Foibles of Biplanes

[Erfolg]

The half tennis ball works well with the lifting bodies I have seen pictures of. Although I have seen little evidence to suggest that lifting bodies are aerodynamically good at lift drag ratios. I am thinking of all the Space Shuttle development aircraft.

 

As suggested the pusher concept probably works well with a tennis ball nose.

 

 

I am not sure that the tennis ball does compromise the nose,. If the hemi-sphere is the starting point of the body shape. I guess it makes no sense to consider other locations for the half ball?

 

I would also suggest that the design, be directed towards being a vehicle to facilitate the design folder and presentation, in addition to actually being up to the lifting task.

 

Although it seems initially that the design all about lifting, it is also about remaining in the competition. A very elegant, theoretically optimised design for lift weight ratio, could fail in that elimination from the comp has a high probability due to lack of durability etc.

 

 

[Tony Richardson]

If a lifting body is the choice why not a delta, pusher configured with, or without a canard??

 

 

Edited By Tony Richardson on 12/02/2012 17:34:08

[Tony Richardson]

Matt take a look Here it may give some ideas.

[Erfolg]

Hmm, I am not sure that a Delta is normally described as a lifting body. Blended body sometimes.

 

I am not sure that Deltas have a good lift drag ratio, do they? Although a foam version could be robust.

 

I have been thinking about commercial high load carrying aircraft. I guess that Agricultural aircraft are the most numerous type.T hey certainly have to be robust. High load ratios are a must. Ease of repair/maintenance is also a given. What is immediately obvious, is the agricultural nature of the whole thing ((joke) . Being serious, slab wings, very angular simple body shape, either biplane or monoplane, but inherently low loaded. Also in the case of agricultural aircraft the need to get the pay load in and then out.

 

To think I just got do a project, for the accreditation requirements, it is not fair !

[Simon Chaddock]

Having read the rules there seems no benefit in flying fast for the heavy lift challenge, unlike the electric one where it is the sum of loads carried around a set course in a given time.

All that is required for the heavy lift is to take off, complete a 360 turn, a circuit and land.

Points are awarded for its mass to payload ratio.

 

This seems to point to the lightest possible structure and wing area within the 2m span. Slow flying with a large diameter fine pitch prop!

 

Lifting bodies? cantilever wings? no.

Something as crude as a DH2 type structure would stand a good chance.

Crash resistant? I wouldn't even worry about it. At the full 4kg payload if it crashes it will almost certainly break so it must have a sufficient reserve of performance so it doesn't crash.

Given the fixed power source that means light weight, very light weight.

 

But then what do I know - I am not doing the challenge!

[Stefan Hafner]

If anyones interested, i still have the photos in my album on here from the plane we built at uni for it. As Simon says, it'll be dead anyway in a crash so we went for full on light weight design, ended up with a plane that weighed 1.8kg if i rmember correctly and lifted about 4.5kg.

 

And important thing to remebre is that your underacrriage is long enough, so that you can rotate to a high angle of attack when your going for the highest load, or you'll never got off the ground.

[Chris Bott - Moderator]

That looks beautifully simple Stefan. Looks like something similar would work as a pusher design to accommodate the tennis ball too?

[Erfolg]

As many others will have been doing, I have though more about the challenge.

 

A significant difference from most model aircraft is that the angle of attack of the wing will be much higher than most models, right from the start. The AOA being a consequence of lifting the max possible.

 

Normally profile drag, is the largest drag element of most models. In this case, by proportion vortex/tip losses will be disproportionally higher compared to most models. Could this be a case for a monoplane or possibly end plates to control these losses. Although they are normally liked, as the drag is high at speed. These models will not normally be going fast.

 

I normally do not like much camber on a wing section, mainly being a glider guider, the reason being, that cambered sections at speed, have high (comparatively) profile drag and at high speed have such a turning moment, that the tailplane works harder. In this case, speed is not the issue, so maybe moderate camber?

 

I ahve also noticed (as I am sure others have) that a good rounded leading edge, works wonders near the stall. Most glider wings are relatively sharp, the stall can happen suddenly. Being at height, this is no great issue, normally, for this competition, a soft stall, could be the difference between doing well and a pile of bits.

 

[Stefan Hafner]

You mentioned there being a pile of bits.......

 

This was the year before I did it, and they were still allowed 3m wingspans. The one at 2:22 is a good example of what happens when the strusture is too flexible, the tail boom on it was too bendy is what we heard from the techies at uni.

[Erfolg]

The video is most instructive.

 

I guess it is the high wing loading and inertial effects which are responsible for the control issues.

 

The three point undercarriage seemed far more practical thana tail drager, as expected.

 

It does seem that control is a major issue, which we RC flyers do not normally encounter to the extent it appears to be the dominant issue. Would Gyros help? They apparently weigh nothing much, I have only just seen my first model with one, on rudder, for taxing. That's other than a Hyper Taxi, which I have not yet seen fly.

 

The totaling from a crash is an issue, there must ways of mitigating the consequences, to some extent.

[Erfolg]

I woke up this morning thinking about this competition.

 

Although vaguely aware that the event took place, I never considered or noted what it really is about. Superficially you tend to think that it is good fun. Yet it potentially provides skills and understanding for all, but particularly young trainee engineers.

 

These are my own thoughts.

 

 

a) It teaches the need and develops the skill of extracting from the "Client Specification" (in this case the rules) what is required, to be "compliant" and how to identify what "success" is from the customer perspective (how to win the overall and components that comprise the competition).

 

b) Competitors learn the skill of constructing a design file, which will typically comprise many sections, and the use of appendix to enable the story to be described simply, yet provide an in depth overview with specific interests. Again typically

 

i) design spec.

ii) option studies

iii) basis of design (configuration, materials, philosophy incl anticipated wing loading and how achieved etc)

iv) Calculations, (structural, aerodynamics, control, weight analysis

v) etc.

 

c) Team Dynamics, which includes,Leadership (Project Management, or whatever form is chosen), team work, use of contractors and Consultants. Particularly contractors, particularly the pilot (assuming that an experienced RC pilot is used?) and how there knowledge can be feed into the project with advantage. With respect to consultants, how to recognise that their role can be many and varied, and can be a valued asset if managed and controlled effectively (I am thinking of the BEB's of this world). Last but not least that each member of the team, has responsibilities to themselves and the team.

 

d) How to effectively sell your idea  to an audience (with varied interests?). Using personal skills, supporting material and question and answer session etc..

 

e) Build the artifact

 

f) Operate the product within the client specification

 

The more you consider the competition the greater the potential value of a seemingly trivial fun event.  It is very close to the real world of an engineer. Where often to get to e) & f) you need to get over all the proceeding hurdles

 

 

Edited By Erfolg on 15/02/2012 11:26:00

Edited By Erfolg on 15/02/2012 11:27:22

Edited By Erfolg on 15/02/2012 11:29:33

Edited By Erfolg on 15/02/2012 11:33:46

[Stefan Hafner]

Thats exactly what we had to do Erfolg. It was dona as part of our course at uni and was worth about 20% of the markd for the year so we had to go through alll of the stuff you described above. We ended up learning a lot , especially about time management (everyone loves Gannt Charts :P). We did have an andvantage though in that I'd built and flown a lot of planes by then

If you're interested in the sort of thing we came up with I can send you our report, don't want to link to it as Matt and team are meant to be doing it themselves.

[Erfolg]

I am interested in your journey/experiences.

 

Now I have retired, i have tended to forget an awful lot of what was second nature.

 

More interestingly from an Engineering perspective, this type of activity does emphasis how complex engineering in the real world is.

 

Until you reminded me, I had overlooked CPA (critical Path Analysis) etc. How important it is in getting from A to B. Additionally the management of resources cannot be achieved without understanding, who is doing what and when.

 

The other aspect that I glossed over is money. How critical is, never enough, the need for an itemized budget and spend profile (S curve), that some will always want to spend more, if the project is to progress.

 

One issue that has been omitted is Safety. Although absolutely necessary, it needs to be seen as built in process. The safety section describing the identified issues, how they are managed and any legislation is accommodated and how compliance is achieved. That is if the overly expensive, and project inhibiting UK approach to H&S is to be halted and the more measured European model embraced.

[Erfolg]

I do think that some one should make the teams aware that avionics are now available, particularly the model speed.

 

The knowledge of the second by second speed, could be a valuable tool, where an observer could manage speed, providing a continuous stream of verbal information, how near the stall the model is.

 

To establish the stall, there is probably a case for having two models. One used to establish stall speeds at various wing loadings.

 

From the Stefan video, stalling these models is a catastrophe, recovery taking an extra 20 foot of height, than is available. Unlike most models, there is no gentle stall, where recovery takes 2 or 3 metres The result of stalling, taking the model out of the competition in ost cases.

[Myron Beaumont]

Erfolg

I have to say this on behalf of the majority of forumites (I suspect) reading your contributions and BEB as well for that matter,I think you should both stand for the post of Prime Minister .Work out who is left wing & who is right wing and I'm sure you would get things sorted & elevate the current situation without a flap .Yaw input would be bound to be an improvement on the present mess up with the knowledge you have obviously accumulated to do with your real life experiences .

[Stefan Hafner]

I think the speed data would make a huge difference, the issue being that most of the high lift profiles that are being used on these models have a nasty stall due to the hysteresis loop on the lift curve, ise stalling and the flow not re attaching untill the AoA has been reduced by about 5 degrees.

 

I can only speak for the Strathclyde prjects, but when we did it we had a budget of £75 that limited us somewhat, and we ended up having to look for sponsorship just to be able to use the carbon fibre stuff that we needed for ours, so while 2 planes would be ideal, it might be difficult, but then I don't know what sort of budget other teams have.

[Erfolg]

Did the £75 just cover the airframe, or extended to servos or even Rx and Tx?

 

I guess with the telemetry of airspeed, it would be possible to take the model from say empty, to say 10, 12,etc up to say 25 oz ft^2, plot and extrapolate the line. I guess you could also be able to plot power to speed at the various wing loadings, with a view of establishing an anticipated max payload with the available power.

 

Even if it proved to be of little value, it would be good for the model design files, presentation. Adding an anticipated to actual result, dimension.

[Former Member]

[This posting has been removed]

[Erfolg]

Had not noticed that Eric. Though it does go to show the merit of attaching to a team a specialist, where appropriate.

 

I guess a scale model pilot would be a good candidate, being better used to handling overweight, high wing loading models. People with my experience only fly low to moderate wing loading, and would be next to useless.

 

Although I am sure crashing because of lack of knowledge/skill/experience can be a useful lesson, it is better in my opinion to acknowledge your own limitations and get some one into the team that can "cut the mustard".

[Simon Chaddock]

Hmmm.

High aspect ratio and high wing loading can be a tricky combination.

Apart from the wing sections natural stall characteristics there is a real danger that the wing twists under load complicating the situation further.

 

Nearly all those crashes had the look of a stall induced incipient spin.

 

What did surprise me was the ease with which the successful ones got off the ground and climbed away, suggesting adequate power. Were they carrying the full 4kg loads?

If they were I would suggest the control problems were airframe related rather than a true speed issue.

 

I have my suspicions that the airframes tend to lack rigidity rather than strength to handle such a huge concentrated load, which neatly brings me back to the braced biplane!

 

I did a little research and found that a single 20AWG (0.8mm diam) spring steel wire can carry no less than 34kg You would obviously want to keep normal loading to say 1/4 of this but that still gives just over 8kg.

It certainly appears that a well designed braced biplane could easily handle the flight loads with absolutely no risk of wing distortion and the extra wing area should allow a wing section with a more benign stall.

 

The final point is it seems no has used one, so either my theory is wrong (quite possible) or no one yet has cottoned on to its benefits in this application.

[Stefan Hafner]

The only biplane i've heard of being used was one of the ones that a team made in 2008. They didn't use any bracing wires which was a shame as it could have done quite well, but none of them had any experience of model building

The pilots that do the flying are all very experienced pilots, and not anyone from the teams, the organisers make sure they are skilled enough.

The ease with which they took off i would guess is becaue they were lightly loaded, but they were allowed up to 3m span till 2007, but the winner that year weighed 14 kg so on safety grounds they reduced the span to 2m. At that time the weight were much higher than they are now i think.

 

The £75 budget was for the airframe and covering, we wer given the radion and engines.

Edited By Stefan Hafner on 15/02/2012 18:28:56

[Erfolg]

If the rules only require a 4 kg load.

 

Using my Fag Packet design ethos and sticking to well known imperial units.

 

The first assumption is that 20 oz ft^-2 would be a tolerable wing loading.

 

Then assuming that a 10" chord is a good size.

 

Following from these assumptions 4 * 2.2 *16 = 141 oz approx.

 

wing area = 141/20 = 7 ft^2

 

at 10" chord = 7 *144 /10 = 102" span

 

If split as a biplane that is only approx 50"

 

No rigging wires needed at all, not even wing dowels. If they were required Carbon Fibre box must be the most appropriate solution.

 

As an aeromodeller it would be a Jabberwock, Waco SRE, Bechcraft Back stagger, Hipperbipe type fuz for me. Then again it is for young engineers without all our experiences, prejudices and hobby horses. Not old Faaa..... like many of us.

[Simon Chaddock]

Erfolg

Do not the rules stipulate a 2m (78.74") span maximum?

What about the weight of the plane itself?

So nearer 6kg in total?

I think you will find 20oz/sqft in a monoplane gets hard to achieve.

My money is still on a DH2!

[Erfolg]

Simon, that is an additional reason to believe that a biplane is the way to go.

 

You are probably correct with the additional 2 kg. In that case I would be looking to increase the span, right up to the 2m perhaps.

 

It will be interesting to hear which way the students go.

 

Although, the rules now do seem to favour a biplane, I guess that a tandem wing, will be considered by the brave.

 

Yes, it will be interesting, in my dotage, I have come to favour incremental improvements of known technology, unless that is a step change is a realistic achievable, and the risks can be understood and measures taken to minimise them.

 

None the less, I would be really interested to receive updates of how the competition progresses, right up to the final event.

[Matt Watts]

Now there's an idea - a design/build blog. We have a meeting tomorrow, so I'll see if I can sort that out. People are still throwing basic design ideas around like hot potatoes, so it will be some time before we'll have anything particularly interesting to say.

 

Oh, and by the way - the telemetry idea. Thank you, thank you, THANK YOU! I had completely forgotten such things even existed! I guess this is what happens when I don't keep up with things. Hell, I haven't even flown since last summer, after I accidentally fried the Raven's receiver...

 

Much catching up to be done. Learning is fun!