Flight Speed

After bumping up the speed on the Twister project from about 51MPH to about 54MPH (62'handle to plane CL plus approx. 2' of my arm--changed from 5.4 sec to 5.0 sec/lap), the plane flies much better. More positive tension and better tracking. (also moved CG per recommendations).

So now I'm still in the prop/engine setting mode of fiddling and wonder if I fly it faster yet--perhaps 57 MPH and 65' lines, will it get better still?

THis is a 480 sq in plane at 52 oz now. (15.6 oz/sq ft)

What type behaviors would indicate that I had too-long a line set or a too-fast flight speed?

Should I try the speed first, then add line length, or length first and then the speed adjustment?

I'm trying to follow the (walker?) trimming flow chart. But I dont recall any reference to speeds in the adjustments.

Suggestions?

Thanks

Our numbers (and problems) are similar. Waiting for the smart guys to respond. Based on what you said, I would leave the line length alone for now, hone in the other variables, and leave line length for last.

That would be my inclination as well since it is pretty easy to change the speed up a bit. I'm running an 11-4 tornado on the Tower forty now. But I put on an altered head to tame the beast and it will run MUCH better now albeit slower. I'm picking up some more props to try like the APC 10 and 11 x 5.

My back-up for the power is the Scott Riese setup with the Macs muffler, and three blade 10-4.

Double backup is probably a SSW rework of the tower.

The trffic here has really dropped off as the big contests draw near. I'm thinking of getting to Muncie for the finals in July for a saturday/sunday since I'm only four or five hours away I think.

Thanks

Hope this helps some?

Thanks for this thread! I just had an epiphany. Combat in the mid 60s was 90 to 100 (at least for me)and at those speeds line tension was pretty much a given. It just hit me that all this trimming hocus-pocus is due to much slower flight speeds. Hate to admit that I didn't fundamentally get it, but I fundamentally didn't get it.

We didn't trim those old combat ships much. Just avoid warps, and everything was golden. I thought the same was true today. Not true, which is why trimming is an art of which I am ignorant.

I am starting over, now that I (think) I understand the problem I need to solve.

YEs, I've been looking at this a bit differently as well--decided to go throught a couple of math examples for my amusement.

Limiting the problem with the assumption that tension is soley a result of accelerations of the circular path, and that in a wingover, 1 g is the minimum required to keep the plane just at zero tension at the very top (weight of plane = centrifugal force) THen the minimum flight speed is only 30 mph for a 62 ft radius! Of course we couldnt fly at this limit but it is interestingly slow. I was surprised I guess.

So some increment in g force is required above the minimum limit, and I now must guess that it is more of a pilot comfort/confidence factor than a hard performance issue. 62' radius requires 37 mph for 1.5 g, 43 mph for 2 g and 53 mph for 3.0 g. If I'm flying alon level with 3 g handle load on a 50 oz plane of about 9 lbs! and I go overhead, tension is going to drop by 1/3 because of gravity. THis is a very uncomfortable feeling and would previously induce an instant panic response to my handle! And if control inputs cause slowing, then lookout below!

Your 80 mph combat ship on those lines is at 6.5 gs ! no problems at all--trimming probably is masked by the tension. the 25oz plane is pulling at 163 oz!

Its also worth noting that, g load wise (tension) that the value is inversely proportional to the line length. Shorter lines =more tension from g loads. But the same loads vary as the SQUARE of the velocity. so speed has a proportionately greater effect on the tension than line length.

This all is a simplified model, especially ignoring influences of line drag and exit angle which could become significant at lower speeds. But it was a good execise for me and it confirms the other guidance in this thread about trying a little more speed.

Hope my math was right! no warranty on quick calculations!

Still in need of some expert trimming guidance on-site, but it continues to get better.

Meanwhile, I'm making it through the beginner pattern in sequence and practicing specific elements at will with the Ukey 40. Hope to compete in August.

Curt --

I didn't catch your g comments on 6/21 until today. My Big Strong Handsome Wife must have been talking to me when I was on-line, and I TOTALLY, TOTALLY missed the post.

Now that I have read it, I think you did indeed make a quickie calc mistake, as you suspected when you posted. Thirty MPH would indeed fall out of the sky on on 62 feet lines, as you down deep know.

There are about seven different ways to skin this cat, and I don't know that any approach is superior to another. Just depends on how you feel comfortable with the data analysis. Regardless of the approach, we should ALL get the same answer, only expressing it differently (laps/second, MPH, etc.)

I got 6.3 seconds per lap as 0.99 g, regardless of line length. If you do raw MPH, then line length becomes a g factor. (If you use lap times in seconds, line length is not a factor, as longer lines result in directly proportional MPH to get everything to add up out.) That translates to 42 MPH at 60 foot line length (plus 2 feet of elbow, held constant for the rest of these comments). On 65 foot lines, that would be 46 MPH.

Also, your comment that an 80 MPH combat ship on 60 foot lines is 6.5 g's makes me think one of us made a mistake. I just looked at lap times from 4.0 to 6.3 which gives an MPH range of 42 to 72, and a g force of 0.99 to 2.47. Although I didn't extends the calcs to anything above 4.0 second lap times, I can't imagine you and I agree on 6.5 g's.

If you want the spreadsheet, e-mail.

Curt was trying to calc the "rock on a string" forces only, knowing full well that trimming could change total tension dramatically. In other words, breaking line tension into its components, we know g force is one of them, and Curt just broke g force out mathematically and looked at it as one organ of the whole being.

Jim --

I haven't yet tried to attach anything here. When I get off work this evening I will try to attach a spreadsheet. Based on the guys attaching pictures, attaching files can obviously be done, and I need to check the old threads. Believe Leonard published some instructions. If unsuccessful, I'll zap you a copy.

Larry: I don't find this to be the case. Just last Sunday, I had to run my Super Ringmaster, with an old OS Twinstack, blubbering rich on 60' lines at 7 seconds per lap, in (admittedly) dead calm conditions. This works out to 36.7 mph and lower. Line tension was 1/2 A-ish, and I just kept it level, but the plane ws rock steady, with no threat of crashing.

Well..I just looked over what I did again--maybe I forgot more physics than I thought but my calcs for your example are 6.3 sec /lap on a 62' radius put the G load at 1.91.

I used the formula a= v^2/r and g=32.2ft/sec^2

so... r=62' c=6.28*62=389.36ft

v=389.36ft/6.3sec = 61.8ft/sec or 42mph

(ft/sec) * (3600/5280)

and then a=v^2/r or (61.8)^2 / 62 = 61.6 ft/sec^2 or 1.91g

Your calcs for the 80 mph example almost sound like a missing square function of the velocity.

Whare are we going wrong here?

Leonard: I agree with your comments about the trim and thrust issues v. these "rock on a string" examples. I try to validate some of my observations with these simple models to aid in my "feel" for the problems and results. In this case, doing so showed me that since the g loads increased by the square of velocity, that small changes in flight speed can make fairly significant load changes. But where are our calcs not agreeing?

No, or formulas are identical! One of us must have a typo. I suspect it is me, as your numbers look "round off in your head correct." I am on my way out the door for dinner, will check when I get back.

1.916g's/61.67 fps^2

It was a typo on my part. Your numbers are correct.

For those of you who do a lot of spreadsheet work, this was a most interesting and troublesome spreadsheet problem to track down, and a darned good lesson. We were all pretty sure the difference in our numbers was due to some faulty physics formulas or some faulty algabric manipulations. I looked high and low for those kinds of errors, and could find none. Each individual cell (I built a table) looked OK to me.

My problem was that in putting in corrected $ cell holders so I could copy across and down, I put in one too many $$, deleted it, and in the process also changed a cell reference from C40 to C4. Dropping a 0 in a cell reference is fatal, as it was in this case. Well, that changed the g reference to the line length reference! My erroneous spreadsheet now had line length twice, and no reference at all the the 32.2 g constant! Unlucky perhaps, as those kinds of errors usually produce a ### cell. My error happened to produce a spreadsheet that worked quite well, and also happened to fit my "real world" picture.

Thanks to all for your help and patience. I would never have found this error without your help.

I am certainly no expert, but I did stay at a Holiday Inn Express last night.

In that vein, I would say this is a most informative thread. We "rassled out" a few things, and here is my summary:

*Total line tension, be it overheads or not, is a function of centrifugal force and aerodynamic forces. We all knew that already, right?

*Isolating the centifugal force numbers from the aerodynamic (trimming) forces is worthwhile.

*In real life, we all have to decide whether to increase speed or to change aerodynamics to change line tension.

*Everyone is tempted to play with line length for a variety of reasons. The numbers here play strongly towards a change in line length should be accompanied by a change in speed, and that you don't have to do much math. Keep LAP TIMES constant, and regardless of line length, tension won't change much.

I think (arguments?) that a particular airplane-fuel-engine-tank-prop has an optimum speed, a very small range of speed where the airplane flys best. What we do with line length changes is to try to generate enough line tension at a lap speed that is comfortable for us. I'd also say that that 0.1 sec change in lap speed will make a small, but detectable, change in line tension. I know that I can tell the difference in tension from a 2-ft change, with the airplane running the same.

I'm not going to just say "Amen," but I will remind Larry and Curt that by including experiments with props in the search for an ideal setup for any new plane being trimmed to fly at its own optimum, you get to see variations in tension within maneuvers, including improved overhead behavior when you can match a prop with better thrust against the power available from your engine.

In the Way Back, some fliers liked wide-bladed props; claimed better performance for them. The first "big" Fox-powered plane I had to trim from scratch flew more poorly with Rev-Up EW's - it could've been the reduced- pitch tips common to Rev-Ups, but what was interesting is the same design plane, a second one, but with a Max-S, did seem to handle better with some of the wider props than with regular width props.

Personally, at 50 Ounces and 490 squares, I'd say that the Twister needs to go on some sort of weight-loss diet! That is really a lot of weight for that size plane. My first Skylark was that heavy, but with 80 more square inches, it had a much better relationship between weight and lift.

The K.

>I'm not going to just say
>"Amen," but I will remind
>Larry and Curt that by
>including experiments with props in
>the search for an ideal
>setup for any new plane
>being trimmed to fly at
>its own optimum, you get
>to see variations in tension
>within maneuvers, including improved overhead
>behavior when you can match
>a prop with better thrust
>against the power available from
>your engine.

I agree. Near as I can tell, most everyone trims with several tools. Choosing which one to tweak at any point is where the good guys have a "touch" us nowhere guys don't have. There are other factors, but some are (in no particular order):

1. Changing aerodynamics, using trim tabs, rudder offset, flap ratios, etc.

2. Changing leadout position.

3. Changing wingtip weight.

4. Changing CG.

5. Changing line length.

6. Changing props.

This is a true DOE problem, but we are pretty much forced to work OFAT, hence the advantage of knowledge, skill, and "touch". Each of these factors affects the others. For example, changing props changes the . . . Well you already got the idea, or you wouldn't have posted your note.

The advantage of the Curt digression is that any change can affect lap times, and we now have a separate tool to measure part of the effect of a change in lap times on a change in line tension.