/4 throttle increasing to maximum doesn't have a noticable effect on the rate of climb or lack of it. Admittedly my MiniPanic is an exception in that it climbs on a neutral elevator when inverted.

I haven't read it yet, but from the previous articles in the series I'm guessing it's aimed at middle of the road flyers with trainers or general sports models wanting to expand their knowledge rather than us hooligans with models in the ballistic category on the basis that the hoolies will already be familiar with trimming for their own flying style and preferences.

 

edit - it took me so long to post what with work getting in the way that BEB has already replied.

Edited By Bob Cotsford on 27/06/2012 11:21:27

[Ben B]

Aha BEB- I wasn't sure whether you were the one and the same. As said, love the articles, really interesting and I personally find them much more engaging that a review of a kit I might not be interested in (although that being said I wasn't overly interested in the mini EDF and now think I might get one!)...

I think I'm going to try re-trimming my sports hack and see how I get on. I do know I prefer a massively forward CoG which I'm going to have a go at adjusting. It makes the planes seem "locked in" but inverted flight is hard work and for aerobatics it's not much fun as the plane won't spin.

All very interesting though! It made me realise that what I thought I was doing re trimming a plane wasn't actually what I was doing! IE if someone asked how I trimmed a plane I'd say as you describe but in reality was doing something different and relying on brute force! Hope there are plenty more such articles in the pipeline.

[Cyclicscooby]

Well put Ben n BEB..

I haven't read the article yet, but was gonna agree with Ben, until I read BEB's reply, which is also valid...

I fall in the Ben camp, cuz all my planes have loadsa power, so falling airspeed at any AoA isn't an issue, but clearly many scale birds haven't got this advantage, so should listen to BEB's advice..

To be fair, i've set-up my UMX Carbon Cub as BEB suggests, as it's not infinate vertical, but the others are level at all throttle settings..

Luv

Chrisie.. xx

[Erfolg]

I am not convinced with respect to the dive test, personally.

I have used with gliders and it does seem to work, although I have never had the CG so far back that the model tucks under.

I have tried it on electric sports models and find it less convincing, to it does not work.

I have tried it on a symmetrical wing stunter. Again I have not moved the CG so far back as to be pretty sensitive in pitch. Although I have moved the CG forward. In these cases it just keeps on going, no pull out. Goes where pointed. I have had similar results with other low camber sectioned wings.

With sports models, I look to how the model behaves in pitch, when disturbed by a gust of wind, whilst in level flight. Does it do a damping fugoid, over a couple of cycles (I consider this ideal). Does the fugoid keep on gonging (defiantly tail heavy, or does the disturbance damp in one cycle(forward CG).

The other check, is inverted flight, how much down elevator is required to stop it diving.

I know this is a concept from "Instrumentation and Control theory", yet it seems to work on models as well. The trouble is, it takes time and observation. The observation open to debate.

I guess the dive test is supposed to work on the basis on the pitching movement of the wing, with speed and AoA, which could alter the loads the tailplane assembly needs to generate, to keep the model on its original path or not.

Edited By Erfolg on 27/06/2012 12:57:04

[Erfolg]

Good Professor

May be your articles would benefit from being in parts?

There is often a lot of ideas and comment as to how ideas impact in the real world. Of course there needs to be the simple introduction and then the analysis of how it works and why.

I just remember as a student, the feeling of "Good as far as you have gone" please resubmit with the meat", that the devil really is in he detail, particularly with respect to understanding. I speak as one who does not believe recirculating theory, other than a general trend, if I was stood on a molecule, going over the upper surface, observing molecules passing underneath. I need to have things made real.

[Lee Smalley]

Good article Beb but as you has already posted I could not trim my aerobats like that I need them to stay level in flight regardless of throttle setting (above 1/3 obviously) and the dive test is for me the only way to do c of g as it takes thrust lines out of the equation