Aerodynamics Questions

[chenng]

Hi all,

Just got a few questions about basic aerodynamics:

1. How many types of drags are there and what are they, and how are they formed?
2. What is boundary layer, how are they formed and what use do they have?
3. How many types of draft are there and how are they created?
4. What is a Mach number used for in terms of scaled down aircraft?
5. How do we work out the Reynolds number and what are they used for?

Also how are they all linked up?

Thanks in advance. Need this for an exam and they are a few concepts I can't really grasp.

Thanks alot
Regards
Brendan

[Lutefisk989]

Blue. No, green.

[Mad (Flt) Scientist]

1. Wiki is your friend.

Types of drag are generally divided into three categories: parasitic drag, lift-induced drag and wave drag. Parasitic drag includes form drag, skin friction and interference drag. Lift-induced drag is only relevant when wings or a lifting body are present, and is therefore usually discussed only in the aviation perspective of drag. Wave drag occurs when a solid object is moving through a fluid at or near the speed of sound in that fluid.

2. It's the layer of air close to the surface of a body, forming the "boundary" between the body itself and the "free stream". It's formed because right next to the body, the air molecules are essentially at rest relative to the body, but in the freestream they are basically going full speed. It's not much "use" - in fact it's a bloody pain at times - but it's unavoidable.

3. Er, what? Are we talking about badly fitting window seals here? Don't understand "draft".

4. Mach No. is ratio between airspeed (speed of body relative to air) and local speed of sound. It's an important factor in how air behaves, so if you're trying to replicate an aircraft with a model - as in a wind tunnel - you want to get the Mach number right (if you can!)

5. Reynolds number denotes the relative importance of viscosity in the flow. A high Re means viscosity isn't so important; a low Re means it dominates. Very important, especially for small scale effects, such as inside the boundary layer. If you want to replicate things like the drag or stalling behaviour of an aircraft with a model, Re is important, so you want that as close as you can to the real value.

You'll note that (4) says wind tunnels try to get mach right, and (5) says they try to get Reynolds number right. Unfortunately, you can't have both with a scale model, unless you use something other than normal air in the tunnel. Hence cryogenic tunnels and pressurised tunnels are sometimes used: expensive, but sometimes necessary.

If you get Re right, you'll get the boundary layer about right, unless there's sonic flow going on, in which case you want Mach right too. Get both Re and Mach, and you'll have a representative boundary layer, and in turn that means the parasite drag will be about right. The right Mach should get the wave drag close. Lift-induced should be ok usually, and be better still with the right Mach and/or Re in certain conditions.

[helicoptertestpilot]

Hi, I don't know how to get the Greek symbols here, but here goes. Reynolds number = rho x v x l / mu. Assuming your wind tunnel is up to mark, rho, v, and mu will match the conditions for which you are testing the model. However, l (lower case of L) will differ (quite a bit) between the model and the actual aircraft. This leads to the 'scale effect'. A dose of AP 3456 (extremely basic stuff, don't worry) would help you as would the Prouty series. Good luck!:)