What determines a planes speed?

[Tony H]

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 B]

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 Chaddock]

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 H]

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 Cole]

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.

[Radge]

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 H]

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 Chaddock]

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!

[Radge]

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!

[twinstar]

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

[twinstar]

more PSS

[Tony H]

Thanks Twinstar, They call them Power Scale Soarers, but they dont seem to have any motors or engines is that right?

[Concorde Speedbird]

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?

You knew I would do that!

CS

[Tony H]

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 P]

Tony - They are called POWER Scale soarers as they are scale models of powered full size - and damn good fun too!

[Tony H]

Thanks Olly I get it now, nothing stopping you giving it some power tho right?

[Tim A]

What determines a planes speed? why at some point in it's life it's GRAVITY!!!

[Tony H]

LOL, Gravity helps me build up speed in a dive nicely.

[Erfolg]

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.

[Greybeard]

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.

[Biggles' Elder Brother - Moderator]

Greybeard hits the nail on the head!

First Erfolg and others are spot on - its all about getting as much thrust as possible and as little drag. A delta wont be particularly beneficial - as Simon says you are no where need compressability issues so a delta brings more problems than benefits. If deltas were good at the speeds you are talking about we'd start seeing a movement towards delta-winged pylon racers!

So:

1. Big motor/engine, prop'ed with a strong bias towards the highest pitch you can reasonably run.
2. Very sleek airframe - as smooth as possible. Shiny!
3. Small control surfaces - at speed you will not want large surfaces with large throws! Put them on rates so you can have more throw for take-off/launch. With the large pitch the model will accelerate quite slowly - but will eventually reach a very high speed (you hope) - at some point you will want to reduce the throws to maintain controllability.
4. A very thin - low lift - wing. This model will generate all the lift it needs simply from speed - it certainly doesn't need a highly cambered wing profile or you will have far too much lift at high speed and will have trim problems.

Finally - consider designing it to be catapult launched. Its very difficult to design an aircraft that can fly well at a wide speed range. If you can get it up to speed quickly - avoiding the slow acceleration phase - so much the better.

BEB

[Tony H]

Thanks BEB,

So do you think a straight thin all balsa solid wing shaped would work well? I was thinking of using a very high Kv 480 sized motor and the plane will be a belly lander for less drag. You are probably right about the catapult, I was hoping not to have to do that tho.

[Biggles' Elder Brother - Moderator]

Assumming that at some point you'll want to turn round (!) a simple solid balsa wing might not stand what are going to be very big G-forces. You could reinforce with glass cloth of course - which would also help with the shiny finish. Or run a carbon rod down the wing.

In this case electric might offer an advantage - you really need to keep the frontal area down as low as possible - that may be easier with a motor.

BEB

[Tony H]

Yeah that's what I was thinking maybe a inrunner with a 5x5 prop on it.

[Biggles' Elder Brother - Moderator]

Let us know how you get on. You could do a lot worse than take a good look at a formula like electric pylon racing - then ask yourself "if I was not bound by the rules - what would I change to get more speed?". Best of luck with it.

BEB