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What determines a planes speed?

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Tony H15/12/2012 14:49:08
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Hi All,

I was thinking of designing my own high speed aircraft and wondered what exactly determines a planes speed??

I assume aerodynamic efficiency, minimal drag and maximum power possible would make for a swift aircraft. What about Mass/Weight? is a heavy plane faster due to inertia or is a lighter plane better? Obviously a heavy plane wouldn't be ideal because of the higher stall speed.

Any thoughts welcome

Ben B15/12/2012 15:08:19
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You do indeed want minimal drag, max power (both thrust and pitch with good efficiency) and low weight.

Weight is involved because if you have a heavy plane for it to fly you have to create more lift and lift inevitably means drag.

Obviously balancing weight and power is a tricky one.

Now the interesting bit (for me anyway) is aspect ratio- you would imagine you would not want a long straight leading edge slamming into the air- you'ld want a more swept back design like a delta. Which begs the question why dynamic soaring planes aren't more like PSS planes!

Simon Chaddock15/12/2012 15:09:35
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Tony

Its going to depend on what you mean by 'speed'.

Up to say 200mph reasonable attention to aerodynamic drag is important but its speed is largely determined by its power and the efficiency of its conversion into thrust.

Above 200 mph attention to aerodynamic drag over rides almost everything as the power required to over come the drag rises more or less by the square of its speed.

It takes power to create lift so with everything else equal and in level flight a light plane is faster than a heavy one.

I hope this helps a bit.

 

I for one am always surprised by the fascination people seem to have with speed.

In RC where you are standing on the ground high speed starts to become a problem of safety and visibility.

Edited By Simon Chaddock on 15/12/2012 15:10:12

Tony H15/12/2012 15:11:43
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Yes exactly my first thought was a delta built up thin wing design or solid balsa shaped, but like you say dynamic gliders are very fast and a very different design. What is PSS?

John Cole15/12/2012 15:12:17
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Power, drag, thrust at speed (i.e. appropriate prop pitch), strength, inherent stability, flutter-free control surfaces (and flying surfaces!). Weight has only a very small effect in straight and level flight.

Oh, and pilot skill if you want it to fly twice.

Radge15/12/2012 15:14:29
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The simplistic answer is the one you have already suggested, thrust, lift, drag and weight. Consider the SR71 Blackbird, A relativley small light aircraft with massive powerplants and every contour calculated to minimise drag.  With regards to dynamic soarers etc, whilst apperently fast,   remember that their VNE is quite low and will remain so unless their profile is changed.(A variable geometry glider?   Now there's a thing!)

Edited By Radge on 15/12/2012 15:23:50

Tony H15/12/2012 15:16:43
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I find speed fun, a fast low pass into an unlimited vertical always feels great!

So I was right to stay light, always seems the best option. I was aiming to achieve 150-200mph absolute max as I feel the size of my flying site would be too small for a faster plane, so in the intrest of safety that is my aim.

I may be better off going electric on this one prop or edf?

Edited By Tony H on 15/12/2012 15:17:46

Simon Chaddock15/12/2012 15:20:25
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Ben

Its all a question of aerodynamic efficiency.

For a given wing area (ie lifting ability) a swept wing (or delta) is only more efficient than a straight one when compressibility comes in to play and you have to be doing 500mph or more for that to happen.

Hence the dynamic soarers use efficient straight wing designs, but the way they are going its only a matter of time!

Radge15/12/2012 15:28:10
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Posted by Simon Chaddock on 15/12/2012 15:20:25:

Ben

Its all a question of aerodynamic efficiency.

For a given wing area (ie lifting ability) a swept wing (or delta) is only more efficient than a straight one when compressibility comes in to play and you have to be doing 500mph or more for that to happen.

Hence the dynamic soarers use efficient straight wing designs, but the way they are going its only a matter of time!

A 2m razor thin airfoil Simon, you've just set yourself your next challenge!wink 2

twinstar15/12/2012 16:32:10
221 forum posts
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What is PSS?

Power scale soaring: planes that look like powered aircraft but that are gliders!

 
Edited by Chris B - the video had been posted too wide despite the instructions that come up when the youtube button is pressed.

 

Edited By Chris Bott - Moderator on 15/12/2012 16:44:40

twinstar15/12/2012 16:37:45
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more PSS

Tony H15/12/2012 16:41:54
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Thanks Twinstar, They call them Power Scale Soarers, but they dont seem to have any motors or engines is that right?

Concorde Speedbird15/12/2012 16:42:11
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How about a Delta? Then you will have speed! Two stroke for power to weight (and drag) ratio, and light weight with powerrr!

And maybe a droop nose, ogival wing, all white colour scheme, a bit like this?

G-BOAF up high

You knew I would do that!

CS

Tony H15/12/2012 16:56:25
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LOL! Yes very good CS. I am considering a delta it's the most likely option a bit like the Westons, they are very expensive kit's for what they are.

Olly P15/12/2012 17:07:10
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Tony - They are called POWER Scale soarers as they are scale models of powered full size - and damn good fun too!

Tony H15/12/2012 17:19:43
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Thanks Olly I get it now, nothing stopping you giving it some power tho right?

Tim A15/12/2012 17:57:12
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What determines a planes speed? why at some point in it's life it's GRAVITY!!!

Tony H15/12/2012 18:05:34
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LOL, Gravity helps me build up speed in a dive nicely.

Erfolg15/12/2012 18:29:15
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At some point I would expect that BEB will post a reply.

To my simple view of the world, it all starts with

thrust = drag at the equilibrium condition.

The next thing that engineers do, is plug in various relationships, which make up either side of the equation.

Yet as a model it really comes down to minimising drag, low cross sectional area, and low coefficients for all the functions expressing drag, the ultimate speed then also being dependant in maximising thrust. Which is best described as having the most efficient drive train at the ultimate speed (incl propeller, if used).

In the real world of modelling speed is very much down to observation of what happens, and determination to go faster, observing what hinders performance and what helps, still whilst having some control. No magic wand.

As as been said, i am not worried about speed, seems of little importance to having a good time.

Greybeard15/12/2012 20:10:06
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The general rule is to stuff the biggest engine that you can afford into the smallest airframe that will enclose it and then see if you can fly it in any direction other than straight down. wink

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