Answering our Adviser’s Initial Questions.
Below is an exchange between our new adviser to the project (to be announced officially soon and myself (Robert Brand). Here are his initial comments and please remember that he has not seen anything yet. Our adviser is a pilot with an aerospace engineering degree.
Our Adviser: Hi Robert, Here are a few questions and thoughts.
At a first glance you may think you don’t have a propulsion problem, because the thing is falling down.
The fact is, you do. The basic forces and their components (lift, weight, thrust and drag) are always in balance as long as the aircraft is not accelerating in any axis.
This is valid the other way around as well: The aircraft will accelerate as long as the forces are not in balance.
For your case, you need to have the capability to accelerate beyond the sound barrier.
The problem is that the parasitic drag increases exponentially as you approach M=1 and because you are going at a certain angle towards the ground, a certain component of this force, or all of it if you dive vertically, adds to your lift. Once your lift becomes greater than your weight, you will start to slow down.
If this happens before M=1, you will never reach supersonic speed. If it happens after M=! you can further accelerate, because the drag drops after transonic. Transonic is the worst place to be. I order to be supersonic, you must achieve M=1 ASAP, before the air becomes dense.
If you drop from 33km, forget it, because at 30km you can already feel the effects of atmosphere.
The first thing you need to do is apply total surface design, or coke-bottling. The total surface of your craft must be consistent, so at the place where you have wings, your fuselage must be narrower. This dictates your fuselage to be in a shape of a coke bottle. This will reduce drag significantly.
Also, center of lift on the wings changes in supersonic flight and you need to cope with that. There are two strategies, variable wings or variable centre of gravity. I have a very original idea how to solve that.
Any object going through a fluid tends to assume a low drag position. Sometimes this low drag position means rotating and spinning.
You can solve this problem by active control (unless you have f-16 engineers on board, forget it) or aircraft design.
I would suggest delta wings, high swept. Delta wing has an inherent auto stability feature and high sweep angle to reduce drag and effect of the wings.
Accept it, your aircraft can be designed either for high speeds or low speeds, unless you have flaps or variable wing geometry.
I look forward to how he views this and I will report back soon. I expect that I will have allayed most of his fears:
Firstly we are already applying the constant area rule. Even the A380 has aspects of the rule in the design. I lectured at Sydney Uni on the subject only a few weeks back. I understand the rule and some other rules to do with supersonic flight, although their effects are much less than the constant area rule.
Yes, the wings may very well be more swept back than in the image on the site. We will do drop tests to a certain the best wing shape and we have access to a wind tunnel.
The wings will be symmetrical (top and bottom)– ie zero lift. They will be therefore not an issue at supersonic speeds. The elevator will provide the “lift” with speed at lower altitudes. Yes, it will land “hot” – we may use “flaperons” ie combined flaps and ailerons. It should be noted that these are less effective as ailerons when they are biased down as flaps, but they will be bigger than needed. They will be symmetrical also. Flaperons are really ailerons that are mixed with the flaps signal on the transmitter to bias them both “on” as flaps/ The ailerons do not work with the same efficiency when they are both biased down, but they do work. We may use separate flaps, we may not use flaps. Testing will determine the stability and best options.
Below is a video that shows how they mix the signals in the transmitter of radio controlled models to adjust the various control surfaces. This is a third party video
The spin will be counteracted by the large ailerons even in low air, the trick is to stop the spin in the first place by making the craft very symmetrical and test that aspect.
Our novel answer to controlling the need for different centres of gravity: We will have serious control of the centre of gravity in the craft and we will be able to move the batteries and electronics with a screw mechanism back and forward in the fuselage. This will keep the craft from being unstable at supersonic speeds. Once it goes back to subsonic, we will begin moving the centre of gravity back as we begin to level out the flight and slow the craft.
At slower speeds, we have air brakes that will slow the craft if needed
The supersonic spike at the front of the aircraft is used to create the shock wave with a pin point device ahead of the fuselage and ensure that the biggest part of the shock misses the wing entirely. A shock wave over the wing creates massive drag and this is why many pilots in the early days, tried to break the sound barrier and failed. The spike doubles as a VHF / UHF antenna
Three weeks ago we launched a payload mainly of wood, covered in bubble wrap for the electronics and, with the parachute deployed, it reached 400kph. For the event we will be using a Zero Pressure Balloon to get to over 40Km altitude. If the 9Kg of the payload are not enough, we will increase the weight and size of the craft. We will break the sound barrier, but need to show it is a fully working aircraft after the dive.
In World War II bombs from high altitude aircraft regularly broke the sound barrier. We will shift the centre of gravity well forward and act like a bomb. We should be able to punch through that barrier with a lot to spare – even Felix Baumgartner broke he sound barrier for his jump altitude of 39Km. He was not very aerodynamic. We expect to terminate supersonic flight at around 31Km
Yes, transonic is a bad place. We do not intend to allow the craft to stay there! Punch through while the air is super thin and keep accelerating!
Will we make Mach 1.5? – it depends on our launch altitude. We will achieve Mach 1 – the sea level speed of sound is our target. About 1200kph.
There is much more, but I expect that I have answered most of your questions in this email. We will be using ITAR controlled GPS units for supersonic tracking and also we will be using radar transponders to warn other aircraft. The Jason and I will be testing a lot of aspects of the flight with drop tests from balloons. I will be launching another balloon in a week’s time.
The picture above shows the constant area rule – efficiency is gained by the cross-sectional area of the aircraft being constant along its length. The fuselage gets thinner where the wings are as there area has to be accounted for. This rule is important as aircraft get close to the sound barrier and this is why Boeing 747 aircraft were so efficient.
Note the light blue area has to be the same as the dark blue area, including the area of the wings. This is the “coke bottle” shape that our adviser mentioned – note the thinner mid section of the central body.