What determines a planes speed?

[Chris Bott - Moderator]

Tony are you sure you want something really fast?

Checkout this video, I certainly wouldn't have the required reactions for it.

[Erfolg]

I think we are all agreed that there are real issues with the power train package.

I can envisage that you could put the set up on a test stand such as the type that Timbo uses. You could extract some information, but how useful would it be? You could use an anemometer to measure the propeller wash, but how near the maximum flying speed would it represent? The watts and volts can easily be measured. Would the thrust be a useful value,not knowing the total airframe drag, at the envisaged speed.

At first sight it seems the easiest way would be to build a airframe. Yet even measuring the speed will not be without issues. Just using stop watches requires a team, some equipment, and some good air. Once you have a value, deciding what can and should be done is the next issue. Again without, single element changes and timed measurements, how do you know what has happened?

The use of telemetry seems difficult, in that I would envisage that so little internal space would be available on a minimal airframe.

Yet it is optimisation of all the elements that is required. The development of the powertrain is just one part. The other elements will also have a significant influence on the maximum speed.

Would a single airframe be sufficient? I would suggest not, as I would envisage some airframe attrition and that it is easier to change separate elements on separate models to investigate performance differences.

Suddenly, it seems a big project, if absolutely ultimate performance is sought.

Or you just look at some one elses successful model, ask yourself, what can be done better, do it and make it your own, as the design develops. Just like Vanwall did with a Ferrari, or all the Formula 1 people do today.

[Erfolg]

Looking at Chris video, I know I could not fly that model for long, yet how fast is it? It seems that this Mega motor is probably doing about 17, 000 revs which is less than 80 mph.

150 mph seems very ambitious, plus a really good pilot and a safe flying space.

[Peter Beeney]

I think my view of ‘slip’ would be slightly different again, nothing much to do with the type of propulsion, rather the relationship between the rotational speed of the prop and the forward speed of the plane through the air. The prop is turning at at certain speed, whatever creates this turning motion seems to me to make little difference.

Let’s say we have a propeller with a pitch of 12 inches. If this could screw into a solid medium for 5 turns it would move forward 5 feet, we would have no slip. But if this was in open air and the motive power was fixed solid the prop would not move forward at all, we would have 100% slip. The air, being fluid, would simply move back instead. Somewhere in between is our model flying though the air. So firstly we need to make the aeroplane present as little resistant to the air as possible. Next we need the propeller to turn as fast as possible. If the power source turning the prop falls down on the job I see that as just reducing the revs, not causing any slip between the propeller speed and the model speed.

Possibly it’s difficult to arrive at a figure for this difference, one way of mechanically doing it might be to monitor the airborne propeller speed with a tacho, this would give us a theoretical forward speed, then compare it with the models actual speed, measured with a pitot. The difference being the percentage slip. If we’ve put in a lot of hard work in trying to get everything perfect then this difference might be pleasingly small, but if we tried it on our bog standard trainer it might be a completely different story.

If we calculate how much power we should get from our power source, and it doesn’t come up to scratch, then we perhaps need to look at this, but this, in my opinion, at least, will only reduce the forward speed, it’s a separate entity, it won’t make any difference to the ratio between the propeller and plane speeds. However, what is important is to overcome these shortcomings so that the propeller turns as fast as possible, because ultimately the speed of the model will always be controlled by this.

Here I’m defining slip as the amount of ‘grip‘ that the propeller has on the air, rather like a tyre on a car’s wheel on the road. Normally this has very little slip, more akin to the prop in the solid air, but on ice this might not have any traction at all, thus it would have 100% slip.

Perhaps the in-flight monitoring situation might be a subject for some telemetry…… 



PB

Edited By Peter Beeney on 17/12/2012 22:48:19

[Martyn K]

Posted by Biggles' Elder Brother - Moderator on 17/12/2012 16:23:52:

looking up the spec the Stryker has a 6x6 prop, 3s battery and a 2200kV motor. So thats a theoretical maximum speed (ie if the motor reaches peak revs and there is no slip) of 139mph - allowing say 20% prop slip it would be 111mph.

The question then becomes is the drag low enough to limit the prop slip to 20% - if so the Stryker does indeed do over 100mph.

BEB

We have measured the speed of a Stryker on fresh batteries at 80-odd mph at our field. Which is pretty fast for a foamie.

Martyn

[Biggles' Elder Brother - Moderator]

That's interesting - if we take your 80-odd as 85mph, that mean the "lumped in" slip figure I talk about about above for the Stryker is around 39%!

(Peter - you are quite right - the motor/battery not being "man" enough is not slip - but for the sake of keeping a simple grip on the maths my point is its convienient to lump that undoubted factor in as a some extra "pseudo-slip" as it where!)

BEB

[Tony H]

Thanks for your input everyone

Chris - to answer your question, yes I can be a speed freak at times and am fairly well known for some quick flights at my club. The video you show is the kind of performance I am looking for.

I have an electric Jamara Roo which is no slouch and my Sig Wonder (0.15 ic) is fairly quick to clocked on telemetry GPS at 106mph out of a dive and around 95mph on flat and level flight path.

I do want it to handle much like a pylon racer. I was wondering if IC might be able to produce the speed? I have clocked planes like the Travel Air and Super Air at around 80mph max on internal GPS.

The Wonder is fairly chunky for it's size, I was thinking if I made really skinny but strong wings and a more streamlined fuslage which and overall lighter weight and maybe upped the engine to a 0.25 how much quicker could I get it to go? This would basicly mean a new design but based on the Sig wonder.

[Former Member]

[This posting has been removed]

[Erfolg]

How are people measuring the speed?

I have only seen stop watches, which in reality give an average speed, after some calculation. Which in most respects can be argued to be more meaningful, in the context that of F3b contest, which seem to be more interested in time.

If done over a flat field, I guess two gates are required and a limit set on the permitted loss of height between gates.

Do some have a radar gun? I guess you just pick a point when estimated to having flown level for some distance.

I guess GPS provides a spot speed, rather than average (or integrated value). I guess you need to know the GPS height before and after the chosen spot reading?

Without a lot of work, does it really matter how fast you can make a model go, given the difficulties, other than if you are seeking to improve a record?

[Erfolg]

To get started going fast why not one of these and only £58, plus taxes etc.