Bouncy landings, particularly EDFs

[Robin Colbourne]

Our club has two sites, one a grass strip, the other a former airfield with a concrete runway.  No problems have been reported landing on the grass, however it is particularly noticeable on the concrete that certain EDF models get positive feedback bounces on the noseleg even after a smooth touchdown, yet engine-powered aerobatic models such as the 1970s Curare, Bullet and Gangster, once down stay down.
In most cases the models are tricycle undercarriage, with the mainlegs mounted on the wing, although an EDF F-16 has been known to suffer too.
Obviously the joins between the concrete runway slabs don't help, but there seems to be more to it than this.  I did wonder if the IC models dissipate the energy by slopping the remaining fuel around the tank, whereas the EDF batteries are a solid lump.

Has anyone else experienced difficulty with getting consistently smooth roll outs after landing EDFs on hard runways, and if so what was the solution?
 

Edited June 26, 2023 by Robin Colbourne

[Phil Green]

I've seen edf's with long noselegs to help take off - might cause what you're describing Robin

[Robin Colbourne]

10 minutes ago, Phil Green said:

I've seen edf's with long noselegs to help take off - might cause what you're describing Robin

Hi Phil,  If you mean like the Phantoms on HMS Ark Royal in the 1970s, then that's not it.  I've seen what you describe on trainers like the Precedent Hiboy.  Repeated hard landings have bent the main legs, but the nose leg stays the same and the model sits nose up, resulting in positive wing incidence after landing.
These are EDF models are built as per the supplied kit, e.g. DH Vampire, DH Venom, BAe Hawk and sitting as the manufacturer intended them.    It did occur to me that a bigger main wheels or a shorter nose leg would give negative wing incidence, but I would have thought it would have to be pretty extreme to make much improvement.

[Cuban8]

Maybe a softer tyre would add a bit of damping? Position of main wheels? A little experimentation to see whether a tad forward or back makes a difference? Not sure if it's aerodynamics at play or weight distribution.

[Denis Watkins]

Softer tyre definately. Hard foam tyres, often provided by the manufacturer, save weight but are difficult to land on tarmac.

Low bounce, rubber tyres are heavier and more expensive but do provide " low bounce "

[Robin Colbourne]

Cuban8 and Denis Watkins, that was my thinking, although the DH Venom owner did try a Radio-Active low bounce airwheel on the nose instead of the foam tyre, but with little improvement.  I did suggest poking some holes in the tyre to let the air out thus improving the damping, but I don't think he's tried this yet.

Changing the leg geometry might help, although as these are ARTFs or RTFs, with retracts, options are limited without significant engineering if the retracts are still to work.

[Chris Walby]

Perhaps its just a case of landing too fast? If I haven't blead all the airspeed off then the model lands, just it still has enough energy and airspeed to tale off again when promoted by a suitable pitch up runway feature. The grass will act as a high drag retard on the wheels and hence keep the model down if landed a bit quick.

 

I am quite surprised and how slow some EDF will fly/glide and how long I can hold off landing....sometimes well beyond the  runway threshold (time to go around)! In part I think its to do with the quite quick flight speed (getting used to quick) and then the pilot not getting accustomed to the low speed characteristics. 

 

A few of the Chris Foss designs reward the pilot with this effect if you have not reduced your air speed suitably, this can be countered with a good dollop of down elevator at the right time, but it does feel weird pushing the nose down especially with a tail dragger.

[Andy Stephenson]

Tail draggers will always tend to bounce because the main gear is in front of the CG. Chris Foss models particularly as they have wheels well out front. Try to land an Acrowot without bouncing, it comes out right about once in 10 landings.

[Zflyer]

Rrr now i know why my Wot 4 mkll lands, lands, lands again!!!

[J D 8]

  By mistake I fitted my WOR 4 UC the wrong way around. This was only pointed out by another club member after I had been flying it some time.

As I have had no problems with any half decent landings it has remained that way.

[Peter Jenkins]

On 26/06/2023 at 12:56, Chris Walby said:

Perhaps its just a case of landing too fast? If I haven't blead all the airspeed off then the model lands, just it still has enough energy and airspeed to tale off again when promoted by a suitable pitch up runway feature. The grass will act as a high drag retard on the wheels and hence keep the model down if landed a bit quick.

 

I am quite surprised and how slow some EDF will fly/glide and how long I can hold off landing....sometimes well beyond the  runway threshold (time to go around)! In part I think its to do with the quite quick flight speed (getting used to quick) and then the pilot not getting accustomed to the low speed characteristics. 

 

A few of the Chris Foss designs reward the pilot with this effect if you have not reduced your air speed suitably, this can be countered with a good dollop of down elevator at the right time, but it does feel weird pushing the nose down especially with a tail dragger.

Spot on Chris.  Most pilots tend to approach too fast for fear of stalling.  But that excess speed has to be bled off as Chris says and it can take time with a slippery airframe.  I can put a 5 Kg aerobatic model down on our tarmac/concrete runway, without bouncing when I get it right, but as the speed is low the bounce is usually very slight and I've stopped by mid runway.  Some of the EDF jets, which are much smaller, can end up overshooting the runway.  Worth doing a series of approaches and getting the speed progressively slower and seeing when the aircraft starts to feel a bit wooly in response to control inputs - that's now too slow!  Also, assuming you don't have trees, hedge of a fence to hop over, try approaching on a 3 deg glide slope which is very shallow and aim to land at the threshold.  Once you have rounded out just above the ground, just hold that attitude till the aircraft sinks onto the ground.  If you got the speed right, as the wheels touch that will knock off the final bit of airspeed and there will be no bounce.

 

Re Chris Foss's designs, I have a Wot 4 and if I follow my own advice above it will land on without bouncing.  Same goes for the Wots Wot.  You just need to keep that elevator stick coming back and keep the aircraft flying for as long as possible just off the ground.

[Martin Harris - Moderator]

Yes, spot on.  
 

I wish I could find a good way to describe - and teach - the feeling of connection with a model on approach at the “right” airspeed where the model almost feels like it’s hanging on a string connected to the throttle lever.  The slightest of adjustments can be seen as changes in the glide path. 
 

This infers that an approach is flown with a relatively high angle of attack (well below the stall angle of course) and a small amount of power to balance the resultant drag.  It’s not appreciated by all pilots that a glide approach will have a faster airspeed than a properly balanced powered one. 
 

In the case of a slippery and possibly high wing loaded EDF these effects are magnified - the preponderance of low wing loaded draggy trainers used these days does little to build good approach technique in new pilots. 
 

Why am I harping on about approach technique on a landings thread?  It’s simply that a good approach makes the transition to the flare and hold off so much easier - and as so eloquently described by Peter, that hold off is the key to a bounceless landing. 

[Chris Walby]

Thanks Peter spot on in my book.

Only thing to add is the issue with depth and speed perception at distance and long low approaches as that it can be difficult to determine the actual air speed of the model.

 

Last weekend with my Habu ss I was surprised that (with nearly no wind) I needed to extend the final circuit (started at half throttle and dropped to quarter while descending) that it needed no throttle before the final descending turn over the trees. For a delta shape wing it does glide very well.

[Peter Christy]

Two comments: "Classic" aerobatic models - as described in the original post - tended to have shorter noselegs. When the model was on the ground, it had a distinct nose-down attitude. This is because, in days of yore, aerobatic contests scored the take-offs and landings as part of the flight. A shorter noseleg (or smaller wheel) meant that the model would very quickly adopt a nose down attitude once the main wheels touched, reducing the tendency to bounce, and improving one's score!

 

Secondly, bouncing primarily occurs because the aircraft still has sufficient airspeed to fly - and will try to do so given the slightest excuse!

 

Many decades ago, my full size flying instructor was teaching me to stall it six inches off the tarmac - something that I found particularly hard to judge! After one attempt, which resulted in a moderate bounce, rather than open the throttle and go around, I did what any RC flyer would do - lower the nose and flare it again. My instructor was rather amazed and asked where I'd learned to do that. "Flying RC models!", I replied. He laughed, but in a very good-natured way!

 

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Pete