Saturday, October 17, 2009

Drag explained for model aeroplanes

This is my attempt to provide a simple explanation of drag for the model builder.

DRAG is the force that resists the forward motion of an object through the air. For model aircraft, there are essentially two types of drag (refer to Figure 1 which is a plot of Drag force against air speed, v):

1. PARASITE DRAG is the resisting force that is due to the shape, roughness and form of the aeroplane. As the model moves through the air, the flow is disrupted by the shape and texture of the model. Its magnitude is given by the expression shown in Figure 1 (red, think of A as a constant). Parasite drag increases as the plane speeds up. Think of it as "High speed drag".

2. INDUCED DRAG is simply the drag force caused by the circular flow of air around the tips of the wing, that is, the vortex. An alternative name is "vortex drag". To get an idea of what it "looks" like, just search on wake turbulence or wing vortex in Google Image Search. Notice also the dependency with the lift force squared, L^2. That is why it is also known as "drag due to lift". So, the induced drag also increases with lift, for example with increasing angle of attack. Its magnitude is given by the expression shown in Figure 1 above (black, think of B as a constant). Since it is inversely proportional to v^2 it means that as the model speeds up, the induced drag decreases. Conversely, as the plane slows down, induced drag increases. Think of it as "Slow speed drag".

The total drag is the sum of the parasite drag and the induced drag (see blue curve in Figure 1 and Equation (1)).

Note for readers who like a bit of mathematics: Equation (2) is the "drag polar". To understand where it comes from you would need to look at an aerodynamics book (see References). The full expression for Equation (1) is Equation (6). You can easily derive Equation (6) by substituting equations (3) and (4) into (2) and recalling the relationship between aspect ratio (AR), span (b) and area (S). Equation (5) is the usual expression for dynamic pressure (q).

So much for the theory...

What does this mean for model aircraft performance and design?

1. It is generally desirable to reduce drag. Lower drag means a flatter glide (see the discussion of glide angle in a previous blog post here). In other words a higher L/D, which is a key performance indicator. Modern open class gliders can achieve L/D of 60 or better. In contrast, the Wright Flyer of 1903 had L/D of 5.7.

2. Mathematically, Equation (1) means that the total drag is a minimum at the air speed where the parasite drag equals the induced drag. That is the air speed where the best L/D is achieved. So if one of these drag contributors (parasite or induced) is low at that speed, then the total drag will be two times that low number. While that's a good thing, the plane's behaviour may suffer at one or other extreme of speed.

3. Take for example a small span pylon racer model. It has a small cross section area, smooth, clean, polished surfaces and therefore low parasite drag. Due to its high wing loading, it will have a high induced drag (this will be explained further below). Since it is fast, it will generally fly on the right hand side of the drag-speed curve shown in Figure 1. The good news is that parasite drag is lowish for this model, so it will perform fine under normal operating conditions; it is not impaired by the high induced drag.

4. For slow flyers for example, thermalling gliders and free flight rubber planes, induced drag is much more important. At low air speeds, parasite drag does not have any appreciable influence - this is the left hand side of Figure 1.

5. For many types of aircraft however, both parasite drag and induced drag should be minimised, for instance hand launched gliders and catapult launched gliders. These travel quite fast on release, so low parasite drag means a higher launch. After the transition to glide, they fly slowly, so low induced drag is required for a flatter glide. Another example is the RC glider. It needs to glide well at slow speed in order to climb in thermals. Then after the climb, it needs to be able to glide at shallow angle to cover lots of ground with little loss of height, in order to catch the next thermal. Induced drag is very important for free flight models too, including rubber power.

Reducing Induced Drag and Parasite Drag

6. The wing is the biggest contributor to both induced drag and parasite drag. So concentrate on the wing before the fuselage and tail feathers!

7. The biggest factor for reducing induced drag is the span loading W/b. This comes from Equation (6), noting that L~W, and see also previous blog. It is not as simple as just increasing AR for reasons explained in that blog post. (The over-emphasis sometimes placed on increasing AR to reduce induced drag probably arises from the dimensionless expression in Equation (2) above).

8. That means keep her light and make her span as big as allowed!

9. Another factor to reduce induced drag is wing planform design (elliptical and similar shapes are good). Non-planar surfaces can also reduce induced drag compared to a same span planar wing. For example, winglets, polyhedral configurations and span-wise camber. Some efficiencies can also be gained from multi-surfaces (e.g. boxplanes), but there is obviously a parasite drag and weight penalty.

10. For reducing parasite drag the biggest factors are the apparent cross section area and the wetted area (the area of the plane that is in contact with the air). Keeping the fuselage as narrow and small as possible is a good start. Sharp corners and junctions between wing and fuselage could be smoothed or "filleted" to reduce the drag. Surface roughness also plays a part (but its not as simple as smoother the better: sometimes a rough surface can keep air flow attached to the wing - "turbulators" are a PhD study on their own!).

11. Note that adding weighty fairings and cowlings in an effort to reduce parasite drag could be counter productive because it may increase induced drag! Fairings and such like may help the high speed flight, but could ruin the low speed glide.

CONCLUSION

I hope this blog has helped you to understand drag. Think about what your plane will be doing most of the time. Flying fast or flying slow? What kind of drag would be most relevant to your model? Having decided that, work to reduce the predominant source of drag. However, concentrate on the wing first. As ever, weight is a major factor especially for induced drag.

REFERENCES

1. Anderson J D (2005) Introduction to Flight, McGraw Hill, 5th edition

2. Simons M (1999) Model Aircraft Aerodynamics, Special Interest Model Books, 4th edition

3. Kroo I (2001) Drag due to lift: Concepts for prediction and reduction, Ann. Rev. Fluid. Mech 33:587-617

Saturday, September 26, 2009

Queen Bee 8" HLG: A simple hand launched glider


Nothing in aeromodelling gives me more pleasure than flying the humble chuckie. Lately, I've been messing around with RC: gliders and electric planes. They're fun, but for some reason, the enjoyment can wear off. I always come back to the chuck glider. Photos here of a simple model that I enjoyed flying yesterday and today, in the light wind and sunny blue sky of September.

Best flight was a 66s today, but alas, that was not one I'd declared for the tinygliders postal competition


I guess it's obvious why she's called QUEEN BEE. She has a bamboo fuselage and simple wings.

Another thing. Chucking gliders is great exercise. I can tell that it has been a while since I did some throwing, because my body really feels it - especially the bicep and shoulder!


Yesterday, I lost WEE BO WEEVIL. It went OOS at ~210s. The DT had triggered - I could see the elastic hanging, but the wing failed to pop up.... :( All the more upsetting because the DT worked perfectly for maxes earlier that day.


Tuesday, August 18, 2009

Albert Hatfull: The Senator, Junior 60 and much, much more!


Albert Edward Hatfull ought to be more famous.

At free flight events in the UK (and around the world) you are likely to see a SENATOR rubber model soar up into the sky for yet another flight over 3 minutes. I never tire of watching the SENATOR. It may well be the most successful rubber model that has ever been sold in kit form (by Keil Kraft). To demonstrate what I mean, here is a great video of a SENATOR being trimmed on a short motor run by US modeller Almico123:





Isn't that superb?!

Many RC afficianados will know and love the Junior 60 design. Originally a free flight "gasser", you'll still see Junior 60s being used by radio control clubs as "workhorses" and camera platforms. It is charming and possesses a functional attractiveness that is difficult to explain to non-modellers. See this fabulous article on an electric conversion by Barry Slater.

The plans for these iconic designs are as beautiful as the models themselves. The SENATOR and JUNIOR 60 were just two of the many models designed by Albert Edward Hatfull. The plans were drawn by him while he was engaged by Keil Kraft as a young man.

Albert was born in 1926 in London. His family had a strong engineering and sea-faring tradition. However, he was into aeroplanes and from the age of 9 he cut his teeth on Megow and Guillow's kits. By all accounts he was a colourful character with a great sense of humour. He was fond of New Orleans jazz, followed the US model aircraft scene (Korda et al) and played the piano. He attended Tottenham Technical College, studied maths and draughtsmanship and developed an interest in aerodynamics. Sadly in 1942 at the tender age of 16, he contracted polio from a local swimming pool. This was a time when vaccination was not yet available. The disease restricted his hand mobility, but the wonderful thing is that he did not let it stop him from being creative.

Soon after the polio infection, the youthful designer created his first model for Keil Kraft. As it was wartime, he named it the INVADER glider and it became a best-seller. Before the age of 20, the JUNIOR 60 was created, quite literally "while the V2s were falling on London". Despite the polio, he built a career as a proficient draughtsman. Engineering drawings and later patent work at the Science Library provided a living, but model aircraft were his passion. He designed dozens of planes, including the SENATOR in ca. 1950 and the SKYSCRAPER around the same time. The two models are very similar, the main difference being an undercambered fuselage on the SKYSCRAPER. Albert was clearly interested in aerofoil shaped "lifting" fuselage profiles. Interestingly, he did not report any difference in the performance of the SENATOR as compared to the SKYSCRAPER and the former became much better known, widely flown.

Apart from gliders (e.g. SOARER series) and rubber power jobs, he also designed many Jetex models. Of the jet designs, his personal favourite was the De Havilland VENOM. Many of his plans were published in the aeromodelling press. He even designed Keil Kraft's triangular trade mark logo!

After Keil Kraft, he did some design work for Worcraft models (Dewsbury) and Elite models (Manchester). These included the Worcraft SCARAB - with lovely elliptical wing profiles and the Elite ELF, which like the SKYSCRAPER, sported a beautiful aerofoil shaped "lifting" fuselage profile. Ill health forced him into retirement in 1984 and he passed away in 2007.

I hope this blog post helps to make Albert E. Hatfull's name as famous as his models. Aeromodellers will not forget him!

(I hope to continue researching and will endeavour to expand this blog post accordingly. For example, I wonder if someone has a good list of all his models - if you do, please email me!).

References

1. Harry Payling (2007), Obituary of Albert Edward Hatfull, New Clarion SAM 1066 newsletter, November 2007, p.20-21.

2. Bill Morley (1996), SAM meets Albert, SAM 35 Yearbook 9 , p.2-7.

3. Andrew Longhurst (1995), Rubber Column No.149, SAM Speaks No.155 June 1995, p.3, 5-6.


Tuesday, July 14, 2009

Mad Scorpio RC glider and Things to do


This is megatastic, a Scorpio rocket glider with RC! :)




I've got many projects that are sitting around to be completed. So much to do, so little time. Work, family and other aspects of life always seem to take higher priority. Here's a potted list:

  • Scorpio 19.25" FF Rapier rocket powered glider, a superb design by Steve Bage. Lovely kit from Shorty's Basement to build. Cool video here.
  • Pearl 1.49m R/E radio control discus launched glider. ARF from South Coast Sailplanes, but kitting it out with radio gear will take some concerted effort and time.
  • Dingbat vintage HLG from plans, which I've been meaning to build for years.
  • AND Lots of flying to do - CLGs, HLGs, DLGs, rubber, RC electric, RC gliders...


Wee Bo Weevil: 12" Catapult Launched Glider CLG


The original Texas Bo' Weevil hand launched glider HLG by Don Chancey was published in American Aircraft Modeler in 1972. It was a 17" span all wood design of traditional layout and very steady, reliable performance.

I thought it would be fun to miniaturise, modernise and catapultise it!

So, here's a 12" span scaled down version, with carbon fibre boom and pop-up wing DT operated by a button timer. Natty rainbow colours anyone?











Side view showing the wing down. To give you a sense of scale, the button timer on the nose is 15mm diameter.

I call it the Wee Bo Weevil.








Another view of the DT system, with wing in the up position.

All up weight including nose ballast is 13.5g. It launches and flys pleasingly well, but is not as super-performing as a "clean" and modern 12" design, a la the experts such as Lee Hines, Tony Matthews, et al.

Fun though, and it's great to watch dog walkers in the park looking puzzled as it DTs down.

Sunday, February 15, 2009

FREE MODEL GLIDER PLAN! Tiny glider, 8 inch Catapult Launched Glider (CLG)

If you downloaded this plan before 28 February 2009, please discard that copy and use this version instead - some important details were either wrong or missing Sorry!

As you can see from earlier blog posts, I've been playing with Charlie Sotich's simple 8" CLG. Necessity really, after the original 1/32 thin wings - as per his plan - shredded on launch.


So here is my "re-mix" of his design. Main changes are conventional HLG style wing, three panel instead of V-dihedral, a slightly longer nose and some mods for a stronger wing fuselage joint.

Sotich named many of his designs [SOMETHING] DIP. For example, his famous 48" glider was called LIL DIP. Since this 8 incher is intended for the Tiny Gliders postal event, I called it the TINY DIP, but I don't know whether he ever made a design with that name.

I hope you enjoy the plan - drawn with a smile on my face and a bit of tongue in cheek. An easy build, lots of fun, pretty in the air and satisfying performance (right click on the plan and select Save Image As). Try a 3/16" rubber loop for the catty. You can use kevlar tow instead of CF. Fix to the model's balsa sides with CA glue. Note the washout on both tips and the washin triangular wedge on the left wing only. There is a slight incidence (decalage) in the design. I visualise it as the bottom of the fuselage being horizontal, and the top line of the fuselage a slope, taller at the front. The closer you get to 0-0, the straighter and higher the launch, but stability decreases. Too much decalage, then the climb becomes a height-sapping spiral (or worse, gets loopy).

Photo below of the TINY DIP glider (4.1g ready to fly).




Sunday, February 8, 2009

Source of Vintage (Old Timer) Plans and Articles


There's an excellent new source of information on "vintage" model gliders and free flight aircraft. The POPULAR SCIENCE magazine series has been digitized and is now available on the internet via Google Book Search.


Screen shot of an article by Frank Zaic on the THERMAL HUNTER rubber model is shown left. It also includes building instructions, tips and drawings.



I had a look through some issues for articles and this is just a selection of the gems that I found:
  1. New Thermal Hunter Plane Model, Frank Zaic, August 1936, p60
  2. International Contest Flyer, Frank Zaic, June 1940, p150
  3. Model Fuselage Building, Edwin T Hamilton, July 1931, p110
  4. Give your model the right propeller, Edwin T Hamilton, July 1930 p96
  5. From ten parts you can build twenty model planes, Edwin T Hamilton, August 1931, p72
  6. Model-Airplane Short Cuts, Frank Zaic, September 1941, p170
  7. Test Flying Model Airplanes, Frank Zaic, June 1942, p196
  8. Gas Models and Towline Gliders, Frank Zaic, September 1942, p.HW230
  9. Novel Inventions aid Model-Plane Champions [no author indicated] November 1939, p67
  10. The modern 300 incher, Zaic, Aug 1942, p.HW174
  11. Hints that will help you build airplane models, Edwin T Hamilton, January 1931, p116
  12. Miss Science (gas model), Frank Zaic, September 1939, p142
Happy reading!

Thursday, February 5, 2009

Tiny Gliders Postal 2009



Alexandre Cruz has created a fabulous blog for the Tiny Gliders postal event this year. Please do enjoy building some 6" and 8" gliders and fly in it. It's a lot of fun and easy to join in! It would make a great club event too, or activity for the kids. There's no excuse....

Left, a photo of my 6" DOGEAR hlg.

Thanks Alexandre!


Tuesday, January 27, 2009

P-51D Mustang - Completed model


Here she is, complete with pilot. One thing I learn from this is to never, I repeat, NEVER, use domestic tissue again. Japanese is so much better. This stuff shrinks in weird ways, is not easy to apply and the finish is lousy.


I love the lines of that wacky Dick Baxter prop - which is pretty much as per plan. I used a yoghurt pot rather than a coffee cup.







Weight without rubber was 8.8g. Prop on its own weighed 1.8g. Here is a photo of two prop centre shafts. The top is a ramp type of freewheel clutch, the bottom has a Garami-style latch. They're both smooth, but the ramp is more compact so I used it because of the spinner. Spinner was made out of tissue pva'd to paper. Cut out a semi-circle and made a cone out of that.




Power is one 1/8" loop. So far, I've only given her ~150 turns. Stab adjustment was required, but no ballast at all (thus far). All indications are that she's a good flyer! Need a good day to try her out properly.


A very enjoyable build.

Saturday, January 10, 2009

Free Flight Postals

Postal events allow competition without the formality of a meet. Each modeller sends in scores by a certain deadline, usually a series of times. Many are run on an honour basis - and I certainly don't know anyone who'd cheat by sending in false times - what's the point? In any case, weather conditions are not the same for everyone. So in my view, postals should not be thought of as "real" competitions.

Click here for a super list of some free flight postals. I was sad to see that the 2008 Tiny Gliders Postal had few entries because it is one of my favourites. The plus side is that Alexandre Cruz did a great job of keeping it going and there is good reason to anticipate that it will be bigger and better in 2009! If I find out the new website for it, I'll post it here.

Finally, how on earth did this patent get granted by the US Patent Office in 1975? There's no way it was novel or inventive at that time!
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