# High altitude aerodynamics and propultion

In summary: The density of air at these altitudes is really low, so a prop would not be able to push the aircraft very fast. The aircraft would be able to travel at a ground speed of 10 m/s with the wind, but it would be really slow.
I am looking for some help and advice on the title topics.

Looking at ideas for a final year project, and currently looking at doing a high altitude and long endurance UAV.

My definition of high altitude is 100k - 200k. I have looked at some other uavs that have done this, including the NASA one, but i can see no technical documentation on any of them.

So, what are the issues with obtaining altitudes over 100k with regards to

1) Propultion systems
2) Aerodynamics

I have been looking at either ducted fans or plain propellors powered by electric motor.

Think a UK venture has got to 120k, was really interested if it was possibe to go much higher using the same design principles and equipement?

Thanks

If this is a final-year school project, you are way over your head, I think. There are tons of problems with high altitude, high endurance flight.

First, there's no air up there. The density is extremely low. Typical engines which work well at sea level or even 20-50k feet have nothing to push against at that altitude. So yes, propulsion is super difficult.

Aerodynamically, you have a shift where viscous forces become much more of a concern.
$$Re = \frac{VD}{\nu}$$
$$\nu = \frac{\mu}{\rho}$$

My senior project was a wooden ramp with a hole in it. If you're going the UAV route, I would stay within a reasonable altitude. The problems that you'll have will be endless.

Very good advice. I think i am going to go with the endurance topic, and have a UAV that can potter about at a reasonable alititude for a long period as after reading what you have said, and a little more research, i can see that the best brains at a few top companies who have spent millions are having issues.

I am interested more in the high altitude, but only from a theory point of view now and in brief just as an addition to further improvements.

I have looked at the designs of the current NASA and Zephr projects, and they seem to be using props rather than and duct fan design.

I found a fair bit out from the NASA report link, has calculations for using props at high altitudes, however, does not talk about the extremees that they are currently at, guess this is to maintain the competative advantage over Zephr?

Using an electric motor with a prop seems to be in use on both platforms.

I am guessing that if there is little air, then the prop won't be able to pull the airframe at a high speed, thus needed a very low stall speed design? The Zephy does around 19km/h, which is very slow.

If at 130k ft an aircraft does 19km/h, what would be is relative ground speed?

Are you planning on building this thing or just writing a design paper? I think the preliminary design of this system is hard enough for a senior project. At high altitudes, low speed flight will be hard to achieve due to lack of air. An a/c needs a lot of speed at these altitudes to capture enough air for the propulsion system to operate and the lifting surfaces to be effective.

Ground speed depends on wind speed. With no wind speed, then true airspeed equals ground speed.

If you ever get a chance to see Pathfinder taking off, you would think your eyes were playing tricks on you it moves so slowly.

I will have to build it, so i think i will stick to low altitudes, i may expand this to high altitude in my masters project.

Wont get a chance to see Pathfinder fly as i live the otherside of the pond.

I have got some videos of it, and it does go very, very slow.

Where could i find what the density of air is at these altitudes?

If nothing else, i would like to do some calculations to show that this project is not a feasible one for the level of study.

Stupid question, but is there any wind at these altitudes around 100k+?

If so, could use the tailwind to help keep the aircraft maintain enough speed to have lift off the wings?

IWhere could i find what the density of air is at these altitudes?
$$\rho = \rho_b \left[\frac{T_b}{T_b + L_b(h-h_b)}\right]^{\left(\frac{g_0 M}{R^* L_b}\right) + 1}$$

If so, could use the tailwind to help keep the aircraft maintain enough speed to have lift off the wings?
Typically you prefer a headwind. Think of it thi way: if you have a ground speed of 10 m/s, and the wind also has a speed of 10 m/s, how fast is the air traveling over your wings?

How big is this team for your group project? Whats your funding? Even building a UAV that flies 200ft from ground level is a very difficult task. There are a lot of proprietary INUs and navigation systems you can buy but they are very expensive.

Also before you get too far, you should check with the agency in charge with controlling aviation, in your country. Usually there are rules for design, manufacturing and where and when they can be flown. Getting something certified through these agencies can be a long, expensive ordeal. At present the rules are changing quite often.

lol, if any of you really think he's going to make something fly above 100k feet, your out of your mind.

His airplane is going to fly in solid works, and that's about it.

Pathfinder and its successors were the craft I thought about when I read high altitude. One either needs a lot of thrust to get speed to get lift, or one needs a large wingspan and very low mass.

http://en.wikipedia.org/wiki/NASA_Pathfinder

http://www.nasa.gov/centers/dryden/history/pastprojects/Erast/pathfinder.html

In August, 2001, the lightweight solar-electric aircraft had reached an official world record altitude for non-rocket powered aircraft of 96,863 feet during a maximum-altitude flight from PMRF, the first of two major flight milestones set for the craft by NASA under ERAST.

http://www.nasa.gov/centers/dryden/news/ResearchUpdate/Helios/index.html

More projects
http://www.nasa.gov/centers/dryden/history/pastprojects/index.html

Think cyrus hit the nail on the head, its only going to fly in solidworks, well i am hopeing to make it fly in X-plane, simulink and matlab.

For this project i will make a model to show what it will look like, and that's about it really. If i get time i might add an engine and prove it can fly.

Last thing, save making a new thread, anyone tell me why a jet engine is more efficient at a higher altitude? Was watching something and the captin said that if they have rapid decompression and need to go down to 10k ft, then they burn "a hell of a lot" more fuel than at 30k ff

that's simply because the engines are optimized for efficiency at cruise altitude, therefore they are less efficient at lower altitudes. If an airplane was designed to cruise at 15,000 ft, then the engine would be designed such that it is most efficient at 15,000 ft.

If you design an engine for 100-200 kft, it probably won't be able to operate at sea level. Hybrid designs can overcome this, read about the SR-71 propulsion system, the pratt & whitney J58.

Building a true UAV would be very hard, but you could try sending up a high altitude balloon. A hobbyist named Alexei Karpenko sent a helium balloon to 100,000 ft (30 km) and took photos and video. You can read about it at the link below:

http://www.natrium42.com/halo/flight2/

There is a lot of engineering involved in something like this, so there should be plenty of things to do.

Hey

I read that ages ago, and forgot so thanks for bringing it back up.

I was thinking over how high helium balloons can go, and like said, they go up high. One i found has a burst hight of 40km, so that's way up.

Is there anyway to push the burst hight even more up?

The balloon bursts due to the helium expanding to the balloons limits? Is it possible to slowly leak out helium to gain more altitude?

Going off now into electronics, but low frequency radio can have a very large range, but if i am right in thinking this is due to the waves bouncing of clouds and the ocean? Not been that into electronics i don't know? But, is it possible to always stay in communication with the balloon so you know raw data such as altitude, long/lat and maybe vert/lat speed?

Would it be possible to fully deflate the balloon before the max altitude so that the balloon can be saved?

Loads of questions, and i am guessing they will be around the weight of such equipement.

Again, thanks for any help.

You could certainly use a stronger material, but the maximum height of a balloon will still be limited by buoyancy considerations. Since the buoyant lift on a balloon is equal to the weight of displaced air, the lift will decrease at higher altitudes until it is insufficient to raise the payload any higher.

I think it would be more interesting to build a balloon with some sort of augmented propulsion system to control the latitude, longitude, and altitude of the craft. The balloon could then be programmed to fly along a specific path, instead of being totally uncontrolled. If you could figure out a way to stabilize the camera, you would have a low cost platform for ground surveillance.

Been reading up on all this, and the problem you state has not really been implemented about stabalizing the payload and camera as i can find. And i can see any that have any type of propultion system, does anyone know different?

Would seem that the rules about sending one up are pretty lax too, as long as the payload is under a value and the volume of helium is under another value, not yet found these values for the UK, but can't see them been too different to USA and Canada.

Balloons don't seem to have propultion systems, where as blimps and the likes do. I am guessing this is because a balloon is not as aerodynamic as a blimp.

Is a high altitude blimp going to be harder to achieve than using a simple balloon shape?

Been reading up on blimps, and would seem they use an airbag so when the helium expands the air dispands.

I am trying to figure out that if you let helium out before the burst point so the burst is avoided, how much higher the balloon can go before loss of boyancy. Is there any formulas for this or would one need to be created?

Thanks again for the help.

At their altitude the aerodynamics is not the first matter because de density of the air is very low. About the propulsion system the solar power could be an idea, especially if the UAV is provided with a wide span wing.

## 1. What is the effect of high altitude on aerodynamics?

The main effect of high altitude on aerodynamics is the decrease in air density. This leads to a decrease in lift and thrust, which can affect the performance of an aircraft. Additionally, the lower air density can also cause the aircraft to stall at higher speeds.

## 2. How does high altitude affect the efficiency of propulsion systems?

High altitude can have a significant impact on the efficiency of propulsion systems. The lower air density means that the engine has to work harder to produce the same amount of thrust, resulting in decreased efficiency. This can also lead to a decrease in fuel efficiency and range.

## 3. What are the challenges of designing aircraft for high altitude flight?

Designing aircraft for high altitude flight poses several challenges. The lower air density and increased speeds can cause issues with stability and control, as well as structural limitations due to decreased lift and increased stress on the aircraft's components. Additionally, the extreme temperatures and lack of oxygen at high altitudes also need to be considered in the design process.

## 4. How does high altitude affect the performance of jet engines?

High altitude has a significant impact on the performance of jet engines. The lower air density means that the engine has to work harder to produce the same amount of thrust, resulting in decreased efficiency. This can also lead to a decrease in thrust and overall engine performance, especially for turbojet engines.

## 5. What are the advantages of high altitude flight?

High altitude flight offers several advantages, such as reduced air resistance, which can lead to increased fuel efficiency. It also allows for faster flight speeds and can reduce the effects of weather conditions on the aircraft. Additionally, high altitude flight allows for more direct and efficient routes, resulting in shorter travel times and cost savings.

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